Characterizing and Classifying Eukaryotes

      Characterizing and Classifying Eukaryotes A unique characteristic of living things is the ability to reproduce themselves. Prokaryotic reproduction typically involves replication of DNA and binary fission of the cytoplasm to produce two identical offspring. Reproduction of eukaryotes is more complicated and varied than reproduction in prokaryotes for a number of reasons: General Characteristics of Eukaryotic Organisms Nuclear Division Reproduction of Eukaryotes LE A R N I N G O U TC O M E S 12.1 State four reasons why eukaryotic reproduction is more complex than prokaryotic reproduction. 12.2 Describe the phases of mitosis, mentioning chromo- somes, chromatids, centromeres, and spindle. 12.3 Contrast meiosis with mitosis, mentioning homologous chromosomes, tetrads, and crossing over. 12.4 Distinguish among nuclear division, cytokinesis, and ­ ­ Eukaryotic reproduction involves two types of division: nuclear division and cytoplasmic division (also called cytokinesis). After we discuss the various aspects of these two types of division, we will consider schizogony. ­ Typically, a eukaryotic nucleus has either one or two complete copies of the chromosomal portion of a cell’s genome. A nucleus with a single copy of each chromosome is called a haploid, 5 or 1n, nucleus, and one with two sets of chromosomes is a diploid,6 or 2n, nucleus. Generally, each organism has a consistent number of chromosomes. For example, each haploid cell of the brewer’s yeast Saccharomyces cerevisiae7 (sak-ă-rō-mī’sēz se-ri-vis’ē-ī) has 16 chromosomes. The cells of most fungi, many algae, and some protozoa are haploid, and the cells of most plants and animals and the remaining fungi, algae, and protozoa are diploid. Typically, gametes are haploid, and a zygote (formed from the union of gametes) is diploid. A cell divides its nucleus so as to pass a copy of its chromosomal DNA to each of its descendants so that each new generation has the necessary genetic instructions to carry on life. There are two types of nuclear division—mitosis (mī-tō’sis) and meiosis (mī-ō’sis). ­ Our discussion of the general characteristics of eukaryotes begins with a survey of the events in eukaryotic reproduction; then we consider some aspects of the complexity of classifying the great variety of eukaryotic organisms. • Most of the DNA in eukaryotes is packaged with histone proteins as chromosomes in the form of chromatin fibers (krō’ma-tin) located within nuclei. The remaining DNA in eukaryotic cells is found in mitochondria and chloroplasts, organelles that reproduce by binary fission in a manner similar to prokaryotic reproduction. In this chapter we will discuss only the nuclear portion of eukaryotic genomes. • Eukaryotes have a variety of methods of asexual reproduction, including binary fission, budding, fragmentation, spore formation, and schizogony (ski-zog’ō-nē) (discussed later). • Many eukaryotes reproduce sexually—that is, via a process that involves the formation of sexual cells called gametes, and the subsequent fusion of two gametes to form a cell called a zygote. • Additionally, algae, fungi, and some protozoa reproduce both sexually and asexually. (Animals generally reproduce only one way or the other.) ­ ­ Eukaryotic microbes include a fascinating and almost bewilderingly diverse assemblage. They include unicellular and multicellular protozoa,1 fungi,2 algae,3 slime molds, and water molds. Additionally, microbiologists study parasitic helminths4 because they have microscopic stages, and they study arthropod vectors because they are intimately involved in the transmission of microbial pathogens. Eukaryotes include both human pathogens and organisms that are vital for human life. For example, one group of marine algae called diatoms, together with a set of protozoa called dinoflagellates (dī’nō-flaj’ĕ-lātz), provide the basis for the oceans’ food chains and produce most of the world’s oxygen. Eukaryotic fungi produce penicillin, and tiny baker’s and brewer’s yeasts are essential for making bread and alcoholic beverages. Among the 20 most frequent microbial causes of death worldwide, six are eukaryotic, including the agents of malaria, African sleeping sickness, and amebic dysentery. Pneumocystis pneumonia, toxoplasmosis, and cryptosporidiosis—common afflictions of AIDS patients—are all caused by eukaryotic pathogens. In previous chapters, we discussed characteristics of cells— their metabolism, growth, and genetics. In this chapter, we discuss eukaryotic organisms of interest to microbiologists— protozoa (single-celled “animals”), fungi, algae, slime molds, and water molds—and conclude with a brief discussion of the relationship of parasitic helminths and vectors to microbiology. We begin by discussing general features of eukaryotic reproduction and classification; the following sections survey some representative members of microbiologically important eukaryotic groups, focusing on beneficial, environmentally significant, and unusual species. (Chapters 19–24 discuss fungal and parasitic agents and vectors of human disease in more detail.) schizogony. M12_BAUM7206_05_SE_C12.indd 351 5From Greek haploos, meaning “single.” Greek diploos, meaning “double.” Greek sakcharon, meaning “sugar,” and mykes, meaning “fungus,” and Latin cerevisiae, meaning “beer.” 6From 7 From  2Plural ­ Greek protos, meaning “first,” and zoion, meaning “animal.” of Latin fungus, meaning “mushroom.” 3Plural of Latin alga, meaning “seaweed.” 4From Greek helmins, meaning “worm.” 1From 11/23/16 3:29 AM 12    Characterizing and Classifying Eukaryotes 1 2 3 4 Prophase. The cell condenses its DNA molecules into visible threads called chromatids (krō’mă-tidz). Two identical chromatids, sister DNA molecules, are joined together in a region called a centromere to form one chromosome. Also during prophase, a set of microtubules is constructed in the cytosol to form a spindle. In most cells, the nuclear envelope disintegrates during prophase so that mitosis occurs freely in the cytosol; however, many fungi and some unicellular microbes (e.g., diatoms and dinoflagellates) maintain their nuclear envelopes so that mitosis occurs completely inside their nuclei. Metaphase. The chromosomes line up on a plane in the middle of the cell and attach near their centromeres to microtubules of the spindle. Anaphase. Sister chromatids separate and crawl along the microtubules toward opposite poles of the spindle. Each chromatid is now called a chromosome. Telophase. The cell restores its chromosomes to their less compact, nonmitotic state, and nuclear envelopes form around the daughter nuclei. A cell might divide during telophase, but mitosis is nuclear division, not cell division. Cell division is called cytokinesis. Though certain specific events distinguish each of the four phases of mitosis, the phases are not discrete steps; that is, mitosis is a continuous process, and there are no clear boundaries between succeeding phases—one phase leads seamlessly to the next. For example, late anaphase and early telophase are indistinguishable. Students sometimes confuse the terms chromosome and chromatid, in part because early microscopists used the word chromosome for two different things. During prophase and metaphase, a chromosome consists of two chromatids (DNA molecules) joined at a centromere. However, during anaphase and telophase, the chromatids separate, and each chromatid is 8Latin, meaning “between phases.” Greek mitos, meaning “thread,” after the threadlike appearance of chromosomes during nuclear division. 10From Greek pro, meaning “before,” and phasis, meaning “appearance.” 11From Greek meta, meaning “in the middle.” 12From Greek ana, meaning “back.” 13From Greek telos, meaning “end.” 9From M12_BAUM7206_05_SE_C12.indd 352 then called a chromosome. In other words, a prophase or metaphase “chromosome” is composed of two chromatids, whereas chromatid and chromosome are synonymous terms during anaphase and telophase. Meiosis In contrast to mitosis, meiosis 14 is nuclear division that involves the partitioning of chromatids into four nuclei such that each nucleus receives only half the original amount of DNA. Thus, diploid nuclei use meiosis to produce haploid daughter nuclei. Meiosis is a necessary condition for sexual reproduction (in which nuclei from two different cells fuse to form a single nucleus) because if cells lacked meiosis, each nuclear fusion to form a zygote would cause the number of chromosomes to double, and their number would soon become unmanageable. Meiosis occurs in two stages known as meiosis I and meiosis II (FIGURE 12.1b). As in mitosis, each stage has four phases, named prophase, metaphase, anaphase, and telophase. The events in meiosis as they occur in a diploid nucleus proceed as follows: 1 2 Early prophase I (prophase of meiosis I). As with mitosis, DNA replication during interphase results in pairs of identical chromatids joined at centromeres, forming chromosomes. But in meiosis I, additional pairing occurs: homologous chromosomes—that is, chromosomes carrying similar or identical genetic sequences—line up side by side. Because these are prophase chromosomes, each of them consists of two identical chromatids; therefore, four DNA molecules are involved in this pairing. An aligned pair of homologous chromosomes is known as a tetrad. Late prophase I. Once tetrads have formed, the homologous chromosomes exchange sections of DNA in a random fashion via a process called crossing over. This results in recombinations of their DNA. It is because of meiotic crossing over that the offspring produced by sexual reproduction have different genetic makeups from their siblings. Prophase I can last for days or longer. Metaphase I. Tetrads align on a plane in the center of the cell and attach to spindle microtubules. Metaphase I differs from metaphase of mitosis in that homologous chromosomes remain as tetrads. Anaphase I. Homologous chromosomes of the tetrads, carrying crossed-over DNA, move apart from one another. In contrast to mitotic anaphase, sister chromatids remain attached to one another. Telophase I. The first stage of meiosis is completed as the spindle disintegrates. Typically, the cell divides at this phase to form two cells. Nuclear envelopes may form, though prophase II may begin immediately without a nuclear envelope forming. Each daughter nucleus is now haploid, though each meiotic haploid chromosome at this stage consists of two chromatids. ­ Mitosis Eukaryotic cells have two main stages in their life cycle: a stage called interphase,8 during which the cells grow and replicate their DNA, and a stage during which the nucleus divides. In the type of nuclear division called mitosis, 9 which begins after the cell has duplicated its DNA such that there are two exact DNA copies (see Chapter 7, Figure 7.6), the cell partitions its replicated DNA equally between two nuclei. Thus, mitosis maintains the ploidy of the parent nucleus; that is, a haploid nucleus that undergoes mitosis forms two haploid nuclei, and a diploid nucleus that undergoes mitosis produces two diploid nuclei. Scientists recognize four phases in mitosis: prophase,10 meta­ phase,11 anaphase,12 and telophase.13 The events of mitosis proceed as follows (FIGURE 12.1a): ­ ter a       Ch p 3 4 5 14From Greek meioun, meaning “to make smaller.” 11/23/16 3:29 AM       General Characteristics of Eukaryotic Organisms (a) Mitosis (b) Meiosis Diploid nucleus (2n) Diploid nucleus (2n) DNA replication 1 Prophase I Tetrad (two homologous chromosomes, four chromatids) DNA replication 2 Late prophase I MEIOSIS I Crossing over 1 Prophase 3 Metaphase I Chromosome (two chromatids) Centromere 4 Anaphase I 2 Metaphase Spindle Chromosome (two chromatids) 5 Telophase I 3 Anaphase Cytokinesis 6 Prophase II 4 Telophase 7 Metaphase II Nuclear envelope MEIOSIS II Two diploid nuclei (2n) 8 Anaphase II ▲ FIGURE 12.1 The two kinds of nuclear division: mitosis and meiosis. The events ­ occurring during each of the numbered phases are described in the text. (a) Mitosis, in which the number of chromosomes (ploidy) in the daughter nuclei is the same as in the parent nucleus. Cell division (cytokinesis) may occur simultaneously with mitosis, but mitosis is not cell division. (b) Meiosis, which results in four nuclei, each with half the number of chromosomes of the parent nucleus. 9 Telophase II Cytokinesis Cytokinesis Nuclear envelope Four haploid nuclei (1n) M12_BAUM7206_05_SE_C12.indd 353 11/23/16 3:29 AM 8 9 Classification of Eukaryotic Organisms LE A R N I N G O UTC O M E S 12.5 Briefly describe the major groups of eukaryotes as they were first classified in the late-18th century and as they were classified in the late-20th century. 