Part-3: Ascomycota

General Characteristics of Ascomycota Members vary in form and structure. Yeast are unicellular, but mostly they have a well-developed, profusely branched and septate mycelium with cross-walls and perforated septa. Multinucleate cells and unicellular forms. Asexual reproduction takes place by various types of non-motile spores such as oidia, chlamydospores, and conidia. In unicellular forms, fission, fragmentation, and budding are the most common methods of vegetative propagation.   They are homothallic or heterothallic. In some heterothallic species, sex organs develop on the same strain. They are self-incompatible. In these species, the male gamete of one mating type fertilizes the ascogonium of another mating type. This process is known as physiological heterothallism. Sexual reproduction takes place by gametangial copulation (yeast), gametangia fusion (Penicillium), or spermatization.   They show the phenomenon of heterokaryosis, the nuclei of two different genotypes are present in the same mycelium. The fruiting body is known as ascocarp. The following four types of ascocarps are known: Apothecium: A disc- or saucer-shaped cup-like structure, brown to black or sometimes red or orange, yellow in color. Cleistothecium: A completely closed, spherical or flask-shaped ascocarp. Perithecium: A flask-shaped ascocarp with a narrow opening. Gymnoascus: A naked ascocarp without any protective covering. The hymenium lines the cup of the apothecium and contains densely packed elongated asci, which are perpendicular to the base. Paraphyses are found interspersed with the asci in some cases. The paraphyses spread over the tips of the asci and form a thin, brightly colored layer called the epithecium. Medulla is present below the hymenium, which contributes to the lateral growth of the apothecium. The basal part of the epithecium is called the cortex, composed of pseudo-parenchymatous hyphae. The apothecium may be stalk or sessile. Ascomycota Apothecium: Cup-shaped, usually brown or orange. Hymenium lines the inner surface of the cavity. Asci are present intermingled with the sterile paraphyses arising from the base of the perithecium. Asci are cylindrical in shape. Cleistothecium: Globular, completely closed fruiting body without any opening. The wall is made up of fungal hyphae and asci are present scattered within the peridium. Asci are globose, pear-shaped, and sessile. Perithecium: Flask-shaped structure with a small opening. Often seen in Gymnoascus. The opening is a single narrow slit and is found in Hysteriales. Gymnoascus: A small, black, elongated structure with a leathery texture. Opens by a single narrow slit. Found in Hysteriales. Ascospores Endogenous in origin The members of Ascomycota are worldwide in distribution. They occur in various ecosystems - terrestrial, fresh, and marine waters. Ascomycetes occur in soil as saprophytes, while some grow as parasites on plants and animals, causing various diseases. Some members of the Ascomycota occur in symbiotic associations with algae to form lichens. They also occur in roots of higher plants. Ascomycetes play major roles in ecosystems. They act as decomposers of organic matter and help in nutrient cycling. Different fruiting bodies produced by various members of Ascomycota are the source of food for insects and snails. Ascomycetes also form symbiotic associations with some animals and plants. Common saprophytic members are Aspergillus, Penicillium, etc., which grow upon rotten vegetables, fruits, soil, and other organic matters and are colored blue and green molds. Life Cycle of Saccharomyces Reproduction in Saccharomyces or budding yeast mainly occurs by vegetative methods of reproduction. 1. Vegetative Method of Reproduction Occurs by two ways: (a) Budding (b) Fission (a) Budding: The most common method of reproduction of yeast when nutrients are present sufficiently in the substrate. The following sequence of events occurs: A portion of the cell wall swells out at one pole of the cell and is known as the bud. The protoplast of this region bulges out as an outgrowth or protuberance. The parent nucleus divides into two, mostly by a mitotic division. One of the daughter nuclei migrates into the enlarging bud. The bud grows and gets constricted at the point of contact and finally pinches off. The detached bud gives rise to a full-fledged yeast cell that again multiplies by budding. Due to rapid budding, sometimes the daughter bud produces a bud before detaching from the parent bud, and this process gives rise to a pseudomycelium-like structure. (b) Fission: Rarely occurs in Saccharomyces species but in other yeast species. The mother cell elongates, the nucleus divides into two, and a septum-like wall growth appears from the middle. This wall stretches from all sides and meets at the middle to form a septum. The cell divides transversely into two daughter cells. However, this process is not common in budding yeast. 