University:
University of Virginia
Course:
BIOL 2200 | Introduction to Biology II: Organisms & Evolution
Academic year:

2024
Views:

372

Pages:

3
Author:

Zoe C.

30: reproduction Monday, April 15, 2024 1:37 PM what determines an animal's sex • in birds, mammals, and most insects, sex is determined by chromosomes ○ mammals: F = XX, M = XY ○ birds: F = ZW, M = ZZ ○ insects: varies • in many fish and reptiles, sex is determined by temperature during development • some animals are hermaphrodites or change sex sexual reproduction in animals • fusion of haploid gametes to form a diploid zygote • gametes formed by meiosis • reproduction is primarily or exclusively sexual for most animals asexual reproduction • fragments can become new individuals ○ budding in corals, hydras ○ fission in sea anemones, Platyhelminthes • parthenogenesis/haplodiploidy ○ eggs develop without fertilization ○ offspring share all (aphids) or half (male bees) mom's genome asexual reproduction or self-fertilization • self-fertilization: male and female gametes from the same organism form a zygote • pros: ○ great for sessile organism ○ great in stable environments ○ may be less costly § the two-fold cost of sex • cons: ○ reduces genetic variation ○ accumulation of deleterious mutations ○ may be less costly § the two-fold cost of sex • cons: ○ reduces genetic variation ○ accumulation of deleterious mutations the two-fold cost of sex • in a population of females that reproduce asexually, every individual can produce young • in a sexual population, only females have young • the asexual population can produce offspring at twice the rate of the sexual population • selection should favor populations with more efficient reproduction • this assumes: ○ asexual and sexual females produce the same number of offspring ○ these offspring have equal fitness disease and asexual reproduction • parasites selectively infect the most common host genotype, causing common genotypes to crash • sexually reproducing hosts experience new gene combinations - can evolve resistance • asexual hosts must rely on random mutation ○ disease vulnerability is a big potential fitness cost switching reproductive modes • species with asexual reproduction also sexually reproduce • in daphnia, factors that lead to a switch from asexual to sexual reproduction include: ○ day length ○ food availability ○ density fertilization • sexual reproduction requires the union of sperm and egg external fertilization • female releases eggs into the environment, male fertilizes them with sperm • this usually requires a moist habitat • how do animals get the timing right ○ environmental cues external fertilization • female releases eggs into the environment, male fertilizes them with sperm • this usually requires a moist habitat • how do animals get the timing right ○ environmental cues ○ chemical signals courtship • synchronous spawning ○ reproduction between many individuals internal fertilization • • • • • sperm deposited in or near female reproductive tract often preceded by courtship behavior requires compatible reproductive morphology embryo develops in an egg or is internally gestated increased parental care strange sexual reproduction strategies • sperm storage: spermathecae hold sperm, female controls fertilization • sperm removal: specialized structures allow male to remove other sperm • sperm plugs: male secretions that prevent females from mating with other males • sex reversal: change sex for increased mating success