Cycle 7: All About Sex Lecture 14: Why Sex PDF

Cycle 7: All About Sex Lecture 14: Why sex? Aspidoscelis uniparens  “Sex” conjures up ideas about copulation, reproduction, gender, gametes…  Sex, recombination, genetic diversity reproducing sexually generates new multilocus combinations of alleles – AaBb...

Cycle 7: All About Sex Lecture 14: Why sex? Aspidoscelis uniparens  “Sex” conjures up ideas about copulation, reproduction, gender, gametes…  Sex, recombination, genetic diversity reproducing sexually generates new multilocus combinations of alleles – AaBb x AaBb  AABB, AaBB, aaBB, AABb, AaBb, aaBb, AAbb, Aabb, aabb – over entire human genome, > 10600 possible combinations of alleles – offspring genetically distinct from either parent and (usually) from each other Many species can reproduce without sex obligately facultatively obligately asexual: sexual: asexual: budding asexual vegetative unfertilized, propagation or sexual diploid eggs reproduction develop into clones of mother  Sexually reproducing species may be isogamous (equal sized gametes) or anisogamous (gametes either small & motile or large & immobile) Some species can self-fertilize (fuse gametes from the same individual to make a zygote), others cannot  Isogamous species may have no distinct ‘mating types’ (any individual can fertilize any other individual, including themselves), or two mating types, or many different mating types Schizophylla commune  Anisogamous species usually have two sexes; individuals can be dioecious (separate sexes) or monoecious (hermaphrodites) Monoecy (hermaphrodism) can be simultaneous or sequential Bluestreak wrasse Clownfish  Size-advantage model of sex reproductive success change size  Size-advantage model of sex reproductive success change size Sex change in Gingko biloba trees   Origins and history of sex 3.5 BYA: the first cells reproduced asexually 2 BYA: bacteria start exchanging genes (parasex) 1.2 BYA: eukaryotic sex appears (specialized gametes, formed by meiosis, fuse to create diploid zygote) Now: most multicellular organisms, especially animals, reproduce sexually at least some of the time. Obligately asexual animals are rare and extinction-prone But bdelloid rotifers have been obligately asexual for >40 mya and still haven’t gone extinct! “An evolutionary scandal”  Sometimes sex seems like a bad idea… Cost of mating: finding a mate is time-consuming and often risky Cost of males: in most sexual species, only females can “grow” the population Cost of meiosis: only pass half your alleles to offspring  Sex is complicated “You only have to think of sex to see how absurd it is. The ‘sexual’ method of reading a book would be to buy two copies, rip the pages out, and make a new copy by combining half the pages from one and half from the other, tossing a coin at each page to decide which original to take the page from and which to throw away. To watch a play, you would go twice, pre- programmed to pay attention to the first performance at one random set of times, amounting to half the total length, and to the second performance at the complementary set of times.” – Ridley, The Ultimate Existential Absurdity One explanation for the persistence of sex: long-term mutational advantage over the long term, sexual recombination benefits the population by getting rid of harmful mutations Without sex, deleterious mutations accumulate in the population (“Muller’s ratchet”) Another explanation for the persistence of sex: immediate benefits in certain environments When is it advantageous to produce a diverse variety of offspring genotypes? – Lottery principle: more sex in unpredictable environments, less sex in stable environments – Evolutionary arms races: keeping up with other species (host-parasite interactions)  Red Queen principle: running to stay in the same place Reproducing sexually allows producing new and rare combinations of alleles Parasites are unlikely to be able to infect these new (rare) host genotypes Sexual reproduction is favoured in environments with lots of parasites  Take-homes: why sex? Many ways to be asexual Many ways to be sexual Sex is costly, risky, and inefficient, but still very widespread – for most multicellular species, the benefits of sex (offspring diversity) outweigh the costs