2. Sexual Reproduction Sexual reproduction in yeast is very rare and occurs during nutrient scarcity. They reproduce like other ascomycetes but they do not have any sex organs like ascogonium and antheridium. The sexual process is extremely simplified and the cells or even the ascospores behave as the sexual units exhibiting the degeneration of sex in this group. The plasmogamy here occurs through the process of conjugation between cells of opposite mating types. Two haploid cells of opposite mating types come together and beak-like outgrowths come out of each cell. These outgrowths touch each other and fuse. This process is called karyogamy, leading to the formation of a diploid zygote. This diploid nucleus divides Life Cycle in Yeast Guilliermond (1940) has recognized three types of life cycles in yeast that occur in different species: haplo-biontic, diplo-biontic, and haplo-diplobiontic. Haplo-biontic Type: In this type, haploid cells are the dominant phase. These species usually multiply vegetatively. Under adverse conditions, they undergo sexual reproduction. When two haploid cells of opposite mating types come together, they form a zygote. The zygote divides by meiosis to form 8 nuclei, which are haploid. These nuclei get organized into ascospores. Ascospores are released by the rupturing of the ascus and give rise to a yeast cell. In this type, the haploid phase is dominant, and the diploid phase is short-lived. Diplo-biontic Type: In this type, diploid cells are the dominant phase. The vegetative cells belong to two mating types (α and α). These cells function as gametangia, and plasmogamy takes place by gametangial copulation. This leads to the formation of a dikaryon, which is a diploid zygote. The dikaryon undergoes meiosis to form 8 haploid nuclei, which get organized into ascospores. Ascospores are not released but fuse within the ascus to form two zygotes. These zygotes germinate to form a small germ tube and give rise to a sprout mycelium, which is diploid. The cells of this sprout mycelium divide mitotically to form yeast cells. In this type, the haploid phase is restricted to ascospores only, and the diploid phase is dominant. Haplo-diplobiontic Type: This type of life cycle is found in Saccharomyces cerevisiae and other related species. There are two types of cells: the dwarf strain and the large strain. The dwarf strain yeast cells belong to two mating types (α and α). These cells function as gametangia, and plasmogamy takes place by gametangial copulation. This leads to the formation of a dikaryon, which is a diploid zygote. The dikaryon undergoes karyogamy to form a diploid nucleus. This diploid nucleus divides by meiosis to form 8 haploid nuclei, which get organized into ascospores. Ascospores are released by the rupturing of the ascus and give rise to a yeast cell. In this type, both the haploid and diploid phases are dominant. Life Cycle in Yeast The diploid zygote, instead of undergoing immediate meiosis, leads an independent vegetative stage. They grow as vegetative yeast cells and multiply by budding. This is the large α strain. When there is an unfavorable environment, the large strain yeast vegetative cell directly functions as an ascus. Here, the diploid nucleus undergoes meiosis to form four ascospores, out of which two are α strain type and two are a strain type. They are released by the rupturing of the ascus wall and develop into haploid yeast cells, the dwarf strains. Since the duration of both haploid and diploid phases are of equal dominance, this life cycle is called the haplo-diplobiontic type Life Cycle of Aspergillus Aspergillus reproduces by asexual and sexual methods. Asexual Reproduction: Conidia are produced in conidiophores, which arise from the vegetative hyphae. The conidiophores are upright branches termed as foot cells. The tip of the conidiophore swells to form a vesicle. A number of nuclei along with protoplasm migrate into the vesicle. The vesicle divides to form a number of sterigmata. From each sterigmata, a chain of conidia is produced. Conidia are branched, aleurosporous, and sometimes septate. They may be black, green, blue, brown, yellow in color. Conidia are light and air-disseminated. A conidium germinates on a suitable substrate by producing a germ tube, which gives rise to the mycelium. Sexual Reproduction: In Aspergillus, sexual reproduction occurs only in different sexes and strains. In some species, the development of the sex organs is suppressed, and they only reproduce asexually. In some other species, there is no development of antheridium, and only the ascogonium develops. In some species, both the sex organs are not developed, and they reproduce only through parasexual cycle. However, all species of Aspergillus reproduce mostly by parasexual methods. The parasexual cycle involves the fusion of two vegetative cells of opposite mating types. This leads to the formation of a heterokaryon, which contains nuclei from both parents. The heterokaryon undergoes karyogamy to form a diploid zygote. The diploid zygote undergoes meiosis to form haploid ascospores. These ascospores are released and germinate to give rise to new haploid strains. Life Cycle of Aspergillus Sexual Reproduction In Aspergillus, the male and female sex organs develop on the same mycelium, called homothallic. The male sex organ, antheridium, develops on the hypha called pollinodium, and the female sex organ, ascogonium, develops on the hypha called archicarp. The archicarp becomes coiled at its terminal region and is divided into three parts: the trichogyne, the central portion, and the hymenium. The pollinodium develops from a vegetative hypha and curls over the apex of the trichogyne. The tip of the pollinodium comes in contact with the trichogyne, and the nucleus of the antheridium passes into the ascogonium. The male and female nuclei in the ascogonium come to lie in pairs but do not fuse. The fusion of male and female nuclei in Aspergillus has not yet been observed. After