12.6 List some of the problems involved in the classification of protists in particular. Historically, the classification of many eukaryotic microbes has been fraught with difficulty and characterized by change. Since the late-18th century, when Carolus Linnaeus (1707–1778) began modern taxonomy, until near the end of the 20th century taxonomists grouped organisms together largely according to readily observable structural traits. Linnaeus classified unicellular algae and fungi as plants, and he classified protozoa—as the name suggests—as animals (FIGURE 12.4a). By the late-20th century, taxonomists had placed fungi in their own kingdom and grouped protozoa and algae together within the kingdom Protista (FIGURE 12.4b), though some taxonomists kept the green algae in the kingdom Plantae. This scheme was troublesome, in part because the Protista included large, photosynthetic, multicellular kelps; nonphotosynthetic unicellular protozoa; and organisms that have both animal-like characteristics and characteristics more typical of plants. An example of the latter is Euglena (yŭ-glēn’ă; see Figure 3.31a), which swims with a flagellum, can move its cytoplasmic membrane, and devours food—all animal-like qualities—but also has a wall-like structure and can photosynthesize sugar in chloroplasts, like plants. Adding to the confusion is the fact that taxonomists who classify plants use the term divisions to refer to the same taxonomic level that zoologists call phyla. More recently, many taxonomists have abandoned classification schemes that are so strongly grounded in large-scale structural similarities in favor of schemes based on similarities in nucleotide sequences and cellular ultrastructure as revealed by electron microscopy. One of the most evident results of such Cytokinesis (Cytoplasmic Division) Cytoplasmic division—also called cytokinesis (sī’tō-ki-nē’sis)— typically occurs simultaneously with telophase of mitosis, though in some algae and fungi it may be postponed or may not occur at all. In these cases, mitosis produces multinucleate cells called coenocytes (sē’nō-sītz). In plant and algal cells, cytokinesis occurs as vesicles deposit wall material at the equatorial plane between nuclei to form a cell plate, which eventually becomes a transverse wall between daughter cells (FIGURE 12.2a). Cytokinesis of protozoa and some fungal cells occurs when an equatorial ring of actin microfilaments contracts just below the cytoplasmic membrane, pinching the cell in two (FIGURE 12.2b). Single-celled fungi called yeasts form a bud, which receives one of the daughter nuclei and pinches off from the parent cell (FIGURE 12.2c). ­ Schizogony Some protozoa, such as Plasmodium (plaz-mō’dē-ŭm)—the cause of malaria—reproduce asexually via a special type of reproduction called schizogony (ski-zog’ō-nē; FIGURE 12.3). In schizogony, multiple mitoses form a multinucleate cell called a Characteristics of the Two Types of Nuclear Division DNA Replication Phases Mitosis Meiosis During interphase, before nuclear division During interphase, before meiosis I begins P TABLE 12.1 ­ ­ ­ In summary, meiosis produces four haploid nuclei from a single diploid nucleus. Meiosis can be considered backto-back mitoses without the DNA replication of interphase between them, though the four phases of meiosis I differ from those of mitosis. The phases of meiosis II are equivalent to those in mitosis. Additionally, crossing over during meiosis I produces genetic recombinations, ensuring that the chromosomes resulting from meiosis are different from the parental chromosomes. This provides genetic variety in the next generation. TABLE 12.1 compares and contrasts mitotic and meiotic nuclear divisions. schizont (skiz’ont); only then does cytokinesis occur, simultaneously releasing numerous uninucleate daughter cells. ­ ter Prophase II. Nuclear envelopes disintegrate, if they formed, and new spindles form. Metaphase II. The chromosomes align in the middle of each cell and attach to microtubules of the spindles. Anaphase II. Sister chromatids separate as in mitosis. Telophase II. Daughter nuclei form. The cells divide, yielding four haploid cells. ­ 7 12    Characterizing and Classifying Eukaryotes ­ 6 a       Ch p rophase, metaphase, anaphase, telophase Meiosis I—prophase I, metaphase I, anaphase I, telophase I Meiosis II—prophase II, metaphase II, anaphase II, telophase II Formation of Tetrads (alignment of homologous chromosomes) Does not occur Early in prophase I Crossing Over Does not occur Following formation of tetrads during prophase I Number of Accompanying Cytoplasmic Divisions That May Occur One Two Resulting Nuclei Two nuclei with same ploidy as the original Four nuclei with half the ploidy of the original M12_BAUM7206_05_SE_C12.indd 354 11/23/16 3:29 AM       P rotozoa Wall of parent cell Nuclei of daughter cells Vesicles forming cell plate ◀ FIGURE 12.2 Different types of cytoplasmic division. (a) Cytokinesis in a plant cell, in which vesicles form a cell plate. (b) Cytokinesis as it occurs in animals, protozoa, and some fungi. (c) Budding in yeast cells. TEM (a) 2 µm ­ Cleavage furrow taxonomic studies is that modern schemes no longer include the taxa “Protozoa” or “Protista”; instead, such eukaryotic microbes belong in several kingdoms. Though no one classification scheme has garnered universal support and more thorough understanding based on new information will almost certainly dictate changes, many taxonomists favor a scheme similar to the one shown in (FIGURE 12.4c). In this scheme, on which the discussions of eukaryotic microbes in this chapter are largely based, the organisms commonly referred to as protozoa are classified in six kingdoms: Parabasala, Diplomonadida, Euglenozoa, Alveolata, Rhizaria, and Amoebozoa; fungi are in the kingdom Fungi; slime molds are in the kingdom Amoebozoa; water molds are in the kingdom Stramenopila; and algae are distributed among the kingdoms Stramenopila, Rhodophyta, and Plantae. As we study the eukaryotic microbes discussed in this chapter, bear in mind that because the relationships among eukaryotic microbes are not fully understood, not all taxonomists agree with this scheme, and new information will shape future alterations in our understanding of the taxonomy of eukaryotic microbes. We begin our survey of eukaryotic microbes with the group of organisms commonly known as protozoa. TELL ME WHY Why is it incorrect to call mitosis “cell division”? Protozoa SEM (b) 100 µm LE A R N I N G O UTC O M E Bud 12.7 List three characteristics shared by all protozoa. The microorganisms called protozoa (prō-tō-zō’ă) are a diverse assemblage defined by three characteristics: They are eukaryotic, unicellular, and lack a cell wall. Note that “protozoa” is not a currently accepted taxon. With the exception of one subgroup (called apicomplexans), protozoa are motile by means of cilia, flagella, and/or pseudopods. The scientific study of protozoa is protozoology, and scientists who study these microbes are protozoologists. In the following sections, we discuss the distribution, morphology, nutrition, reproduction, and classification of various groups of protozoa. Distribution of Protozoa ­ Protozoa require moist environments; most species live worldwide in ponds, streams, lakes, and oceans, where they are critical members of the plankton—free-living, drifting (c) M12_BAUM7206_05_SE_C12.indd 355 SEM 2 µm 11/23/16 3:29 AM 12    Characterizing and Classifying Eukaryotes ◀ FIGURE 12.3 Schizogony. Multiple mitoses Cytokinesis Plantae Most protozoa are chemoheterotrophic; that is, they obtain nutrients by phagocytizing bacteria, decayed organic matter, other protozoa, or the tissues of a host; a few protozoa absorb nutrients from the surrounding water. Because the protozoa called dinoflagellates and euglenids are photoautotrophic, botanists historically classified them as algal plants rather than as protozoa. Animalia dop a men opil Stra zoa ebo Amo aria Rhiz a olat Alve zoa leno Eug mon adid Dipl o la basa Para Early 21st Century Fun gi (c) Animalia Plantae hyta Protista a Fungi Plantae Rho (b) Late 20th Century Nutrition of Protozoa Animalia Anim alia ­ Though protozoa have most of the features of eukaryotic cells (discussed in Chapter 3, pp. 352, and illustrated in Figure 3.3), this group of eukaryotic microbes is characterized by great morphological diversity. Some ciliates have two nuclei: a larger macronucleus, which contains many copies of the genome (often more than 50n) and controls metabolism, growth, and sexual reproduction, and a smaller micronucleus, which is involved in genetic recombination, sexual reproduction, and regeneration of macronuclei. Protozoa also show variety in the number and kind of mitochondria they contain. Several groups lack mitochondria, whereas all the others have mitochondria with discoid or tubular cristae rather than the platelike cristae seen in animals, plants, fungi, and many algae. Additionally, some protozoa tae Morphology of Protozoa have contractile vacuoles that actively pump water from the cells, protecting them from osmotic lysis (FIGURE 12.5). All free-living aquatic and pathogenic protozoa exist as a motile feeding stage called a trophozoite (trof-ō-zō’īt), and many have a hardy resting stage called a cyst, which is characterized by a thick capsule and a low metabolic rate. Cysts of protozoa are not reproductive structures: One trophozoite forms one cyst, which later becomes one trophozoite. Such cysts allow intestinal protozoa to pass from one host to another and to survive harsh environmental conditions such as desiccation, nutrient deficiency, extremes of pH and temperature, and lack of oxygen. ­ organisms that form the basis of aquatic food chains. Other protozoa live in moist soil, beach sand, and decaying organic matter, and a very few are pathogens—that is, disease-causing microbes—of animals and humans. Linnaeus (a) Late 18th Century Sequential mitoses without intervening cytokineses produce a multinucleate schizont, which later undergoes cytokinesis to produce many daughter cells. Schizont ­ Nucleus  ­ Daughter cells Plan ter a       Ch p P ▲ FIGURE 12.4 The changing classification of eukaryotes over the centuries. (a) In the late-18th century, Linnaeus classified all organisms as either plants or animals. (b) In the late-20th century, taxonomists placed fungi in their own group and recognized a new kingdom, rotista. (c) Today, microbial eukaryotes are classified into numerous kingdoms based largely on their genetic relatedness. Though not all taxonomists would agree about every detail of this scheme, it forms the basis for the discussion of eukaryotic organisms in this chapter. M12_BAUM7206_05_SE_C12.indd 356 11/23/16 3:29 AM       P rotozoa As we have seen, over two centuries ago Linnaeus classified protozoa as animals; later taxonomists grouped protozoa into kingdom Protista. Furthermore, some taxonomists divided the protozoa into four groups based on the organisms’ mode of locomotion: Sarcodina (motile by means of pseudopods), Mastigophora (flagella), Ciliophora (cilia), and Sporozoa (nonmotile). Other taxonomists lumped the first two groups together into a single group called Sarcomastigophora. Grouping of the protozoa according to locomotory features is in common usage for many practical applications. Taxonomists today recognize that grouping protozoa by locomotion does not reflect genetic relationships either among protozoa or between protozoa and other organisms. Accordingly, taxonomists continue to revise and refine the classification of protozoa based on nucleotide sequencing and features made visible by electron microscopy. One genetic scheme classifies protozoa into the six taxa Parabasala through Amoebozoa shown in Figure 12.4c, which various taxonomists consider kingdoms (as here), subkingdoms, phyla, or other taxa. In the following sections, we will briefly discuss members of these six taxa of protozoa, formed largely according to similarities in nucleotide sequences and ultrastructure. We begin with parabasalids. Open vacuole LM 10 µm ­ ▲ FIGURE 12.5 Contractile vacuoles. Many protozoa, such as Paramecium, have this prominent feature. Open vacuoles fill with water that entered the cell via osmosis; closed vacuoles are contracted, pumping the water out of the cell. Is the environment in this case hypertonic, hypotonic, or isotonic to the cell? Explain. Parabasala Parabasalids lack mitochondria, but each has a single nucleus and a parabasal body, which is a Golgi body–like structure. Trichonympha (trik-ō-nimf’ă), a parabasalid with numerous ­f lagella (FIGURE 12.6), inhabits the guts of termites, where it ­ Closed vacuole Figure 12.5 The cell is pumping water out, so water has moved into the cell. Water moves into a cell by osmosis when the environment is hypotonic to the cell. Reproduction of Protozoa Most protozoa reproduce asexually only, by binary fission or schizogony; a few protozoa also carry out sexual reproduction in which two individuals exchange genetic material. Some sexually reproducing protozoa become gametocytes (gametes) that fuse with one another to form a diploid zygote. Ciliates reproduce sexually via a complex process called conjugation, which involves the coupling of two compatible mating cells and the exchange of nuclei. Classification of Protozoa LE A R N I N G O U TC O M E S 12.8 Discuss the reasons for the many different taxonomic schemes for protozoa. Identify several features of a typical euglenid. Compare and contrast three types of alveolates. Compare and contrast three types of amoebas. Describe characteristic features of parabasalids, diplomonads, rhizaria, and amoebozoa. SEM 6.5 µm ▲ FIGURE 12.6 Trichonympha acuta, a parabasalid with prodigious flagella. ­ ­ 12.9 12.10 12.11 12.12 M12_BAUM7206_05_SE_C12.indd 357 11/23/16 3:29 AM E       Chapter 12    Characterizing and Classifying ukaryotes assists in the digestion of wood. Another well-known parabasalid is Trichomonas (trik-ō-mō’nas; see p. 770), which lives in the human vagina. When the normally acidic pH of a vagina is raised, Trichomonas proliferates and causes severe inflammation that can lead to sterility. Sexual intercourse spreads Trichomonas; however, trichomoniasis is usually asymptomatic in males. Flagellum Diplomonadida “Eyespot” Because members of the group Diplomonadida15 lack mitochondria, Golgi bodies, and peroxisomes, biologists once thought these organisms were descended from ancient eukaryotes that had not yet phagocytized the prokaryotic ancestors of mitochondria. More recently, however, geneticists have discovered rudimentary mitochondria-like structures called mitosomes in the cytoplasm as well as mitochondrial genes in the nuclear chromosomes, a finding that suggests that diplomonads might be descended from typical eukaryotes that somehow lost their organelles. Diplomonads have two equal-sized nuclei and multiple flagella. A prominent example is Giardia (jē-ar’dē-ă), a diarrhea-causing pathogen of animals and humans that is spread to new hosts when they ingest resistant Giardia cysts (see Disease in Depth: Giardiasis, pp. 734–735). Euglenozoa Short second flagellum Contractile vacuole Paramylon granule (euglenoid starch) Nucleus Chloroplasts Pellicle (a) Part of the reason that taxonomists established the kingdom Protista in the 1960s was to create a “dumping ground” for euglenids, eukaryotic microbes that share certain characteristics of both plants and animals. More recently, based on similar 18S rRNA sequences, the presence of a crystalline rod of unknown function in the flagella, and the presence of mitochondria with disk-shaped cristae, some taxonomists have created a new taxon: kingdom Euglenozoa. The euglenozoa include euglenids and some flagellated protozoa called kinetoplastids. Nucleus ­ Kinetoplast M12_BAUM7206_05_SE_C12.indd 358 10 µm E T ▲ FIGURE 12.7 Two representatives of the kingdom euglenozoa. (a) he euglenid Euglena. uglenids have characteristics that are similar to both plants and animals. (b) Kinetoplastids called Trypanosoma in blood. What is the function of the kinetoplast in rypanosoma? T that direction. Euglenids reproduce by mitosis followed by longitudinal cytokinesis. They form cysts when exposed to harsh conditions. Kinetoplastids Euglenozoa called kinetoplastids (FIGURE 12.7b) each have a single large mitochondrion that contains a unique region of mitochondrial DNA called a kinetoplast. As in all mitochondria, this DNA codes for some mitochondrial polypeptides, which join with polypeptides coded by nuclear DNA to make functional proteins. ­ ­ 15From Greek diploos, meaning “double,” and monas, meaning “unit,” referring to two nuclei. LM (b) Figure 12.7 A kinetoplast is mitochondrial DNA, which codes for some mitochondrial polypeptides. Euglenids The group of euglenozoa called euglenids, which are named for the genus Euglena (FIGURE 12.7a), are photoautotrophic, unicellular microbes with chloroplasts containing light-absorbing pigments—chlorophylls a and b and carotene. For this reason, botanists historically classified euglenids in the kingdom Plantae. However, one reason for not including euglenids with plants is that euglenids store food as a unique polysaccharide called paramylon instead of starch, as in true plants. Euglenids are similar to animals in that they lack cell walls, have flagella, are chemoheterotrophic phagocytes (in the dark), and move by using their flagella as well as by flowing, contracting, and expanding their cytoplasm. Such a squirming movement, which is similar to amoeboid movement but does not involve pseudopods, is called euglenoid movement. A euglenid has a flexible, proteinaceous, helical pellicle that underlies its cytoplasmic membrane and helps maintain its shape. Typically, each euglenid also has a red “eyespot,” which plays a role in positive phototaxis by casting a shadow on a photoreceptor at the flagellar base, triggering movement in 06/03/2018 09:11       P rotozoa Alveoli Flagellum Cytoplasmic membrane Cytoplasm SEM 25 µm ▲ FIGURE 12.9 A predatory ciliate, Didinium (left), devouring another ciliate, Paramecium. 0.2 µm ▲ FIGURE 12.8 Membrane-bound alveoli found in some protozoa. Even though their function is not yet known, alveoli are present in eukaryotic microbes with similar 18S rRNA sequences, indicating genetic relatedness and forming the basis of the group of eukaryotes called alveolates. ­ ­ Kinetoplastids live inside animals, and some are pathogenic. Among the latter are the genera Trypanosoma (tri-pan’ō-sō-mă) and Leishmania (lēsh-man’ē-ă), certain species of which cause potentially fatal diseases of mammals, including humans (see Chapter 23). Alveolates Alveolates (al-vē’ō-lātz) are protozoa with alveoli16 (al-vē’ō-lē), which are small membrane-bound cavities packed as a continuous layer beneath the cytoplasmic membrane (FIGURE 12.8). Scientists do not know the purpose of alveoli, though alveoli are essential for some parasitic alveolates’ motility and invasion of host cells. Alveolates share at least one other characteristic—tubular mitochondrial cristae. This group is further divided into three subgroups: ciliates, apicomplexans, and dinoflagellates. Apicomplexans The alveolates called apicomplexans (ap-i-komplek’sănz) are all chemoheterotrophic pathogens of animals. The name of this group refers to the complex of special intracellular organelles, located at the apices of the infective stages of these microbes, that enables them to penetrate host cells. Many apicomplexans reproduce using schizogony. Examples of apicomplexans are Plasmodium, Cryptosporidium (krip-tō-spō-rid’ē-ŭm), and Toxoplasma (tok-sō-plaz’mă), which cause malaria, cryptosporidiosis, and toxoplasmosis, respectively. (Chapters 21 and 23 consider in more detail these apicomplexans and the diseases they cause.) Dinoflagellates The group of alveolates called dinoflagellates are unicellular microbes that have photosynthetic pigments, such as carotene and chlorophylls a, c1, and c2. As with many plants and algae, their food reserves are starch and oil, and their cells are often strengthened by plates of cellulose within their alveoli. Even though botanists historically classified dinoflagellates as algae because dinoflagellates are photoautotrophic and share other characteristics with algae, taxonomists today note that their 18S rRNA sequences and the presence of alveoli indicate that dinoflagellates are more closely related to ciliates and apicomplexans than they are to either plants or algae. Interestingly, unlike other eukaryotic chromosomes, dinoflagellate chromosomes lack histone proteins. Dinoflagellates make up a large proportion of freshwater and marine plankton. Motile dinoflagellates have two flagella of unequal length (FIGURE 12.10). The transverse flagellum wraps around the equator of the cell in a groove in the cell wall, and its beat causes the cell to spin; the second flagellum extends posteriorly and propels the cell forward. Many dinoflagellates are bioluminescent—that is, able to produce light via metabolic reactions. When luminescent dinoflagellates are present in large numbers, the ocean water lights up ­ Ciliates As their name indicates, ciliate (sil’ē-āt) alveolates have cilia by which they either move themselves or move water past their cell surfaces. (The structure and function of cilia are discussed on p. 82.) Some ciliates are covered with cilia, whereas others have only a few isolated tufts. All ciliates are chemoheterotrophs and have two nuclei—one macronucleus and one micronucleus. Some taxonomists consider them the sole members of phylum Ciliophora. Notable ciliates include Vorticella (vōr-ti-sel’ă), whose apical cilia create a whirlpool-like current to direct food into its “mouth”; Balantidium (bal-an-tid’ē-ŭm), which is the only ciliate pathogenic to humans; and the carnivorous Didinium (dī-di’nē-ŭm), which phagocytizes other protozoa, such as the well-known pond-water ciliate Paramecium (FIGURE 12.9). ­ TEM 16Latin, meaning “small hollows.” M12_BAUM7206_05_SE_C12.indd 359 11/23/16 3:29 AM 12    Characterizing and Classifying Eukaryotes with every crashing wave, passing ship, or jumping fish. Other dinoflagellates produce a red pigment, and their abundance in marine water is one cause of a phenomenon called a red tide. Some dinoflagellates, such as Gymnodinium (jim-nō-din’ē-um) and Gonyaulax (gon-ē-aw’laks), produce neurotoxins—poisons that can act against the human nervous system. Humans can become exposed when they eat shellfish that have ingested planktonic dinoflagellates and concentrated their toxins. The neurotoxin of another dinoflagellate, Pfiesteria 17 (festēr’ē-ă), may be even more potent: It has been claimed that the toxin poisons people who merely handle infected fish or breathe air laden with the microbes, resulting in memory loss, confusion, headache, respiratory difficulties, skin rash, muscle cramps, diarrhea, nausea, and vomiting. The Centers for Disease Control and Prevention (CDC) calls such poisoning possible estuary­associated syndrome (PEAS). ­ ­ ter a       Ch p ­ Cellulose plate ­ Transverse flagellum Rhizaria ­ Unicellular eukaryotes called amoebas (or amoebae)18 are protozoa that move and feed by means of pseudopods (see Figure 3.30). Beyond this common feature and the fact that they all reproduce via binary fission, amoebas exhibit little uniformity. Some taxonomists currently classify amoebas into two kingdoms: Rhizaria and Amoebozoa. We consider them in order. Rhizaria is a group of amoebas with threadlike pseudopods. A major taxon is composed of armored marine amoebas known as foraminifera. A foraminiferan has a porous shell composed of calcium carbonate arranged on an organic matrix in a snail-like manner (FIGURE 12.11). Pseudopods extend through holes in the shell. Commonly, foraminifera live attached to sand grains on the ocean floor. Most foraminifera are microscopic, though scientists have discovered species several centimeters in diameter. SEM ­ Longitudinal flagellum ­ 7 µm ▲ FIGURE 12.10 Gonyaulax, a motile armored dinoflagel- late. Dinoflagellates have two flagella. The transverse flagellum spins the cell; ­ the longitudinal flagellum propels the cell forward. SEM (a) 0.25 mm LM (b) 0.1 mm ▲ FIGURE 12.11 Rhizaria called foraminifera (here, Globigerina) have multichambered, snail-like shells of calcium P ­ carbonate. (a) seudopods are not present in this dead specimen. (b) Long, thin pseudopods of a living specimen. 17Named 18From M12_BAUM7206_05_SE_C12.indd 360 for dinoflagellate biologist Lois Pfiester. Greek ameibein, meaning “to change.” 11/23/16 3:29 AM       Fungi  ­ ◀ FIGURE 12.12 Radiolarians, a type of rhizaria. (a) Radiolarians have ornate shells of silica. (b) Thin pseudopods extend through the holes in the shells. SEM (a) 0.1 mm LM (b) Over 90% of known foraminifera are fossil species, some of which form layers of limestone hundreds of meters thick. The great pyramids of Giza outside Cairo, Egypt, are built of ­foraminiferan limestone. Geologists correlate the ages of sedimentary rocks from different parts of the world by finding identical foraminiferan fossils embedded in them. Amoebas called radiolaria make up another group of rhizaria, but they have ornate shells composed of silica (SiO2, the mineral found in opal) (FIGURE 12.12a) and live unattached as part of the marine plankton. Radiolarians reinforce their pseudopods with stiff internal bundles of microtubules so that the pseudopods radiate from the central body like spokes of a spherical wheel (FIGURE 12.12b). The dead bodies of radiolarians settle to the bottom of the ocean, where they form ooze that is hundreds of meters thick in some locations. In summary, protozoa are a heterogeneous collection of single-celled, mostly chemoheterotrophic organisms that lack cell walls. Some taxonomists classify them in Parabasala, Diplomonadida, Euglenozoa, Alveolata, R hizaria, and Amoebozoa, though their relationships with one another and with other eukaryotic organisms are still unclear. TABLE 12.2 on p. 362 summarizes the incredible diversity of these microbes. We next turn our attention to another group of chemoheterotrophs, the fungi, which differ from the protozoa in that fungi have cell walls. TELL ME WHY Why did early taxonomists categorize such obviously different microorganisms as parabasalids, diplomonads, euglenozoa, alveolates, rhizaria, and amoebozoa in a single taxon, Protozoa? Amoebozoa Fungi LE A R N I N G O UTC O M E 12.13 Cite at least three characteristics that distinguish fungi from other groups of eukaryotes. ­ Amoebozoa constitute a second kingdom of amoebas distinguished from rhizaria by having lobe-shaped pseudopods and no shells. Amoebozoa include the normally free-living amoebas Nae­ gleria (nā-glē’rē-ă) and Acanthamoeba (ă-kan-thă-me’bă), which can each cause diseases of the eyes or brains of humans and animals that swim in water containing them. Other amoebozoa, such as Entamoeba (ent-ă-me’bă), always live inside animals, where they produce potentially fatal amebic dysentery. (Chapters 20 and 23 examine these pathogenic amoebas in more detail.) Taxonomists formerly considered another group of amoebozoa—slime molds—to be fungi, but their nucleotide sequences, as well as the lobe-shaped pseudopods by which they feed and move, show that they are amoebozoa. Scientists have identified two types of slime molds: plasmodial slime molds and cellular slime molds. 0.2 mm • They lack cell walls, more closely resembling the amoebas in this regard. • They are phagocytic rather than absorptive in their nutrition. Species in the two groups of slime molds differ based on their morphology, reproduction, and 18S rRNA sequences. Slime molds are important to humans primarily as excellent laboratory systems for the study of developmental and molecular biology. M12_BAUM7206_05_SE_C12.indd 361 ­ ­ Slime molds differ from true fungi in two main ways: Organisms in the kingdom Fungi (fŭn’jī), such as molds, ­ ushrooms, and yeasts, are like most protozoa in that they m are chemoheterotrophic; however, unlike protozoa, they have cell walls, which typically are composed of a strong, flexible, nitrogenous polysaccharide called chitin. (The chitin in fungi is chemically identical to that in the exoskeletons of insects and other arthropods, such as grasshoppers, lobsters, and crabs.) Fungi differ from plants in that they lack chlorophyll and do not perform photosynthesis; they differ from animals by having cell walls, although genetic sequencing of fungal and animal genomes has shown that fungi and animals are related. The study of fungi is mycology,19 and scientists who study fungi are mycologists. 19From Greek mykes, meaning “mushroom,” and logos, meaning “discourse.” 