Origins and maintenance of sex

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Origins and maintenance of sex
Dr. Sally Otto, UBC
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Asexuality vs. self-fertilization

• Self-fertilization
• Asexuality

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Costs of sex
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Advantages of asexuality which sex limits
population growth?
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Advantages of asexuality fitness
SEX
ASEX
Frequency of individuals
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So, is sex good?
99.9 of species cant be wrong
Cnemidophorus
Daphnia
Dandelions
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(but a few asexual lineages persist)
Bdelloid rotifers freshwater filter feeders
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Hypotheses to explain the maintenance of sex
Harmful mutations Mllers ratchet Speed of
adaptation and the Red Queen
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Disadvantages of asexuality Müllers ratchet
Mutations happen
mutation-free chromosome
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Müllers ratchet
LLC least-loaded class
CLICK
frequency
frequency
of deleterious mutations
of deleterious mutations
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Müllers ratchet in sexuals? No.
CLICK
frequency
frequency
of deleterious mutations
of deleterious mutations
frequency
of deleterious mutations
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Müllers ratchet an experiment

• Set up 444 cultures of Salmonella
• Transfer one individual every 24 hours
• 1700 generations
• Prediction lower fitness
• Test generation time
• parental
• Experimental populations

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Evidence for Müllers ratchet the human Y
chromosome

• In XX females, recombination
• Y chromosome does not pair with X

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Müllers ratchet overall
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Reasons for sex adaptation
A1
B1
parental
B2
A1
A1
B1
recombinant
meiosis
A2
B1
A2
B2
recombinant
A2
B2
parental
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Does recombination increase genetic variance for
fitness?
D 0.25
D -0.25
D 0
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If parents have higher than average fitness, what
effect of recombination?
The results of sex cost of recombination
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Experimental test of recombination and adaptation

• Adaptation may require new combinations of
  alleles
• Asexuality does not allow this
• Experiment flour beetles (Tribolium)
• Have stock population
• Allow one population to evolve
• The other is restocked from original population,
  as if asexual.
• Asexual has 3x reproductive advantage
• Asexuals start 0.5 of population
• Selection pesticide Malathion
• What proportion are sexual?

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Advantage of sex adaptation
Malathion concentration
Proportion sexual
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Generations
figure 7.18
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Do organisms need to adapt? Red queen hypothesis
Red Queen to Alice Now, here, you see, it takes
all the running you can do, to keep in the same
place. --Lewis Carrol, Through the Looking
Glass (1872) Concept constant adaptation
needed. Why?
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Parasites and hosts

• Imagine four parasite genotypes, four host
  defense genotypes

Parasite
Host
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Parasites and host Red queen
Parasite
Host
start
after selection on host
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Parasites and host Red queen
Parasite
Host
after selection on host
after selection on parasite
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Evolution over time cycling genotypes
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Red queen, evidence topminnows of Mexico

• Interspecific hybrids from asexual triploids
• or sexual diploid fish
• Infected by parasites that cause spots

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Data I sexuals vs. asexuals which has more
parasites?
Asexuals have more parasites
Parasites
Fish length (mm)
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What if there is no variation in sexual
population?

• Heart pool dried up in 1976
• Recolonized by just a few sexual minnows and a
  few asexual minnows.
• Which will have the higher fitness?

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Why sex, summary
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Readings and questions

• Reading Freeman and Herron chapter 8. (chapter
  7, 3rd edn)
• 1. In the beetle evolution experiment (figure
  8.18) Dunbrack et al did not actually asexual
  beetles, as there aren't any. Instead they used
  two different lines of beetles that differed in
  color, treating one line as if it were asexual by
  replacing individuals with individuals from a
  stock population. The researcher's simulated
  asexual population was not allowed to evolve at
  all in response to competition and the presence
  of the insecticide. Is this realistic?
• 2. In general, would you expect asexual lineages
  to persist longer with small population sizes or
  large population sizes? Why?
• 3. If the offspring of sexual and asexual
  individuals have equivalent fitness, why would
  asexuals take over a population?
• 4. Explain how Muller's ratchet affects sexual
  populations differently from asexual populations.
  
• 5. Explain how the Red Queen hypothesis relates
  to the maintenance of sex. Why might sex be
  advantageous in the face of parasites or disease?