Explaining the evolution of sex and recombination/media/people/new-fellows-2014/... ·...
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Explaining the evolution of sex and recombination Peter Keightley Institute of Evolutionary Biology University of Edinburgh
Transcript of Explaining the evolution of sex and recombination/media/people/new-fellows-2014/... ·...
Explaining the evolution ofsex and recombination
Peter Keightley
Institute of Evolutionary BiologyUniversity of Edinburgh
Sexual reproduction is ubiquitous in eukaryotes
Meiosis with recombination
Syngamy
Sex and recombination have potential disadvantages
● Meiotic recombination breaks apart combinations of genes that have been built up by selection.
Sex and recombination have potential disadvantages
● Meiotic recombination breaks apart combinations of genes that have been built up by selection.
● Sexual reproduction requiresfinding a mate, which carries risks.
Sex and recombination have potential disadvantages
● Meiotic recombination breaks apart combinations of genes that have been built up by selection.
● Sexual reproduction requiresfinding a mate, which carries risks.
● Sex can carry a two-fold costif population growth is limited bythe number of females.
● Why are sex andrecombination almostubiquitous in nature?
● Why are sex andrecombination almostubiquitous in nature?
● Why don’t asexualstake over sexualpopulations?
Is there a general explanation for the evolution of recombination?
Collaborator: Sally OttoUniversity of British Columbia
Association between beneficial (+) and deleterious alleles (-)
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Four chromosomes in a population
Association between beneficial (+) and deleterious alleles (-)
● Alleles at the loci are negatively associated.● The coupling types ++ and -- are absent.
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Association between beneficial (+) and deleterious alleles (-)
● Alleles at the loci are negatively associated.● The coupling types ++ and -- are absent.
● The effectiveness of selection acting on variation at the loci is weakened.
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m
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Mutations that increase recombination are favoured if there are negative allelic associations
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M
m
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Mutations that increase recombination are favoured if there are negative allelic associations
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M
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m+ + M
● A mutation (M) at a linked locus that increases recombination is favoured because it hitchhikes on chromosomes carrying multiple beneficial alleles, generated by recombination.
Mutations that increase recombination are favoured if there are negative allelic associations
Felsenstein & Yokoyama 1976
Non-random associations between linked loci arise in two ways
1. Directional selection.
size is very large.+ +
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Rapidly fixed
Rapidly eliminated
Non-random associations between linked loci arise in two ways
1. Directional selection.
size is very large.- +
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Non-random associations between linked loci arise in two ways
1. Directional selection.
size is very large.
Pop. size
Disequilibrium
BUT non-random associations are expected to be absent in large populations if there is some recombination.
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Non-random associations between linked loci arise in two ways
2. Synergistic epistatic interactions between loci.
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Unfit combination that is rapidly eliminated
Non-random associations between linked loci arise in two ways
2. Synergistic epistatic interactions between loci.
BUT there is little empirical evidence for this kind of epistasis.
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Is recombination favoured in the presence of selection at multiple linked loci?
Simulating the fate of a recombination modifier in the presence of selection and mutation at multiple linked loci
Population of N haploid individuals at equilibrium between linked deleterious mutations and selection.
Introduce a mutation that increases recombination.Frequency = 1/N
Simulating the fate of a recombination modifier in the presence of selection and mutation at multiple linked loci
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M
Possible fate of a recombination modifier mutation
Modifier lost
M
MM MMM
Possible fate of a recombination modifier mutation
MMM
M
M
MM
MMM
M
MMMMM
MMM
Modifier lost
Modifier fixed
● Happens at a frequency of 1/N by chance
Modifiers mutations that increase recombination were always observed to be
favoured
Recombination is strongly favoured in nonrecombining systems
0 0.25 0.5 0.75 11
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Initial recombination rate (map length, L)
Relative fixation rate of modifier
Recombination modifier increasing map length by 0.1M
The advantage of recombination increases with population size
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L = 0L = 0.1L = 1
Population size, N
Relative fixation rate of modifier
Map length
The advantage of recombination increases with population size
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L = 0L = 0.1L = 1
Population size, N
● This property “emerges” because of the multi-locus nature of the model.
● In systems with small numbers of loci, recombination is favoured only in small populations.
Relative fixation rate of modifier
Map length
Why does the advantage of a recombination modifier increase with population size?
● Selection generates multi-locus associations.
Why does the advantage of a recombination modifier increase with population size?
● Selection generates multi-locus associations.
● Large populations maintain more polymorphic loci, and more negative associations, increasing the hitchhiking effect favouring recombination modifier mutations.
Why does the advantage of a recombination modifier increase with population size?
● Selection generates multi-locus associations.
● Large populations maintain more polymorphic loci, and more negative associations, increasing the hitchhiking effect favouring recombination modifier mutations.
● The infinite population prediction that negative associations should be absent does not apply when there are multiple loci under selection.
Summary
● Simple simulations show that interference between selected mutations favours mutations that increase recombination.
Summary
● Simple simulations show that interference between selected mutations favours mutations that increase recombination.
● Theory developed for two- or three-locus systems proved to be misleading.
Summary
● Simple simulations show that interference between selected mutations favours mutations that increase recombination.
● Theory developed for two- or three-locus systems proved to be misleading.
● The mechanism described is likely to be the general explanation for the evolution of recombination.
Mutations affecting fitness are universal and natural populations tend to be very large.