11/23/16 3:29 AM 12    Characterizing and Classifying Eukaryotes Characteristics of Protozoa Category Parabasala Representative Genera Mentioned in the Text Distinguishing Features P TABLE 12.2 ter a       Ch p arabasal body; single nucleus; lack mitochondria Two equal-sized nuclei; lack mitochondria, Golgi bodies, and peroxisomes; multiple flagella Euglenozoa Flagella with internal crystalline rod; disk-shaped mitochondrial cristae Euglenids Kinetoplastids Alveolates P Diplomonadida hotosynthesis; pellicle; “eyespot” Giardia Euglena Single mitochondrion with DNA localized in kinetoplast Trypanosoma, Leishmania Alveoli (membrane-bound cavities underlying the cytoplasmic membrane); tubular cristae in mitochondria Cilia Balantidium, Paramecium, Didinium Apicomplexans Apical complex of organelles Plasmodium, Cryptosporidium, Toxoplasma hotosynthesis; two flagella; internal cellulose plates Foraminifera Shells of calcium carbonate Radiolarians Shells of silica Amoebozoa P Free-living and parasitic forms lasmodial (acellular) slime molds Cellular slime molds Lobe-shaped pseudopods; no shells Do not form aggregates Naegleria, Acanthamoeba, Entamoeba Multinucleate body (called plasmodium) Physarum Cells aggregate but retain individual (cellular) nature Dictyostelium The Significance of Fungi LE A R N I N G O U TC O M E 12.14 List five ways in which fungi are beneficial. Fungi are extremely beneficial microorganisms. In nature, they decompose dead organisms (particularly plants) and recycle their nutrients. Additionally, the roots of about 90% of vascular plants form mycorrhizae,20 which are beneficial associations between roots and fungi that help the plants absorb water and dissolved minerals. Humans use fungi for food (mushrooms and truffles), in religious ceremonies (because of their hallucinogenic properties), and in the manufacture of foods and beverages, including bread, alcoholic beverages, citric acid (the basis of the soft drink industry), soy sauce, and some cheeses. Fungi also produce antibiotics, such as penicillin and cephalosporin; the immunosuppressive drug cyclosporine, which makes organ transplants possible; and mevinic acids, which are cholesterol-reducing agents. Fungi are also important research tools in the study of metabolism, growth, and development and in genetics and biotechnology. For instance, based on their work with the fungus Neurospora (noo-ros’pōr-ă) in the 1950s, George Beadle (1903– 1989) and Edward Tatum (1909–1975) developed their Nobel Prize–winning theory that one gene codes for one enzyme. Because of similar research, Saccharomyces (brewer’s yeast) is the best-understood eukaryote and the first eukaryote to have its entire genome sequenced. (Chapter 25 highlights some uses of fungi in agriculture and industry.) Not all fungi are beneficial—about 30% of known fungal species produce mycoses (mī-kō’sēz), which are fungal diseases of plants, animals, and humans (see Chapters 19–24). For example, Dutch elm disease is a mycosis of elm trees, and athlete’s foot is a fungal disease of humans. Because fungi tolerate concentrations of salt, acid, and sugar that inhibit bacteria, fungi are responsible for the spoilage of fruit, pickles, jams, and jellies exposed to air. In the following sections, we will consider the basic characteristics of fungal morphology, nutrition, and reproduction before turning to a brief survey of the major groups of fungi. Morphology of Fungi LE A R N I N G O UTC O M E 12.15 Distinguish among septate hyphae, aseptate hyphae, and mycelia. Fungi have two basic body shapes. The bodies of molds are relatively large and composed of long, branched, tubular filaments called hyphae.21 Hyphae are either septate (divided into cells by cross walls called septa22; FIGURE 12.13a) or aseptate (not divided by septa; FIGURE 12.13b). Aseptate hyphae are multinucleate. The bodies of yeasts are typically small, globular, and composed of a single cell, which may produce buds (FIGURE 12.13c). 21From Greek rhiza, meaning “root.” M12_BAUM7206_05_SE_C12.indd 362 Gymnodinium, Gonyaulax, Pfiesteria Threadlike pseudopods ­ Rhizaria P Ciliates Dinoflagellates 20From Trichomonas Greek hyphe, meaning “weaving” or “web.” 22Latin, meaning “partitions” or “fences.” 11/23/16 3:29 AM       Fungi LM LM 25 µm (a) Septate hypha 25 µm (b) Aseptate hypha Moldlike (filamentous) form Yeastlike form Bud LM 3 mm ▲ FIGURE 12.14 A fungal mycelium. 6 µm LM (d) Dimorphic fungus 50 µm ▲ FIGURE 12.13 Fungal morphology. (a) Septate hyphae have cross walls; (b) aseptate hyphae do not. (c) Cells of Saccharomyces (baker’s or brewer’s yeast), which are unicellular and spherical to irregularly oval in shape. (d) The bodies of a dimorphic fungus, Mucor rouxii, showing both yeastlike and moldlike growth in response to environmental conditions. ­ ­ ­ ­ ­ In response to environmental conditions such as temperature or carbon dioxide concentration, some fungi produce both yeastlike and moldlike shapes (FIGURE 12.13d); fungi that produce two types of body shapes are said to be dimorphic (which means “two-shaped”). Many medically important fungi are thermally dimorphic; that is, they change growth habits in response to the temperature in their immediate vicinity. Such fungi include Histoplasma capsulatum (his-tō-plaz’mă kap-soo-lă’tŭm), which causes a respiratory disease called histoplasmosis, and Coccidioides immitis (kok-sid-ē-oy’dēz im’mi-tis), which causes a flulike disease called coccidioidomycosis (koksid-ē-oy’dō-mī-kō’sis). Generally, the yeast form of a dimorphic fungal pathogen causes disease, whereas the filamentous form does not. M12_BAUM7206_05_SE_C12.indd 363 Nutrition of Fungi Fungi acquire nutrients by absorption; that is, they secrete catabolic enzymes outside their bodies to break large organic molecules into smaller molecules, which they then transport throughout their bodies. Most fungi are saprobes 23 (sap’rōbz)—they absorb nutrients from the remnants of dead organisms—though some species trap and kill microscopic soil-dwelling nematodes (worms; FIGURE 12.15). Fungi that derive their nutrients from living plants and animals usually have modified hyphae called haustoria 24 (haw-stō’rē-ă), which penetrate the tissue of the host to withdraw nutrients. Absorptive nutrition is important in the role that fungi play as decomposers and recyclers of organic waste. Cytoplasmic ­ LM ­ (c) Yeast cells’ buds The body of a mold is composed of hyphae intertwined to form a tangled mass called a mycelium (plural: mycelia; FIGURE 12.14). Mycelia are typically subterranean and thus usually escape our notice, though they can be very large. In fact, the largest known organisms on Earth are fungi in the genus Armillaria, the mycelia of which can spread through thousands of acres of forest to a depth of several feet and weigh many hundreds of tons. (In contrast, blue whales, the largest living animals, weigh only about 150 tons.) Fruiting bodies, such as puffballs and mushrooms, are the hyphae of the reproductive structures of molds and are only small visible extensions of vast underground mycelia. 23From 24From Greek sapros, meaning “rotten,” and bios, meaning “life.” Latin haustor, meaning “someone who draws water from a well.” 3/9/18 8:16 PM E (kan’did-ă al’bi-kanz), which causes oral thrush and vaginal yeast infections in humans, a series of buds remain attached to one another and to the parent cell, forming a long filament called a pseudohypha (see Disease in Depth: Candidiasis on pp. 758–759). Candida invades human tissues by means of such pseudohyphae, which can penetrate intercellular cracks. Filamentous fungi reproduce asexually by producing lightweight spores, which enable a fungus to disperse vast distances on the wind. Researchers have isolated fungal spores from wind currents many miles above the surface of the Earth. Scientists categorize the asexual spores of molds according to their mode of development. Following are examples of some asexual spores: Hyphal loop SEM 75 µm T ▲ FIGURE 12.15 Predation of a nematode by the fungus Drechslerella. he fungus produces special looped hyphae that constrict T when the worm contacts the inside of the loop. he fungus secretes enzymes that digest the nematode and then absorbs the resulting nutrients. What is the more typical mode of nutrition found in fungi? Figure 12.15 Most fungi are saprobic. streaming frequently transports nutrients and organelles, including nuclei, throughout a mycelium. Streaming between cells of septate mycelia occurs through pores in the septa. Scientists have discovered that some fungi use ionizing radiation (radioactivity) as an energy source for metabolism. The fungi absorb radiation with a black pigment, melanin, and transform absorbed gamma radiation into chemical energy, which the fungi then use to grow. Most fungi are aerobic, though many yeasts (e.g., Saccharomyces) are facultative anaerobes that obtain energy from fermentation, such as occurs in the reactions that produce alcohol. Anaerobic fungi are found in the digestive systems of many herbivores, such as cattle and deer, where they assist in the catabolism of plant material. • Sporangiospores form inside a sac called a sporangium,25 which is often borne on a spore-bearing stalk, called a sporangiophore,26 at either the tips or the sides of hyphae (FIGURE 12.16a). • Chlamydospores form with a thickened cell wall inside hyphae (FIGURE 12.16b). • Conidiospores (also called conidia) are produced at the tips or sides of hyphae, but not within a sac. There are many types of conidia, some of which develop in chains on stalks called conidiophores (FIGURE 12.16c). Medical lab technologists use the presence and type of asexual spores in clinical samples to identify many fungal pathogens. Sexual Spore Formation Scientists can arbitrarily designate fungal mating types as “plus (+)” and “minus (-)” rather than as male and female, in part because their bodies are morphologically indistinguishable. The process of sexual reproduction in many fungi has four basic steps (FIGURE 12.17): ­ Nematode ­       Chapter 12    Characterizing and Classifying ukaryotes 1 Many fungi form dikaryons in the course of sexual reproduction. Haploid (n) cells from a plus fungus and a minus fungus fuse to form a dikaryon, a cell containing two different nuclei, both + and - nuclei. A dikaryotic stage is neither diploid nor haploid but instead is designated (n + n). Dikaryotic cells can reproduce to form daughter cells, each of which is also a dikaryon. 2 After a period of time that typically ranges from hours to years but can be centuries, a pair of nuclei within a dikaryon fuse to form one diploid (2n) nucleus. Meiosis of the diploid nucleus restores the haploid state. The haploid nuclei are partitioned into + and - spores, which reestablish + and - fungi by mitoses and cell divisions. Reproduction of Fungi LE A R N I N G O U TC O M E S 12.16 Describe asexual and sexual reproduction in fungi. 12.17 List three basic types of asexual spores found in molds. Whereas all fungi have some means of asexual reproduction involving mitosis followed by cytokinesis, most fungi also reproduce sexually. In the next sections we briefly examine asexual and sexual reproduction in fungi. Budding and Asexual Spore Formation Yeasts typically bud in a manner similar to prokaryotic budding. Following mitosis, one daughter nucleus is sequestered in a small bleb (a blisterlike outgrowth) of cytoplasm that is isolated from the parent cell by the formation of a new wall (see Figure 12.13c). In some species, especially Candida albicans M12_BAUM7206_05_SE_C12.indd 364 3 4 Fungi differ in the ways they form dikaryons and in the site at which meiosis occurs. 25From 26From Greek spora, meaning “seed,” and angeion, meaning “vessel.” Greek phoros, meaning “bearing.” 3/7/18 7:32 PM       Fungi Sporangiophore Sporangiospore Sporangium ◀ FIGURE 12.16 Representative asexual spores of molds. (a) Sporangiospores, which develop within a sac called a sporangium that is borne on a sporangiophore, here of Mucor. (b) Chlamydospores are thickwalled spores that form inside hyphae, here of Aureobasidium. (c) Conidiospores (conidia) develop at the ends of hyphae, here of Penicillium. How do conidia differ from sporangiospores? Figure 12.16 Conidia are never enclosed in a sac. Tips fuse + LM (a) – Dikaryotic stage (n + n) – + 10 µm Chlamydospore Mitoses and cell divisions Two nuclei per cell – + 1 Dikaryon 4 + + – + + – – – 3 Meiosis LM (b) Haploid stage (n) 10 µm Conidia 2 Nuclei fuse ▲ FIGURE 12.17 The process of sexual reproduction in fungi. The steps in the process are ­ Conidiophore Diploid stage (2n) H described in the text. aploid cells appear against a blue background, diploid cells against a green background, and dikaryotic cells against an orange background. tor ideo V Tu Classification of Fungi LE A R N I N G O UTC O M E S 12.18 Compare and contrast the three divisions of fungi with respect to the formation of sexual spores. 