Why Sex and Recombination

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Why Sex and Recombination?
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Two features that distinguish sex from asex
meiosis and syngamy
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Sex typically associated w/ anisogamy
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SexRecombination
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Recombination
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Recombination
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Species with separate sexes in separate
individuals are dioecious. Most animals, some
angiosperms (gingko tree). Plants with separate
male and female flowers on same individual are
monoecious. Most conifers, some
angiosperms. Plants with both sexual functions
in the same flower are called perfect. Most
angiosperms. Animals in which the same
individual can produce both eggs and sperm are
either simultaneous hermaphrodites or sequential
hermaphrodites.
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Hermaphrodites
Simultaneous many snails, annelid
worms Sequential echinoderms, some fish (e.g.,
gobies, wrasses, parrot fish)
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Asexual Reproduction
Common in plants, less common in animals. Found
in all groups of vertebrates, except
mammals Many organisms capable of both sexual
and asexual reproduction Many plants, algae,
yeasts Aphids, Daphnia, nematodes, hydra
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Sexual reproduction is costly
Gonads are expensive organs to produce and
maintain. Mating is risky and costly, often
involving elaborate structures and behaviors,
risk of injury and disease. Recombination breaks
up beneficial combinations of alleles at
different loci. So why does sexual reproduction
exist at all?
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Asexual reproduction is evolutionarily
advantageous

• Assuming that
• reproductive mode does not affect how many
  offspring a female can produce
• reproductive mode does not affect the survival or
  reproductive success of the offspring,
• then

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So, sexual reproduction is evolutionarily
advantageous.
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Therefore

• Are the assumptions valid?
• Does reproductive mode affect how many offspring
  a female can produce?
• Does reproductive mode affect the survival or
  reproductive success of the offspring?

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(Dunbrack et al. 1995)
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Otto and Lenormand 2002
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Reproductive mode may affect survival and
reproduction of offspring

• Deleterious mutation and drift --gt Mullers
  ratchet
• Deleterious mutation alone --gt Kondrashovs
  hatchet
• Faster adaptive evolution

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Mullers Ratchet Model
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Theoretical and experimental tests

• Asexual bacteria do accumulate deleterious
  mutations over many generations.
• Ratchet turns quickly in populations with Ne lt
  10.
• Ratchet turns very slowly in populations with Ne
  gt 1000.
• Advantage of recombination is a long-term, but
  not a short-term advantage. So why so few asexual
  spp? How to account for Dunbrack et al.
  experiment?

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Kondrashovs Hatchet Model
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Theoretical and experimental tests

• Attractive because of universality.
• Evidence for high mutation rates and synergistic
  interactions exists, but is not universal.
• Slow--requires thousands of generations.

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Faster adaptive evolution
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Red Queen an evolutionary arms race
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Livelys Red Queen Study

• Freshwater snail, Potamopyrgus antipodarum
• Both sexual and asexual females in populations
• Trematode worm parasites (several spp.)
• Parasites render snails sterile (destroy gonads)

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Recombination breaks up favorable allele
combinations