3. Evolution of Sex 2010

3. Evolution of Sex 2010 - Evolution of Sex Evolution No...

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Unformatted text preview: Evolution of Sex Evolution No area of evolutionary biology offers … a more fascinating offers mixture of strange phenomena … than the evolution of sex… (Stearns, 1987) Sex Sex • Refers to union (SYNGAMY) of two genomes • Individuals need not have distinct sexes – usually carried by gametes – followed later by reduction (meiosis) – defined as differences in size or structure of gamete Sexual Dimorphism Sexual • sexual dimorphism occurred late in evolution of life – major onset with the development of eukaryotes – feature of most vertebrates Isogamous Organisms Organisms • Isogamous - gametes are not distinct Isogamous gametes in size or shape – Many fungi (yeasts) – Algae • Same size gametes unite if different mating type ( + or -) Anisogamous Organisms Organisms • Anisogamous - large (egg) and small large (sperm) gametes – GONOCHORISTIC - individual can only be male or female – HERMAPHRODITIC - individual can be HERMAPHRODITIC individual male and female • sequential • simultaneous Gonochoristic • Sexual phenotype is determined early in the life history of these animals • Once established it persists throughout adult life • Most common pattern of sex allocation Sequential hermaphroditism • producing both male or female gametes sequentially – not at same time • Undergo sex reversal – -Protoandry: male first – -Protogyny: female first • East Asian swamp eel – Presently invading Florida wetlands (protogyny) – Also observed in hagfish, reef fishes Simultaneously hermaphroditic Rivulus marmoratus • Simultaneously male and female • Possess an ovotestis containing oogenic and spermatogenic tissue • Most are not self fertilizing • -2x the chance of meeting a potential mate – – Not common important in low density populations Acquired 'hermaphroditism' • Deep sea ceratioid angler fish • Male fish fuses body with female fish • - shares blood supply and all his organs degenerate (except male reproductive organs) Two parts to the evolution of sex • 1) the origin of sexual reproduction (cellular evolution) • 2) the evolution and maintenance of sexual reproduction and recombination • Recombination – like sex in that it re-assorts genetic material Advantages with Sex Advantages • Recombination increases rate of adaptive evolution – Does sex increase the rate of evolution? – Does this explain the origin versus persistence of sex? Problems with Sex Problems • Even if rate of evolution increased two problems – 1) Recombination destroys adaptive combinations of genes – 2) cost of sex • Half of the offspring are males! • “cost of males” Cost of Anisogamy • Male contributes only the genome of the sperm • Female ovum provides – Nuclear genes – Mitochondrial genes – Cellular environment necessary for embryonic development Females invest more energy per viable gamete than males Cost of meiosis • Loss of genetic information • Gametes contain ½ of the parental genotype - Clonally reproducing form passes 100% of its genetic material on to offspring - Sexually reproducing form must produce 2 fit offspring to pass on the same amount of genetic material to the next generation Cost of genetic recombination • Genetic recombination dismantles two successful genotypes and recombines them into a novel combination in the next generation • Adaptive allele combinations may be lost • Recombined genome may not be as successful – May contain deleterious recessive alleles Cost of mating • Energetic costs of mating are associated with sexual dimorphisms and mating behaviors • Increased risk of predation? Immortality versus Mortality Immortality • individual protozoans are potentially individual are immortal • multicellularity has a heavy price multicellularity has individuals become mortal Sex Must Be An Sex Advantage • Sexual reproduction persists in many, many populations • Must be great enough to offset 2X disadvantage Sexual Reproduction • 1: can bring together beneficial mutations – aids in the spread of advantageous traits • 2: can bring together deleterious mutations – severely unfit individuals eliminated – removal of deleterious genes • 3: creates new gene combinations – maybe more fit than existing ones Immediate Benefit Hypothesis Immediate • Immediate benefit hypothesis • (Bernstein and Bernstein, 1991) – Molecular recombination facilitates DNA repair • Breaks/lesions in DNA molecule can be repaired by copying homologous chromosome – Formation of new gene combinations are a by-product of DNA repair • not reason for evolution of recombination/sex Problem? Problem? • DNA repair does not require meiosis or syngamy DNA syngamy (fertilization) • Permanent diploid species exist - thus can repair DNA without the above • Origin of recombination could have been a response driven by need for DNA repair – but what about meiosis & syngamy? but syngamy DNA Repair Why Sex? • The Red Queen Theory – "It takes all the running you can do, to keep in the same place." Red Queen - 'Alice in Wonderland' – Sexual reproduction persists because it enables species to rapidly evolve new genetic defenses against parasites • Guppy and snail species exhibit sexual reproduction when higher level of parasitism – (Dybdahl and Lively 1995; Howard and Lively 1994). • Parasite-host interactions Additional Hypotheses Additional • Many – – – Fixation of rare beneficial mutations Heterogeneous habitats Deleterious mutations removed • Muller’s ratchet Muller’s Ratchet Muller • Herman Muller (1964) – Nobel Prize for mutagenic effects of radiation • Back mutation from deleterious to wild-type alleles is extremely rare – In asexual population mutations accumulate over time – Can’t be removed – Zero mutation genomes become rare, then extinct Muller’s Ratchet Muller 40 40 40 40 40 40 30 30 30 30 30 20 30 20 20 20 20 20 10 10 10 10 100 10 0 0 00 0 40 40 40 40 40 40 30 30 30 30 30 30 20 20 20 20 20 20 10 10 10 10 100 10 0 0 00 0 40 40 40 40 40 40 30 30 30 30 30 30 20 20 20 20 20 20 10 10 10 10 100 10 0 0 00 0 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 0 11 22 33 44 55 66 77 88 99 10 0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 10 0 0 012345 012345 0 11 22 33 44 55 00 1 2 3 4 5 0 12345 6 7 8 9 10 6 7 8 9 10 6 77 88 99 10 6 7 8 9 10 7 8 9 10 10 6 6 • Frequency of asexual individuals with different numbers of mutations at three time periods. 01234567 01234567 0 11 22 33 44 55 66 77 00 1 2 3 4 5 6 7 0 1234567 8 9 10 8 9 10 8 99 10 8 9 10 9 10 10 8 8 Evolution of Sexes Evolution • Anisogamy evolved from isogamy isogamy – evolution of large versus small gametes • If: – Large size enhances survival of offspring – Selection for ‘transport’ of second gamete Selection • Movement difficult Sex vs. Gender Sex: either of the two major forms of individuals that occur in many species and that are distinguished respectively as female or male - based on type of gonad/gamete produced Gender: the behavioral, cultural, or psychological traits typically associated with one sex Merriam-Webster online Evolution of Gender Evolution • Anisogamy gives rise to different mating types Anisogamy gives – (+ / -) (female/male) • Recognition of opposite type has advantages – chemical (pheromones) – visual signals • secondary sex characters – coloration, horns, behavioral display ...
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