12.19 Describe the deuteromycetes, and explain why this group no longer constitutes a formal taxon. (c) M12_BAUM7206_05_SE_C12.indd 365 SEM 15 µm In the following sections, we will consider the four major ­s ubgroups into which taxonomists traditionally divided the kingdom Fungi. Three of these subgroups, which are taxa called divisions that are equivalent to phyla in other kingdoms, are based on the type of sexual spore produced (divisions 11/23/16 3:30 AM E       Chapter 12    Characterizing and Classifying ukaryotes ­ Zygomycota, Ascomycota, and Basidiomycota); the fourth (the deuteromycetes) refers to an abandoned taxon that was a repository of fungi for which no sexual stage was known. We begin by considering the Zygomycota. Division Zygomycota morel, Morchella esculenta. hough it might look like a mushroom, it isn’t; Morchella is a delectable edible ascomycete. he pits visible in this photograph are lined with asci, sacs that contain numerous ascospores. T as silkworms and honeybees. The Environmental Protection Agency has approved one species of Nosema as a biological control agent for grasshoppers. Seven genera of microsporidia, including Nosema, Encephalatizoon (en-sef-a-lat-e’zō-an), and Microsporidium (mī-krō-spor-i’dē-ŭm), are known to cause diseases in immunocompromised patients (see Emerging Disease Case Study: Microsporidiosis, in Chapter 16 on p. 494). Division Ascomycota ­ ­ The division Ascomycota contains about 32,000 known species of molds and yeasts that are characterized by the formation of haploid ascospores within sacs called asci (singular: ascus).27 Asci occur in fruiting bodies called ascocarps, which have various shapes (FIGURE 12.19). Ascomycetes (as’kōmī-sēts), as they are called, also reproduce asexually by conidiospores. Sexual reproduction of an ascomycete proceeds when hyphal tips of opposite mating types fuse to form a dikaryon. The dikaryon undergoes mitosis and cell division to produce a hypha composed of dikaryotic cells. Eventually, nuclei of opposite mating types within the dikaryons fuse to form diploid nuclei that then undergo meiosis and cytokinesis to form four haploid cells lined up within an ascus. The four haploid cells may each undergo mitosis and cytokinesis to form eight haploid ascospores. The asci open to release their ascospores, which germinate to produce either a + or a - hypha. Ascomycetes are familiar and economically important fungi. For example, most of the fungi that spoil food are ascomycetes. This group also includes plant pathogens, such as the causative agents of Dutch elm disease and chestnut blight, which have almost eliminated their host trees in many parts of  ­ ­ Mature zygosporangium ­ Fused tips ­ ­ Microsporidia Microsporidia are small organisms that are difficult to classify. Until 2003, taxonomists thought microsporidia were protozoa, but genetic analysis indicates they are more similar to zygomycetes. They are obligatory intracellular parasites, that is, organisms that must live within their hosts’ cells. Microsporidia spread from host to host as small, resistant spores. An example is Nosema (nō-sē’mă), which is parasitic on insects, such ▲ FIGURE 12.19 Ascocarps (fruiting bodies) of the common T Fungi in the division Zygomycota are multinucleate molds called zygomycetes (zī’gō-mī-sēts). Of the approximately 1100 species known, most are saprobes; the rest are obligate parasites of insects and other fungi. Typical zygomycetes reproduce asexually via sporangiospores, but the distinctive feature of most zygomycetes is the formation of sexual structures called zygosporangia (sometimes incorrectly termed “zygospores”) (FIGURE 12.18). The dikaryon of a zygomycete is typically a single cell that develops from the fusion of sexually compatible hyphal tips. The dikaryon develops into a zygosporangium where nuclei from one hypha (+) fuse with nuclei from the other hypha (-) to form many diploid nuclei. Each diploid nucleus undergoes meiosis, but only one of the four meiotic daughters of each nucleus survives. The zygosporangium then produces a haploid sporangium, which is filled with haploid spores (true zygospores). The sporangium releases these spores, each of which germinates to produce either a + or a - mycelium. This completes the life cycle. LM 10 mm T ▲ FIGURE 12.18 Zygosporangium. he tips of + and - hyphae (here, of the bread mold Rhizopus) fuse and form a dark, diploid zygosporangium that matures and undergoes meiosis to produce internal haploid sporangiospores. M12_BAUM7206_05_SE_C12.indd 366 27From Greek askus, meaning “wineskin.” 3/9/18 8:21 PM       Fungi d t S u y ase C isease EMERGING D Aspergillosis Division Basidiomycota A walk through fields and woods in most parts of the world may reveal mushrooms, puffballs, stinkhorns, jelly fungi, bird’s nest fungi, or bracket fungi, all of which are the visible fruiting bodies of the almost 22,000 known species of fungi in the division Basidiomycota. Poisonous mushrooms are sometimes called toadstools, but there is no sure way to always distinguish an edible “mushroom” from a poisonous “toadstool” except by eating them—a truly risky practice! M12_BAUM7206_05_SE_C12.indd 367 H H 1. How could a microbiologist determine that the fungus in Matt’s lung was an ascomycete rather than a basidiomycete? 2. Why is aspergillosis emerging as a “new” disease? 3. Why is a powerful drug like voriconazole effective against fungi but relatively harmless to humans? The major mycelium of a basidiomycete is dikaryotic; every cell is a dikaryon. The aboveground mushrooms or other fruiting bodies of basidiomycetes (ba-sid’ē-ō-mi-sēts) are called basidiocarps (FIGURE 12.20). The entire structure of a basidiocarp consists of tightly woven dikaryotic hyphae that extend into multiple, often club-shaped projections called basidia, the ends of which produce sexual basidiospores (typically four on each basidium) as a result of meiosis and cell division. Basidiospores disperse in the wind and germinate in suitable, moist environments to form small haploid hyphae that have either + or - nuclei. The tips of opposite mating types can fuse to form dikaryons that develop into the expansive mycelia of these fungi. Besides the edible mushrooms—most notably, cultivated Agaricus (a-găr’i-kus)—basidiomycetes affect humans in several ways. Most basidiomycetes are important decomposers that digest chemicals such as cellulose and lignin in dead plants and return nutrients to the soil. Many mushrooms produce toxins or hallucinatory chemicals. An example of the latter is Psilocybe cubensis (sil-ō-sī’bē kŭ-ben’sis), which produces psilocybin, a hallucinogen. The basidiomycete yeast Cryptococcus neoformans (krip-tō-kok’ŭs nē-ō-for’manz) is the leading cause of fungal meningitis. Cryptococcus is an unusual basidiomycete in that it doesn’t form hyphae, but it does form sexual spores in a basidium—it is a true basidiomycete. Other basidiomycetes are rusts and smuts, which attack crop plants, causing millions of dollars of damage each year. ­ the United States. Claviceps purpurea (klav’i-seps poor-poo’rē-ă) growing on grain produces lysergic acid, also known as ergot, which causes abortions in cattle and hallucinations in humans. Aspergillus can also infect humans (see Emerging Disease Case Study: Aspergillosis). On the other hand, many ascomycetes are beneficial. For example, Penicillium (pen-i-sil’ē-ŭm) mold is the source of penicillin; Saccharomyces, which ferments sugar to produce alcohol and carbon dioxide gas, is the basis of the baking and brewing industries; and truffles (varieties of Tuber) grow as mycorrhizae in association with oak and beech trees to form culinary delights (see Beneficial Microbes: Fungi for $10,000 a Pound on p. 368). As previously noted, another ascomycete, the pink bread mold Neurospora, has been an important tool in genetics and biochemistry. Many ascomycetes partner with green algae or cyanobacteria to form lichens (discussed in more detail shortly). and delirium. All would be his daily companions. e might also be paralyzed on one side of his body. Matt spent four weeks in the hospital. The worst days were those when he was aware. The sheer terror of knowing his brain was being pierced by thin hyphal threads, digesting his personality away, was almost more than Matt could stand. e looked forward to the times he would lapse into unconsciousness, even though he knew that each period of wakefulness might be his last. A medical miracle in the form of a new antifungal drug (voriconazole) brought Matt back to life. The invasive mold was defeated, and Matt returned home grateful, aware that life is precious and hopeful that no more spores floated his way. ­ ­ U Matt was not in good shape, and it had nothing to do with his physique or time spent at the gym; it had to do with the ball of fungus in his right lung. That fungal sphere, an aspergilloma, had started as a single spore of an ascomycete fungus Aspergilloma called Aspergillus. The mold had formed the mass of fungal hyphae that was invading the airways of his lung and slowly killing him. Such bronchopulmonary aspergillosis is a rare but increasingly frequent pathogen of immunocompromised people. Besides his difficulty in breathing, fever, and chest pain, Matt most hated coughing up wads of bloody mucus—as if he were expelling the very fabric of his life. In fact, he literally was coughing up pieces of his lung. nfortunately for Matt, the disease had not yet peaked. Aspergillus had invaded his blood and was even now progressing toward his brain. Soon the signs and symptoms of invasive aspergillosis would be his—extreme tiredness, excessive weakness, severe headaches, 11/23/16 3:30 AM 12    Characterizing and Classifying Eukaryotes ter a       Ch p BENEFICIAL MICROBES Fungi for $10,000 a Pound H pigs are more likely to eat the truffles.) Dogs may be trained at the niversity of Truffle unting Dogs, founded in 1880 in Roddi, Italy. The underground habitat of truffles requires them to form symbiotic relationships with trees for nutrients and with animals for spore dispersal—the animals dig up the truffles and thus help spread the spores to other locations. Incidentally, chocolate truffles derive only their name from their prized fungal counterparts. U Truffles—rare, intensely flavored ascomycetes that grow underground—are one of the most luxurious and expensive foods on Earth, selling on average for more than $800 per pound. There are many different varieties of truffles. The most coveted include Tuber melanosporum, a black truffle that is also known as the “black diamond,” and Tuber magnatum, a white truffle that can sell for $10,000 per pound! Because truffles are very difficult to find, truffle hunters often use pigs and dogs trained to sniff them out. (Dogs are preferred because Deuteromycetes As noted previously, the divisions Zygomycota, Ascomycota, and Basidiomycota are based on type of sexual spore produced. Because scientists have not observed sexual reproduction in all fungi, taxonomists in the middle of the 20th century created the division Deuteromycota (also called imperfect fungi) to contain the fungi whose sexual stages are unknown—either because they do not produce sexual spores or because their sexual spores have not been observed. More recently, however, the analysis of rRNA sequences has revealed that most deuteromycetes in fact belong in the division Ascomycota, and thus modern taxonomists have abandoned Deuteromycota as a formal taxon. Nevertheless, many medical laboratory technologists, health care practitioners, and scientists continue to refer to “deuteromycetes” or “imperfect fungi” because these are traditional names. (a) LM 1 mm Lichens LE A R N I N G O UTC O M E S 12.20 Describe a lichen’s members. 12.21 List several beneficial roles or functions of lichens. ­ A discussion of fungi is incomplete without considering lichens. A lichen is a partnership between fungi and photosynthetic microbes—commonly cyanobacteria or, less frequently, green algae. For over a century, scientists thought that each lichen contained one fungal species and one photosynthetic species, but recent genetic research shows that most lichens have two fungi—a filamentous ascomycete and a basidiomycete yeast. The relationship between the fungi and their photosynthetic partner is so intimate that scientists classify lichens as distinct species, even though a lichen is composed of distinct organisms. In a lichen, the hyphae of the ascomycete surround the yeast and photosynthetic cells (FIGURE 12.21) and provide them nutrients, water, and protection from desiccation and harsh light. In return, each alga or cyanobacterium provides the fungi with products of photosynthesis—carbohydrates and spores, looking like flattened eggs in birds’ nests, develop inside basidiocarps of the bird’s nest fungus, Crucibulum. (b) The familiar shapes of bracket fungi and mushrooms are also basidiocarps of extensive mycelia. Shown here is Laetiporus sulphureus growing on a tree. M12_BAUM7206_05_SE_C12.indd 368 ­ ▲ FIGURE 12.20 Basidiocarps (fruiting bodies). (a) Basidio- ­ ­ (b) 11/23/16 3:30 AM       Fungi Ascocarp of fungus Soredium Algal cell Fungal hyphae Fungal hyphae Phototrophic layer Substrate SEM 125 µm ▲ FIGURE 12.21 Makeup of a lichen. The hyphae of a fungus (most commonly an ascomycete) constitute the major portion of the body of the lichen. A basidiomycete yeast (not shown) is also present. Cells of the photosynthetic member of the lichen are concentrated near the lichen’s surface. Soredia, which are bits of fungus surrounding phototrophic cells, are the means by which some lichens propagate themselves. Members of what three groups of photosynthetic microbes can be part of a lichen? Figure 12.21 Lichens are composed of an ascomycete, a basidiomycete, and a cyanobacterium or an alga. oxygen. In some lichens, the phototroph releases 60% of its carbohydrates to the fungi. The partnership in a lichen is not always mutually beneficial; in some lichens, the ascomycete produces haustoria that penetrates and kills the alga. Such lichens are maintained only because the alga reproduces faster than the fungus can devour it. The ascomycete of a lichen reproduces by spores, which must germinate and develop into hyphae that capture an appropriate partners. Alternatively, wind, rain, and small animals disperse bits of lichen called soredia, which contain both phototrophs and bits of fungi, to new locations where they can establish a new lichen if there is suitable substrate. Scientists have identified over 14,000 species of lichens, which are abundant throughout the world, particularly in pristine unpolluted habitats, growing on soil, rocks, leaves, tree bark, other lichens, and even the backs of tortoises. Indeed, lichens grow in almost every habitat—from high-elevation alpine tundra to submerged rocks on the oceans’ shores, from frozen Antarctic soil to hot desert climes. The only unpolluted places where lichens do not consistently grow are in the dark depths of the oceans and the black world of caves—after all, lichens require light. Lichens grow slowly, but they can live for hundreds and possibly thousands of years. They occur in three basic shapes (FIGURE 12.22): • Foliose lichens are leaflike, with margins that grow free from the substrate. • Crustose lichens grow appressed to their substrates and may extend into the substrate for several millimeters. • Fruticose lichens are either erect or are hanging cylinders. Lichens create soil from weathered rocks, and lichens containing nitrogen-fixing cyanobacteria provide significant M12_BAUM7206_05_SE_C12.indd 369 amounts of usable nitrogen to nutrient-poor environments. Many animals eat lichens; for example, reindeer and caribou subsist primarily on lichens throughout the winter. Birds use lichens for nesting materials, and some insects camouflage themselves with bits of living lichen. Humans also use lichens in the production of foods, dyes, clothing, perfumes, medicines, Foliose Crustose Fruticose 10 mm ▲ FIGURE 12.22 Gross morphology of lichens. Crustose forms are flat and tightly joined to the substrate, here a tree branch; foliose forms are leaflike with free margins; and fruticose forms are either erect (as shown) or pendulant. 