Maintenance of sex Flashcards
What is sex?
‘A system for recombining genomes’
What are the 3 main processes that sex relies on?
- Meiosis
- Syngamy
- Segregation (of homologous chromosomes) and recombination
What is meiosis?
A two-step reduction division
What is syngamy?
The fusion of two haploid gametes
What happens during segregation and recombination?
The reshuffling of genes within the diploid zygote
Does sex change allele frequencies?
No
Does recombination break down linkage disequilibrium between alleles?
Yes
What can sex do to genetic variation?
Increase or decrease it
Is sex an ancient process in eukaryotes?
Yes
Do asexual lineages persist in evolutionary time?
Apparently not; they are relatively young and do not persist
Why don’t asexual lineages persist?
They cannot recombine and so irreversibly accumulate deleterious mutations, cannot escape Muller’s Ratchet.
What is the mutational deterministic hypothesis and who came up with it?
Alexey Kondrashov
Assumes that the majority of deleterious mutations are only slightly deleterious and allowed to accumulate. The additive effects of these mutations have an increasingly large effect on phenotype. Therefore populations are generally composed of individuals with a small number of mutations. Sex recombines genotypes, causing some individuals with less or more mutations. Individuals with more mutations die out due to fitness effects.
BASICALLY sex brings together crappy alleles so they can be removed, and brings together good alleles so they can be fixed.
What is clonal interference?
Occurs in asexual population with linkage disequilibrium. When two or more beneficial mutations arise independently in different individuals, these cannot come together in sex. Therefore clones compete against each other until one is outcompeted.
Sex helps to avoid clonal interference. True or false?
True
Sex makes natural selection more efficient at selecting beneficial mutations. How?
(Presumably) because there is more variation due to recombination and so more disparity in fitness between genotypes.
What is the cost of sex?
In an asexual lineage, every individual can give rise to new individuals by clonal division.
In a sexual lineage, only females are physically able to produce offspring whilst males only contribute sperm, are a ‘waste’
How can sex reduce genetic variation?
If extreme genotypes are common, sex can generate intermediates in recombination.
How can sex increase genetic variation?
If extreme genotypes are rare, then recombination can generate extremes.
When does recombination spread?
When selection favours extremes OR when extremes are rare (as recombination responsible for creating extremes)
What is a modifier?
A factor that increases recombination rate
When are modifiers favoured? Give 3 examples.
- When selection is continuously directional, making extremes common
- When selection makes extremes rare
- There is negative epistasis
What is negative epistasis and why does that favour the evolution of modifiers?
When the combined effects of multiple deleterious mutations produce an even worse phenotype than they would if they are expressed alone.
Recombination is needed to remove this by breaking up alleles that interact in negative epistasis.
Under which dynamics are extremes favoured?
Red Queen
Give a referenced example showing how sex is beneficial under RQD.
Morran et al., 2011
Host = C. elegans Parasite = S. marcescens
Mating system genetically manipulated so that worms existed in either sexual, self-fertilising or mixed populations.
Addition of parasite = self-fertilising worms driven to extinction whilst sexual populations displayed reciprocal coevolution.
Sex = creation of new genotypes that were fitter than the parents in response to parasite invasion
