Reproduction Lecture PDF

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Notes from Lecture 31 with Professor Jessamyn Manson...

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BIOL 2200 Lecture 31

4/13/2018

Reproduction The liver is involved in chemical digestion and absorption of nutrients

What determines an animal’s sex? ● In birds and mammals, sex is determined by genes ○ Mammals: F=XX M=XY ○ Birds: F=ZW M=ZZ ● In many fish and reptiles, sex is determined by temperature ○ After the eggs are laid, while they are being developed ○ In turtles at lower temperatures there is a higher proportion of males and vice versas ○ In lizards there is lower percentage of males at the extreme temperatures Asexual and Sexual rEproduction ● Sexual REproduction in Animals ○ Fusion of two haploid gametes to form a diploid zygote ○ Gametes formed by meiosis ○ Reproduction is primarily or exclusively sexual for most animals. ● Asexual Reproduction and Hermaphrodites ○ Fragments can become new individuals ■ Budding in corals ■ Fission in sea anemones, Platyhelminthes ○ Parthenogenesis: egg develops without fertilization ○ Hermaphrodites: both male and female reproductive organs ● Asexual Reproduction: plants vs animals ○ Sexual diversity is the same ● Asexual/Hermaphrodite reproduction in animals ○ Pros ■ Any mate will do / no mate required ■ Great for sessile organisms ■ Great in stable environments ■ May be less costly * ● * The two-fold cost of sex ○ In a population of females that produce asexually, each 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

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Asexual and sexual females produce the same number of offspring These offspring have to have equal fitness

■ Cons ■ May lead to reduced genetic variation ■ Accumulation of deleterious mutations

The Red Queen Hypothesis ○ In a population challenged by a parasite, some individuals may have genes that confer resistance ○ Parasites quickly evolve to overcome host resistance ○ Sexual reproduction can lead to new gene combinations more likely to provide a new resistance strategy ○ Asexual species must rely on random mutation alone Switching Reproductive Modes ○ Many asexual species can switch to sexual reproduction - what triggers the switch? ○ In Daphnia, there are several factors ■ Day length ■ Food availability ■ Density - competition and resources availability ○ Daphnia move to sexual reproduction when conditions are stressful ■ More genetic diversity

○ Fertilization ○ Sexual reproduction requires the union of sperm and egg ○ Spermatogenesis

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■ Look at textbook pages(260-261) and slides ■ Primordial germ cell in embryo - mitotic divisions ■ Spermatogonial stem cell - mitotic division ■ Spermatogonium - mitotic division ■ Primary spermatocytes - meiosis I ■ Secondary spermatocyte - Meiosis II ■ Early spermatid - differentiation ■ Sperm cells (4) Sperm production occurs throughout a male’s life Oogenesis

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Primordial germ cell - mitotic division Oogonium - mitotic division (IN EMBRYO) Primary oocyte (present at birth), arrested in prophase of meiosis I ■ (STARTING AT PUBERTY) - completion of meiosis I and onset of meiosis II ■ Secondary oocyte, arrested at metaphase of meiosis II - ovulation, sperm entry - completion of meiosis II ■ Fertilized egg (1) ○ In humans, all primary oocytes have been produced before birth External Fertilization ○ Female releases eggs into the environment, male fertilize them with sperm ○ This usually requires a moist habitat ○ How do animals get the timing right?

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■ Environmental cues ■ Chemical signals ■ Courtship Internal Fertilization ● Sperm deposited in or near female reproductive tract ● May still require courtship behavior to ensure successful fertilization ● Embryo in egg or internally gestated ● Increased parental care Strange sexual reproduction strategies ○ Hermaphroditism - mate with anybody (and sometimes themselves)! ○ Sex reversal - change sex for increased mating success ■ Slipper shell snails born male, become female ○ Sperm storage - spermatheca hold sperm, female controls fertilization ○ Sperm plugs - male secretions that prevent females from mating with other males...