11/23/16 3:30 AM 12    Characterizing and Classifying Eukaryotes Distribution of Algae Even though some algae grow in such diverse habitats as in soil and ice, in intimate association with fungi as lichens, and on plants, most algae are aquatic, living in the photic zone (penetrated by sunlight) of fresh, brackish, and salt bodies of water. This watery environment provides some benefits and also presents some difficulties for photosynthetic organisms. Whereas most bodies of water contain sufficient dissolved chemicals to provide nutrients for algae, water also differentially absorbs longer wavelengths of light (including red light), so only shorter (blue) wavelengths penetrate more than a meter below the surface. This is problematic for algae because their primary photosynthetic pigment—chlorophyll a—captures red light. Thus, to grow in deeper waters, algae must have accessory photosynthetic pigments that trap the energy of penetrating, short-wavelength light and pass that energy to chlorophyll a. Members of the group of algae known as red algae, for example, contain a red pigment that absorbs blue light, enabling red algae to inhabit even the deepest parts of the photic zone. The Romans used the word alga (al’ga) to refer to any simple aquatic plant, particularly one found in marine habitats. Their usage thus included the organisms we recognize as algae (al’jē), cyanobacteria, sea grasses, and other aquatic plants. Today, the word algae properly refers to eukaryotic photoautotrophic organisms that have sexual reproductive structures in which every cell becomes a gamete. In plants, by contrast, a portion of the reproductive structure always remains vegetative. Algae are not a unified group; rather, they differ widely in distribution, morphology, reproduction, and biochemical traits. Moreover, the word algae is not synonymous with any taxon; in the taxonomic scheme shown in Figure 12.4c, algae can be found in several kingdoms, including kingdoms Stramenopila, Rhodophyta, and Plantae. The study of algae is phycology,28 and the scientists who study them are called phycologists. TABLE 12.3 Characteristics of Fungi Division and Type of Sexual Spore Distinguishing Features Representative Genera Zygomycota Multinucleate (aseptate) Rhizopus Septate; some associate with cyanobacteria or green algae to form lichens Claviceps, Neurospora, Penicillium, Saccharomyces, Tuber Septate Agaricus, Amanita, Cryptococcus Zygospores Ascomycota Ascospores Basidiomycota Basidiospores 28From Greek phykos, meaning “seaweed,” and logos, meaning “discourse.” M12_BAUM7206_05_SE_C12.indd 370 ­ Morphology of Algae Algae can be unicellular or colonial, or they can have simple multicellular bodies, which are commonly composed of branched filaments or sheets. The bodies of large marine algae, commonly called seaweeds, can be relatively complex, with branched holdfasts to anchor them to rocks, stemlike stipes, and leaflike blades. Many of the larger marine algae are buoyed in the water by gas-filled bulbs called pneumocysts (see Figure 12.26). Though the bodies of some marine algae can surpass land plants in length, they lack the well-developed transport systems common to vascular plants.  ­ 12.22 Describe the distinguishing characteristic of algae. ­ LE A R N I N G O U TC O M E Reproduction of Algae LE A R N I N G O UTC O M E 12.23 Describe the alternation of generations in algae. In unicellular algae, asexual reproduction involves mitosis followed by cytokinesis. In unicellular algae that reproduce sexually, each algal cell acts as a gamete and fuses with another such gamete to form a zygote, which then undergoes meiosis to return to the haploid state. Multicellular algae may reproduce asexually by fragmentation, in which each piece of a parent alga develops into a new individual, or by motile or nonmotile asexual spores. As noted previously, in multicellular algae that reproduce sexually, every cell in the reproductive structures of the alga becomes a gamete—a feature that distinguishes algae from all other photosynthetic eukaryotes. Many multicellular algae reproduce sexually with an alternation of generations of haploid and diploid individuals (FIGURE 12.23). In such life cycles, diploid individuals undergo meiosis to produce male and female haploid spores that develop into haploid male and female bodies, which may ­ Algae ­ Why isn’t a fungal dikaryon—with its two haploid (n) nuclei— considered diploid? ­ TELL ME WHY ­ ­ and the litmus of indicator paper. Because lichens will not grow well in polluted environments, ecologists use them as sensitive living assays for monitoring air pollution. In the preceding sections, we have seen that fungi are chemoheterotrophic yeasts and molds that function primarily to decompose and degrade dead organisms. Some fungi are pathogenic, and others associate with cyanobacteria or algae to form lichens. All fungi reproduce asexually either by budding or via asexual spores, and most fungi also produce sexual spores, by which taxonomists classify them. TABLE 12.3 summarizes the characteristics of fungi. ­ ter a       Ch p 11/23/16 3:30 AM       Algae into several groups that are named for the colors of their photosynthetic pigments: green algae, red algae, brown algae, golden algae, and yellow-green algae. The following sections present some of these groups. We begin with the green algae of the division Chlorophyta. Diploid (2n) generation Division Chlorophyta (Green Algae) Zygote Diploid (2n) individual Meiosis Male haploid (1n) spore Fusion Male gamete Female gamete Female haploid (1n) spore Haploid (1n) generation Female haploid individual ▲ FIGURE 12.23 Alternation of generations in algae, as occurs in the green alga Ulva. A diploid individual meiotically produces haploid spores that germinate and grow into male and female bodies. These haploid algae produce gametes that fuse to form a diploid zygote, which grows into a new diploid individual. Each generation can also reproduce asexually by fragmentation and by spores. look identical to the diploid body. Every cell of the reproductive portions of the haploid bodies becomes a gamete—the definition of an alga. Some haploid algae produce gametes that fuse to form a zygote, which grows into a new diploid alga. Both haploid and diploid algae may reproduce asexually as well. Kingdom Rhodophyta (Red Algae) Algae of division Rhodophyta, 30 which had been placed historically in kingdom Plantae and then Protista, are now in their own kingdom—Rhodophyta. They are characterized by the red accessory pigment phycoerythrin; the storage molecule glycogen; cell walls of agar or carrageenan (kar-ă-gē’nan), sometimes supplemented with calcium carbonate; and nonmotile male gametes called spermatia. Phycoerythrin allows red algae to absorb short-wavelength blue light and photosynthesize at depths greater than 100 meters. Because the relative proportions of phycoerythrin and chlorophyll a vary, red algae range in color from green to black in the intertidal zone to red in deeper water (FIGURE 12.24). Most red algae are marine, although a few freshwater genera are known. The gel-like polysaccharides agar and carrageenan, once they have been isolated from red algae such as Gelidium (jelli’dē-ŭm) and Chondrus (kon’drŭs), are used as thickening agents for the production of solid microbiological media and numerous consumer products, including ice cream, toothpaste, syrup, salad dressings, and snack foods. Some studies suggest that ingested carrageenan can induce inflammation of the colon. ­ Classification of Algae Chlorophyta29 are green algae that share numerous characteristics with plants—they have chlorophylls a and b and use sugar and starch as food reserves. Most have cell walls composed of cellulose, though some have walls of glycoprotein or lack walls entirely. In addition, the 18S rRNA sequences of green algae and plants are comparable. Because of the similarities, green algae are often considered to be the progenitors of plants, and in some taxonomic schemes the Chlorophyta are placed in the kingdom Plantae. Most green algae are unicellular or filamentous (see Figure 1.7a) and live in freshwater ponds, lakes, and pools, where they form green to yellow scum. Some multicellular forms grow in the marine intertidal zone—that is, in the region exposed to air during low tide. Prototheca (prō-tō-thē’kă) is an unusual green alga in that it lacks pigments, making it colorless. This chemoheterotrophic alga causes a skin rash in sensitive individuals. Codium (kō’dē-ŭm) is a member of a group of marine green algae that do not form cross walls after mitosis; thus, its entire body is a single, large, multinucleate cell. Some Polynesians dry and grind Codium for use as seasoning pepper. The green alga Trebouxia (tre-book’sē-a) is the most common alga found in association with fungi in lichens. LE A R N I N G O U TC O M E S 12.24 List four groups of algae, and describe the distinguish- ing characteristics of each. 12.25 List several economic benefits derived from algae. The classification of algae is not yet settled. Historically, ­taxonomists have used differences in photosynthetic pigments, storage products, and cell wall composition to classify algae M12_BAUM7206_05_SE_C12.indd 371 29From 30From Greek chloros, meaning “green,” and phyton, meaning “plant.” Greek rhodon, meaning “rose.” 11/23/16 3:30 AM ter a       Ch p 12    Characterizing and Classifying Eukaryotes Pneumocyst 20 mm LM 5 µm ▲ FIGURE 12.24 Pterothamnion plumula, a red alga. Flagellum “Hairs” ▲ FIGURE 12.26 Portion of the giant kelp Macrocystis, a brown alga. A kelp’s blades are kept afloat by pneumocysts. some of the giant kelps, such as Macrocystis (FIGURE 12.26), form “kelp forests.” Kelps can surpass the tallest trees in length though not in girth. Brown algae use the polysaccharide laminarin and oils as food reserves and have cell walls composed of cellulose and alginic acid (alginate). Food processors use alginic acid as a thickening agent, and dental hygienists use it in the preparation of dental impressions. Chrysophyta (Golden Algae, Yellow-Green Algae, and Diatoms) ­ Chrysophyta33 is a group of algae that are diverse with respect to cell wall composition and pigments. They are unified in using the polysaccharide chrysolaminarin as a storage product. Some additionally store oils. Modern taxonomists group these algae with brown algae and water molds (discussed shortly) in the kingdom Stramenopila based on similarities in nucleotide sequences and flagellar structure. Whereas some chrysophytes lack cell walls, others have ornate external coverings, such as scales or plates. Members of one taxon of chrysophytes, Bacillariophyceae—the diatoms (dī’ă-tomz)—are unique in having silica cell walls composed of two halves called frustules that fit together like a Petri dish (FIGURE 12.27). Most chrysophytes are unicellular or colonial. All chrysophytes contain more orange-colored carotene pigment than they do chlorophylls a and c, accounting for the common names of two major classes of chrysophytes—golden algae and yellow­green algae. Diatoms are a major component of marine phytoplankton: free-floating photosynthetic microorganisms that form the basis of food chains in the oceans. Further, because of their enormous number, diatoms are the major source of the world’s oxygen. The silica frustules of diatoms contain minute holes for LM 1 µm ▲ FIGURE 12.25 Hairy flagellum. The “hairy” flagellum of gametes Phaeophyta (Brown Algae) The Phaeophyta31 are in kingdom Stramenopila32 based in large part on their gametes being motile by means of two flagella— one whiplike and one with hollow projections giving it a “hairy” appearance (FIGURE 12.25). They have chlorophylls a and c, carotene, and brown pigments called xanthophylls (zan’thō-fils). Depending on the relative amounts of these pigments, brown algae may appear dark brown, tan, yellow-brown, greenish brown, or green. Most brown algae are marine organisms, and 31From Greek phaeo, meaning “brown.” 32From Latin stramen, meaning “straw,” and pilos, meaning “hair.” M12_BAUM7206_05_SE_C12.indd 372 ­ ­ of brown algae and other stramenopiles is distinctive. 