Maintenance of sex

Question 1

What is sex?

Answer 1

‘A system for recombining genomes’

Question 2

What are the 3 main processes that sex relies on?

Answer 2

1. Meiosis
2. Syngamy
3. Segregation (of homologous chromosomes) and recombination

Question 3

What is meiosis?

Answer 3

A two-step reduction division

Question 4

What is syngamy?

Answer 4

The fusion of two haploid gametes

Question 5

What happens during segregation and recombination?

Answer 5

The reshuffling of genes within the diploid zygote

Question 6

Does sex change allele frequencies?

Answer 6

No

Question 7

Does recombination break down linkage disequilibrium between alleles?

Answer 7

Yes

Question 8

What can sex do to genetic variation?

Answer 8

Increase or decrease it

Question 9

Is sex an ancient process in eukaryotes?

Answer 9

Yes

Question 10

Do asexual lineages persist in evolutionary time?

Answer 10

Apparently not; they are relatively young and do not persist

Question 11

Why don’t asexual lineages persist?

Answer 11

They cannot recombine and so irreversibly accumulate deleterious mutations, cannot escape Muller’s Ratchet.

Question 12

What is the mutational deterministic hypothesis and who came up with it?

Answer 12

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.

Question 13

What is clonal interference?

Answer 13

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.

Question 14

Sex helps to avoid clonal interference. True or false?

Answer 14

True

Question 15

Sex makes natural selection more efficient at selecting beneficial mutations. How?

Answer 15

(Presumably) because there is more variation due to recombination and so more disparity in fitness between genotypes.

Question 16

What is the cost of sex?

Answer 16

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’

Question 17

How can sex reduce genetic variation?

Answer 17

If extreme genotypes are common, sex can generate intermediates in recombination.

Question 18

How can sex increase genetic variation?

Answer 18

If extreme genotypes are rare, then recombination can generate extremes.

Question 19

When does recombination spread?

Answer 19

When selection favours extremes OR when extremes are rare (as recombination responsible for creating extremes)

Question 20

What is a modifier?

Answer 20

A factor that increases recombination rate

Question 21

When are modifiers favoured? Give 3 examples.

Answer 21

1. When selection is continuously directional, making extremes common
2. When selection makes extremes rare
3. There is negative epistasis

Question 22

What is negative epistasis and why does that favour the evolution of modifiers?

Answer 22

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.

Question 23

Under which dynamics are extremes favoured?

Answer 23

Red Queen

Question 24

Give a referenced example showing how sex is beneficial under RQD.

Answer 24

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

Question 25

Under the Muller’s Ratchet hypothesis, sex only provides an advantage for sex in small populations. Why?

Answer 25

As mutations accumulate, there is a chance loss (drift) of the least-fit individuals from the population. This is more of a problem in small populations.

Question 26

What is synergistic epistasis?

Answer 26

The relationship between the number of mutations and known fitness effects (i.e. they all contribute to phenotype).

Question 27

In what kind of population is the mutational deterministic hypothesis for sex applicable? What does sex allow?

Answer 27

A large population, as mutations segregate further apart (as there are more people to mate with). Sex brings them back together. If these mutations are beneficial they become fixed, allow for adaptive evolution.

Question 28

What does the mutational deterministic hypothesis rely on?

Answer 28

Synergistic epistasis, e.g. the phenotype is more extreme when multiple mutations act together (allows for proper shit and well proper good combinations that selection can act on)