pairing of nuclei, the ascogonium begins to produce ascogenous hyphae. These hyphae may be branched or unbranched and are multinucleate. The ultimate and penultimate cells of each ascogenous hypha give rise to asci. Each ascus contains 8 ascospores. At this time, several sterile hyphae develop from the stalk of the ascogonium and surround the ascocarp completely, forming the fruiting body. The ascocarp is ovate, rounded, yellow, and a hollow ball with a smooth wall. It contains several asci, and the ascocarp has no opening. This type of ascocarp is called cleistothecium or cleistocarp. The two haploid nuclei (male and female) fuse to form a synkaryon, which normally divides mitotically to form 8 haploid ascospores. After the formation of ascospores, the ascus wall dissolves, and the ascospores are liberated inside the ascocarp. At maturity of the ascocarp, all the vegetative structures, including the wall of the ascus, undergo degeneration to form a nutritive fluid that provides nutrition to the developing ascospores. The ascospores are liberated by the decay of the wall of the cleistothecium. Life Cycle of Aspergillus Asexual Reproduction Each ascospore is a lens-shaped, uninucleate structure, swelling at one end. The ascospore germinates, giving rise to a germ tube, which grows on a suitable substrate through germ pore. The germ tube develops into conidiophore. The conidiophore gives rise to a cluster of sterigmata. From each sterigmata, conidia are produced. Conidia are light and air-disseminated. When a conidium falls on a suitable substrate, it germinates to form a germ tube, which develops into a new mycelium. Life Cycle of Penicillium Reproduction in Penicillium mainly occurs by asexual methods. Sexual reproduction is rare and occurs only under special conditions, leading to the formation of ascospores and fruiting bodies. Asexual Reproduction Conidia are produced on conidiophores. Conidiophores can arise from any vegetative cell as an erect, tubular hypha and grow in length. The conidiophores are septate, branched, and multinucleate. At the ends of the branches of the conidiophores, bottle-like sterigmata or phialides are produced. Usually, a single whorl of phialides is present, but in some species, two or more whorls are present. The first-formed branches are called primary branches, and the secondary branches give rise to tertiary branches. The ultimate branches that give rise to sterigmata are called metulae. Conidia are produced from the tips of the sterigmata in a basipetal manner. Conidia are ovoid, globose, elliptical, or pyriform in shape and color. They are conidial, dry, and light. Conidia are dispersed by wind and germinate upon getting a suitable substrate. The germ tube develops into a new mycelium. Sexual Reproduction Almost all species of Penicillium are homothallic except one, which is heterothallic. The male sex organ is called antheridium, and the female sex organ is called ascogonium. An ascogonium originates from an ordinary cell of the mycelium as a short protuberance that elongates further. Initially, it is uninucleate, but during elongation, it divides repeatedly to form 20-64 nuclei. It becomes a tubular, non-septate multinucleate structure. The antheridium also develops from an ordinary vegetative cell of the mycelium. The antheridium is a short, uninucleate hypha that grows towards the ascogonium. The tip of the antheridium penetrates the wall of the ascogonium, and the nucleus of the antheridium fuses with the nucleus of the ascogonium. This process is called karyogamy and results in the formation of a diploid zygote. The zygote undergoes meiosis to form 8 haploid nuclei, which get organized into ascospores. These ascospores are arranged in a linear or sac-like structure called the ascus. The ascus is surrounded by sterile hyphae, which form the peridium or the wall of the fruiting body. The fruiting body is called the cleistothecium in Penicillium. Ascospores are released by the rupture of the peridium. After liberation, the ascospores germinate to form new mycelia. Life Cycle of Penicillium Sexual Reproduction The antheridium may originate from another hypha close to the ascogonium or from the same hypha. The antheridium elongates and curves towards the ascogonium. The tip of the antheridium comes in contact with the ascogonium, and the septum between them dissolves. The protoplasts of the antheridium and ascogonium mix, resulting in karyogamy. According to some researchers, the male nucleus may not enter the ascogonium and may remain in the antheridium. The nuclei in the ascogonium arrange themselves in pairs and undergo karyogamy. After karyogamy, the dikaryotic cells in the ascogonium give rise to lateral outgrowths called ascogenous hyphae. These hyphae develop into branched or unbranched structures. The tip of the ascogenous hypha swells to form a sac-like structure called the ascus. The ascus contains a single diploid nucleus. The diploid nucleus undergoes meiosis, followed by one mitotic division, resulting in the formation of 8 haploid nuclei. These haploid nuclei get organized into ascospores. The ascospores are arranged in a linear or sac-like structure called the ascus. The ascus is surrounded by sterile hyphae, which form the peridium or the wall of the fruiting body. The fruiting body is called the cleistothecium in Penicillium. Ascospores are released by the rupture of the peridium. After liberation, the ascospores germinate to form new mycelia.