33From Greek chrysos, meaning “gold.” 11/23/16 3:30 AM       Water Molds TELL ME WHY Why aren’t there large numbers of pathogenic algae? Water Molds LE A R N I N G O UTC O M E 12.26 List four ways in which water molds differ from true fungi. SEM Scientists once classified the microbes commonly known as water molds as fungi because they resemble filamentous fungi in having finely branched filaments. However, water molds are not true molds—they are not fungi. Water molds differ from fungi in the following ways: 30 µm • They have tubular cristae in their mitochondria. • They have cell walls of cellulose instead of chitin. • Their spores have two flagella—one whiplike and one “hairy.” • They have true diploid bodies rather than haploid bodies. ▲ FIGURE 12.27 Paralia sulcata, a diatom. Diatoms have P frustules, composed of silica and cellulose, that fit together like a etri dish. ­ ­ Characteristics of Various Algae Group (Common Name) Chlorophyta Kingdom P TABLE 12.4 Because water molds have “hairy” flagella and certain similarities in rRNA sequence to sequences of diatoms, other chrysophytes, and brown algae, taxonomists classify all these organisms in kingdom Stramenopila (see Figure 12.4c). Water molds decompose dead animals and return nutrients to the environment (FIGURE 12.28). Some species are detrimental pathogens of crops, such as grapes, tobacco, and soybeans. In 1845, the water mold Phytophthora infestans (fī-tof’tho-ră in-fes’tanz) was accidentally introduced into Ireland and devastated the potato crop, causing the great famine that killed over 1 million people and forced a greater number to emigrate to the United States and Canada. ­ the exchange of gases, nutrients, and wastes with the environment. Organic gardeners use diatomaceous earth, composed of innumerable frustules of dead diatoms, as a pesticide against harmful insects and worms—the frustules appear to draw water from the cells of the pests. Diatomaceous earth is also used in polishing compounds, detergents, and paint removers and as a component of firebrick, soundproofing products, swimming pool filters, and reflective paints. In summary, algae are unicellular or multicellular photoautotrophs characterized by sexual reproductive structures in which every cell becomes a gamete. The colors produced by the combination of their primary and accessory photosynthetic pigments give them their common names and provide the basis of at least one classification scheme. TABLE 12.4 summarizes the characteristics of the major groups of algae. lantae (green algae) Pigments Chlorophylls a and b, carotene, xanthophylls Storage Product(s) Cell Wall Component(s) Sugar, starch Cellulose or glycoprotein; absent in some Freshwater, brackish water, and saltwater; terrestrial Spirogyra Habitat Representative Genera Prototheca Codium Trebouxia Rhodophyta (red algae) Rhodophyta Chlorophyll a, phycoerythrin, phycocyanin, xanthophylls Glycogen Agar or carrageenan, some with calcium carbonate Mostly saltwater Chondrus Gelidium Antithamnion Phaeophyta (brown algae) Stramenopila Chlorophylls a and c, carotene, xanthophylls Laminarin, oils Cellulose and alginic acid Brackish water and saltwater Macrocystis Chrysophyta (golden algae, yellowgreen algae, diatoms) Stramenopila Chlorophylls a, c1, and c2; carotene; xanthophylls Chrysolaminarin, oils Cellulose, silica, calcium carbonate Freshwater, brackish water, and saltwater; terrestrial; ice Stephanodiscus M12_BAUM7206_05_SE_C12.indd 373 11/23/16 3:30 AM 12    Characterizing and Classifying Eukaryotes TELL ME WHY How do scientists know that water molds are more closely related to brown algae than to true molds? Other Eukaryotes of Microbiological Interest: Parasitic Helminths and Vectors LE A R N I N G O U TC O M E S 12.27 Explain why microbiologists study large organisms such as parasitic worms. 12.28 Discuss the inclusion of vectors in a study of microbiology. Microbiologists are also interested in two other groups of eukaryotes, although they are not microorganisms. The first group is the parasitic helminths, commonly called parasitic worms. Microbiologists became interested in parasitic helminths because they observed the microscopic infective and diagnostic stages of the helminths—usually eggs or larvae (immature forms)—in samples of blood, feces, and urine. Thus, microbiologists study parasitic helminths in part because they must distinguish the parasites’ microscopic forms from other microbes. (Chapters 21 and 23 discuss some helminth parasites of the blood and of the digestive system.) The second group of macroorganisms of interest to ­m icrobiologists is arthropod vectors 34—animals that carry pathogens and have segmented bodies, hard external skeletons, and jointed legs. Some arthropods are mechanical vectors, meaning they merely carry pathogens; others are biological vectors, meaning they also serve as hosts for microbial pathogens. Given that arthropods are small organisms (so small that we generally don’t notice them until they bite us) and given that they produce numerous offspring, controlling arthropod Arachnids LE A R N I N G O UTC O M E S 12.29 Describe the distinctive features of arachnids. 12.30 List five diseases vectored by ticks and two diseases vectored by mites. All adult arachnids (ă-rak’nidz) have four pairs of legs. Both ticks and mites (commonly known as chiggers) go through a six-leg stage when they are juveniles, but they display the ­c haracteristic eight legs as adults. Ticks and mites resemble each other morphologically, having disk-shaped bodies. Ticks are roughly the size of a small rice grain, whereas mites are usually much smaller (see Figure 12.29b). Ticks are the most important arachnid vectors. They are distributed worldwide and serve as vectors for bacterial, viral, and protozoan diseases. Ticks are second only to mosquitoes in the number of diseases that they transmit. Hard ticks—those with a hard plate on their dorsal surfaces—are the most prominent tick vectors. They wait on stalks of grasses and brush for their hosts to come by. When a human, for example, walks past them, the ticks leap onto the person and begin searching for exposed skin. They use their mouthparts to cut holes in the skin and attach themselves with a gluelike compound to prevent being dislodged. As they feed on blood, their bodies swell to several times normal size. Some tick-borne diseases are Lyme disease, Rocky Mountain spotted fever, tularemia, relapsing fever, and tick-borne encephalitis. Parasitic mites of humans live around the world, wherever humans and animals coexist. A few mite species transmit rickettsial diseases (rickettsial pox and scrub typhus) among animals and humans. ­ ­ H nutrients in aquatic habitats. ere a water mold decomposes a dead goldfish. ­ ▲ FIGURE 12.28 Water molds help recycle organic vectors to eliminate their role in the transmission of important human diseases is an almost insurmountable task. Disease vectors belong to two classes of arthropods: Arachnida and Insecta. Ticks and mites are arachnoid vectors. Spiders are also arachnids, but they do not transmit diseases. Insects account for the greatest number of vectors, and within this group are fleas, lice, flies (such as tsetse flies and mosquitoes), and true bugs (such as kissing bugs). FIGURE 12.29 shows representatives of major types of arthropod vectors. Most vectors are found on a host only when they are actively feeding. Lice are the only arthropods that may spend their entire lives in association with a single individual host. (Chapters 20, 21, 23, and 25 discuss in more detail vectors and some diseases they carry.) ­ ter a       Ch p Insects LE A R N I N G O UTC O M E S 12.31 Describe the general physical features of insects and the features specific to the various groups of insect vectors. 12.32 List diseases transmitted by fleas, lice, true flies, mos34From Latin vectus, meaning “carried.” M12_BAUM7206_05_SE_C12.indd 374 quitoes, and kissing bugs. 11/23/16 3:30 AM (a) (e) Actual size Actual size Actual size (b) (f) (c) Actual size Actual size (d) (g)       H P Other Eukaryotes of Microbiological Interest: arasitic elminths and Vectors Actual size Actual size ­ ▲ FIGURE 12.29 Representative arthropod vectors. Arachnid vectors include (a) ticks and (b) mites; insect vectors include (c) fleas, (d) lice, (e) true flies, such as this tsetse fly, (f) mosquitoes (a type of fly), and (g) true bugs. As adults, all insects have three pairs of legs and three body regions—head, thorax (chest), and abdomen. Adult insects are, however, far from uniform in appearance. Some have two wings, others have four wings, and others are wingless; some have long legs, some short; and some have biting mouthparts, whereas others have sucking mouthparts. Many insects have larval stages that look very different from the adults, complicating their identification. The fact that many insects can fly has epidemiological implications. Flying insects have broader home ranges than nonflying insects, and some migrate, making control difficult. Fleas are small, vertically flattened, wingless insects that are found worldwide, though some species have geographically limited ranges. Most are found in association with wild rodents, bats, and birds and are not encountered by humans. A few species, however, feed on humans. Cat and dog fleas are usually just pests (to the animal and its owner), but they can also serve as the intermediate host for a dog tapeworm, Dipylidium (dipĭ-lid’ē-ŭm). The most significant microbial disease transmitted by fleas is plague, carried by rat fleas. Lice (singular: louse) are parasites that can also transmit disease. They are horizontally flat, soft-bodied, wingless insects found worldwide among humans and their habitations. M12_BAUM7206_05_SE_C12.indd 375 They live in clothing and bedding and move onto humans to feed. Lice are most common in unsanitary conditions or overcrowded communities. Lice are the vectors involved in epidemic outbreaks of typhus in developing countries. Flies are among the more common insects, and many different species are found around the world. Flies differ greatly in size, but all have at least two wings and fairly well-developed body segments. Not all flies transmit disease, but those that do are usually bloodsuckers. Female sand flies (Phlebotomus; fle-bot’ō-mŭs) transmit leishmaniasis in North Africa, the Middle East, Europe, and parts of Asia. Tsetse flies (Glossina; glo-sī’nă) are limited geographically to tropical Africa, where they are found in brushy areas and transmit African sleeping sickness. Mosquitoes are a type of fly, though they are morphologically distinct from other fly species. Female mosquitoes are thin and have wings, elongated bodies, long antennae, long legs, and a long proboscis for feeding on blood. Male mosquitoes do not feed on blood. Mosquitoes are found throughout the world, but particular species are geographically limited. Mosquitoes are the most important arthropod vectors of diseases, and they carry the pathogens that cause malaria, yellow fever, dengue fever, filariasis, viral encephalitis, and Rift Valley fever. 11/23/16 3:30 AM 12    Characterizing and Classifying Eukaryotes Too Much Wine? U At the ER, Becky’s physical exam is normal. The mental assessment confirms confusion and disorientation. The doctor orders blood and urine lab analysis and admits Becky to the hospital. The blood and urine cultures come back negative for any microbes. Becky’s confusion gets worse, and she begins experiencing weakness in her arms and legs. Even though Becky and Tim told the doctor she hadn’t suffered any head injuries, the doctor orders computed tomography (CT) of her head. The CT scan, also known as a CAT scan, consists of a series of X-ray images taken from multiple angles and then combined by a computer to provide cross-sectional and three-dimensional views. The results come back normal. rgently seeking a diagnosis, the doctor sits down with Becky and Tim to go over the preceding days again—where had they gone, what had they done, what had they eaten? Becky is having a hard time remembering anything from the last few days. As Tim recounts their meals, the doctor asks where they got their crab and TELL ME WHY Why are large eukaryotes such as mosquitoes and ticks considered in a microbiology class? clams and how much they ate. Tim said he had only a bite of crab, but Becky ate a lot. The doctor suspects that Becky was poisoned by the crabs and orders a lab test for a toxin. The results of the test come back positive. Becky has amnesic shellfish poisoning (AS ) from the contaminated crab. Although there is no antidote for the toxin, in nonsevere cases such as Becky’s, the toxin will eventually pass from the system. Becky receives supportive care in the hospital for three weeks; by the time she is discharged, she has made a complete recovery. 1. Why did Becky’s doctor provide only supportive care? Why weren’t antimicrobials part of her treatment? 2. Becky’s illness was caused by a diatom from the Pseudo-nitzchia genus. How are diatoms different from other eukaryotic cells? Do they have any features found in prokaryotes? Check your answers to Micro in the Clinic Follow- p questions in the MasteringMicrobiology Study Area. U Kissing bugs are relatively large, winged, true bugs with cone-shaped heads and wide abdomens. They are called kissing bugs because of their tendency to take blood meals near the mouths of their hosts. Both sexes feed nocturnally while their victims sleep. Kissing bugs transmit disease in Central and South America. The most important disease they transmit is Chagas’ disease. P ter a       Ch p CHAPTER SUMMARY ­ General Characteristics of Eukaryotic Organisms (pp. 351–355) 1. A typical eukaryotic nucleus may be haploid (having a single copy of each chromosome) or diploid (having two copies). It divides by mitosis in four phases—prophase, metaphase, anaphase, and telophase—resulting in two nuclei with the same ploidy as the original. 2. Meiosis is nuclear division that results in four nuclei, each with half the ploidy of the original. 3. A cell’s cytoplasm divides by cytokinesis either during or after nuclear division. 4. Coenocytes are multinucleate cells resulting from repeated mitoses but postponed or no cytokinesis. 5. Some microbes undergo multiple mitoses by schizogony to form a multinucleate schizont, which then undergoes cytokinesis to produce numerous daughter cells. M12_BAUM7206_05_SE_C12.indd 376 6. The classification of eukaryotic microbes is problematic and has changed frequently. Historical schemes based on similarities in morphology and chemistry have been replaced with schemes based on nucleotide sequences and ultrastructural features. Protozoa (pp. 355–361) 1. Protozoa (studied by protozoologists) are eukaryotic, unicellular organisms that lack cell walls. 2. A motile trophozoite is the feeding stage of a typical protozoan. Some protozoa form a cyst, a resting stage that is resilient to environmental changes. 3. Most protozoa are chemoheterotrophs, getting their energy from chemicals and their carbon from other organisms, but dinoflagellates and euglenoids are photoautotrophs. 4. A few protozoa undergo sexual reproduction by forming gametocytes that fuse to form a zygote. 11/23/16 3:30 AM       Chapter Summary 5. Protozoa may be classified into six groups: parabasalids, diplomonads, euglenozoa, alveolates, rhizaria, and amoebozoa. 6. Parabasalids (e.g., Trichomonas) are characterized by a Golgi body–like structure called a parabasal body. 7. Members of the Diplomonadida lack mitochondria, Golgi bodies, and peroxisomes. 8. Unicellular flagellated euglenids are euglenozoa that store food as paramylon, lack cell walls, and have eyespots used in positive phototaxis. Because they exhibit characteristics of both animals and plants, euglenids are a taxonomic problem. 9. A kinetoplastid is a euglenozoan with a single, large, apical mitochondrion that contains a kinetoplast, which is a region of DNA. 9. Fungi in the division Ascomycota, a group of economically important fungi, produce ascospores within sacs called asci. 10. Fungi of the division Basidiomycota have fruiting bodies called basidiocarps that include mushrooms, puffballs, and bracket fungi. Basidiocarps produce basidiospores at the ends of basidia. 11. Deuteromycetes (imperfect fungi) are an informal grouping of fungi having no known sexual stage. 12. Lichens are economically and environmentally important organisms composed of fungi living in partnership with photosynthetic microbes, either green algae or cyanobacteria. Algae (pp. 370–373) 1. Algae are photosynthetic organisms in which every cell of the reproductive structures becomes a gamete. 11. Protozoa that move and feed with pseudopods are amoebas, which are classified into two kingdoms: Rhizaria and Amoebozoa. 3. Most algae are aquatic, but they can live in moist soil and in ice. ­ 10. Alveolates, with cavities called alveoli beneath their cell surfaces, include ciliate alveolates (characterized by cilia), apicomplexans (all of which are pathogenic), and dinoflagellates (responsible for red tides). 12. Rhizaria include foraminifera, which have threadlike pseudopods and calcium carbonate shells, and radiolaria, which have threadlike pseudopods and silica shells. 13. Amoebozoa have lobe-shaped pseudopods. They include ­free-living amoebas, parasitic amoebas, and slime molds. Slime molds, despite their name, are not fungi but are a kind of amoeba. They lack cell walls and are phagocytic in their nutrition. 14. Plasmodial slime molds (acellular slime molds) are composed of multinucleate cytoplasm. Cellular slime molds are composed of myxamoebas that phagocytize bacteria and yeasts. Fungi (pp. 361–370) 4. Small algae grow as single cells, in colonies, or as simple multicellular bodies. Large algae are multicellular with stemlike stipes, leaflike blades, and holdfasts that attach them to substrates. 5. Algae typically reproduce by an alternation of generations, in which a haploid body alternates with a diploid body. 6. Division Chlorophyta contains green algae, which are metabolically similar to land plants. 7. Rhodophyta, red algae, contain the pigment phycoerythrin, the storage molecule glycogen, and cell walls of agar or carrageenan, substances used as thickening agents. 8. Phaeophyta, brown algae, contain xanthophylls, laminarin, and oils. They have cell walls composed of cellulose and alginic acid, which is another thickening agent. A brown algal spore is motile by means of one “hairy” flagellum and one whiplike flagellum. 9. Chrysophyta—the golden algae, yellow-green algae, and diatoms—contain chrysolaminarin as a storage product. The silica cell walls of diatoms are arranged in nesting halves called frustules. ­ 1. Fungi (studied by mycologists) are chemoheterotrophic eukaryotes with cell walls usually composed of chitin. 2. The study of algae is phycology, and the scientists who study them are phycologists. 2. Most fungi are beneficial, but some produce mycoses (fungal diseases). 3. Mold bodies are composed of tubular filaments called hyphae. Yeasts are globular, single cells. 4. Hyphae are described as either septate or aseptate, depending on the presence of cross walls. A mycelium is a tangled mass of hyphae. 5. A dimorphic fungus is either moldlike (with hyphae) or yeastlike, depending on environmental conditions. 6. Most fungi are saprobes—they acquire nutrients by absorption from dead organisms. Other fungi get nutrients from living organisms using haustoria that penetrate host tissues. 7. Fungi reproduce asexually either by budding or via asexual spores, which are categorized according to their mode of development. Most fungi also reproduce sexually via sexual spores. tor ideo V Tu 8. Most fungi in the division Zygomycota produce rough-walled zygosporangia. Microsporidia are intracellular parasites formerly classified as protozoa but now classed with zygomycetes based on genetic analysis. M12_BAUM7206_05_SE_C12.indd 377 Water Molds (pp. 373–374) 1. Water molds have tubular cristae in their mitochondria, cell walls of cellulose, spores having two different flagella, and diploid bodies. They are placed in the kingdom Stramenopila along with chrysophytes and brown algae. Other Eukaryotes of Microbiological Interest: Parasitic Helminths and Vectors (pp. 374–376) 1. Parasitic helminths are significant to microbiologists in part because their infective stages are usually microscopic. 2. Also important to microbiologists are arthropod vectors, animals that carry and transmit pathogens. Mechanical vectors merely carry microbes; biological vectors also serve as microbial hosts. 3. Ticks and mites (chiggers) are arachnids—eight-legged arthropods. 4. Insect vectors include fleas; lice; bloodsucking flies, including mosquitoes; and kissing bugs. 11/23/16 3:30 AM ter a       Ch p 12    Characterizing and Classifying Eukaryotes U Q ESTIONS FOR REVIEW Answers to the Questions for Review (except Short Answer questions) begin on p. A-1. 2. Which of the following sequences reflects the correct order of events in mitosis? c. telophase, prophase, a. telophase, anaphase, metaphase, anaphase metaphase, prophase d. prophase, metaphase, b. prophase, anaphase, anaphase, telophase metaphase, telophase 3. Which of the following statements accurately describes prophase? c. The cell constructs a. The cell appears to have microtubules to form a a line of chromosomes spindle. across the midregion. d. Chromatids separate b. The nuclear envelope and become known as becomes visible. chromosomes. 4. Multiple nuclear divisions without cytoplasmic divisions result in cells called __________________. a. mycoses c. haustoria b. coenocytes d. a pseudohypha 5. Tubular filaments with cross walls found in large fungi are ________________. a. septate hyphae c. aseptate haustoria b. aseptate hyphae d. dimorphic mycelia 6. The type of asexual fungal spore that forms within hyphae is called a __________________. a. sporangiospore c. blastospore b. conidiospore d. chlamydospore 7. A phycologist studies which of the following? c. rusts, smuts, and yeasts a. classification of eukaryotes eukaryotes d. parasitic worms b. alternation of generations in algae 8. The stemlike portion of a seaweed is called its _________________. a. trunk c. stipe b. holdfast d. blade 9. Carrageenan is found in the cell walls of which group of algae? a. red algae c. dinoflagellates b. green algae d. yellow-green algae 10. Chrysolaminarin is a storage product found in which group of microbes? a. dinoflagellates c. golden algae b. euglenids d. brown algae M12_BAUM7206_05_SE_C12.indd 378 12. Amoebas include microbes with __________________. a. threadlike pseudopods c. parabasal bodies b. eyespots d. alveoli 13. The motile feeding stage of a protozoan is called a(n) __________________. a. apicomplexan c. cyst b. gametocyte d. trophozoite 14. Which of the following is common to mitosis and meiosis? a. spindle c. tetrad of chromatids b. crossing over d. cytokinesis 15. Which taxon is characterized by “hairy” flagella? a. Apicomplexa c. Alveolata b. Euglenozoa d. Stramenopila Matching 1. _____ Mitosis A. Cytoplasmic division 2. _____ Meiosis B. Diploid nuclei producing haploid nuclei 3. _____ Homologous chromosomes 4. _____ Crossing over 5. _____ Cytokinesis ­ contain half a set of chromosomes are found in the cytosol of eukaryotic organisms C. Results in genetic variation D. Carry similar genes E. Diploid nuclei producing diploid nuclei ­ 1. Haploid nuclei __________________. c. a. contain one set of chromosomes d. b. contain two sets of chromosomes 11. Which of the following features characterizes diatoms? c. chlorophylls a and c and a. laminarin and oils as food carotene reserves d. paramylon as a food b. protective plates of storage molecule cellulose in their cells  Multiple Choice 1. _____ Chitin A. Fungal cell wall component 2. _____ Basidiospore B. Fungus + alga or bacterium 3. _____ Zygosporangium 4. _____ Hypha 5. _____ Ascospore 6. _____ Lichen C. Fungal filament D. Fungal spore formed in a sac E. Diploid fungal zygote with a thick wall F. Fungal spore formed on clubshaped hypha 1. _____ Chlorophyta A. Foraminifera 2. _____ Rhodophyta B. Yellow-green algae 3. _____ Chrysophyta 4. _____ Phaeophyta 5. _____ Rhizaria C. Green algae D. Brown algae E. Red algae 11/23/16 3:30 AM       Questions for Review 2. Describe the features of a general fungal life cycle. VISUALIZE IT! 1. Label the photos below with the type of fungal spore, and indicate whether the spore is asexual or sexual. Ascus Basidium a. ___________________ (event) + – b. ___________________ (stage) + – g. _______________ (events) c. __________________ (nucleus state) + a. ______________, _______________ b. ______________, _______________ Conidiophore Sporangiophore + – + – – + + – d. ___________________ (stage) – f. ___________________ (stage) e. ___________________ (event) c. _____________, ________________ d. ______________, _______________ 7. Name two ways that slime molds differ from true fungi. Short Answer 1. Compare and contrast the following closely related terms: Chromatid and chromosome Mitosis and meiosis II Hypha and mycelium Algal body and fungal body Water mold and slime mold 2. How do fungi transport nutrients? 3. How are lichens useful in environmental protection studies? 4. What are the taxonomic challenges in classifying euglenids? 5. List several economic benefits of algae. 6. Why are relatively large animals such as parasitic worms studied in microbiology? 8. What is the role of rRNA sequencing in the classification of eukaryotic microbes? 9. Describe the nuclear divisions that produce eight ascospores in an ascus. Fill in the Blanks 1. The study of protozoa is called __________________. 2. The study of fungi is called __________________. 3. The study of algae is called __________________. 4. Fungal diseases are called __________________. 5. Amoebas with stiff pseudopods and silica shells are __________________. H CRITICAL T INKING 1. How are cysts of protozoa similar to bacterial endospores? How are they different? 4. How can one distinguish a filamentous fungus from a colorless alga? 2. The host of a home improvement show suggests periodically emptying a package of yeast into a drain leading to a septic tank. Explain why this would be beneficial. 5. Why do scientists as a group spend more time and money studying protozoa than they do studying algae? 3. Why doesn’t penicillin act against any of the pathogens discussed in this chapter? M12_BAUM7206_05_SE_C12.indd 379 6. Why are there more antibacterial drugs than antifungal drugs? 11/23/16 3:30 AM E       Chapter 12    Characterizing and Classifying ukaryotes cell wall. However, taxonomists today classify Pneumocystis as a fungus. Why do you think this pathogen has been reclassified as a fungus? 7. Which metabolic pathways are present in protozoa that lack mitochondria (amoebas, diplomonads, and parabasalids)? Which metabolic pathways are absent? 10. Explain why both dinoflagellates and euglenids were originally classified by zoologists as protozoa and by botanists as algae. 8. Without reference to genetic sequences, mycologists are certain that none of the septate, filamentous deuteromycetes will be shown to make zygospores. How can they be certain when the sexual stages of deuteromycetes are still unknown? 11. Fungi tend to reproduce sexually when nutrients are limited or other conditions are unfavorable, but they reproduce asexually when conditions are more ideal. Why is this a successful strategy? 9. Twenty years ago, Pneumocystis jirovecii, a pathogen of immunocompromised patients that causes pneumonia, was classified as a protozoan because it is a chemoheterotroph that lacks a ING APP M EPT CONC 12. Given that Prototheca is colorless, how do scientists know that it is really a green alga? Using the following terms, fill in the following concept map that describes eukaryotic microorganisms. Unicellular (2) Yeasts Giardia Molds Plasmodium Protozoa Chitin Colonial Cryptosporidium Fungi Algae Alginic acid Carrageenan Cell walls Eukaryotic microorganisms include 1 are include 2 6 lack Cell walls 4 3 5 may be 10 have are Photosynthetic Unicellular Multicellular Cell walls which are which are may be composed of 7 8 9 may be have 13 which may be composed of 11 Agar 12 14 Multicellular 15 Cellulose M12_BAUM7206_05_SE_C12.indd 380 06/03/18 4:55 PM