Recombination and Multigene Evolution Flashcards Preview

Conservation Genetics > Recombination and Multigene Evolution > Flashcards

Flashcards in Recombination and Multigene Evolution Deck (38)
Loading flashcards...
1
Q

How is genetic variation maintained in a population

A
Balancing selection (overdominanc and frequency dependent selection)
Mutation-drift balance
Migration-drift balance (and hybridisation, genetic introgressions)
2
Q

What dos MHC stand for

A

Major Histocompatibility Complex

3
Q

What is the MHC

A

The MHC is a gene dense region that is present in all jawed vertebrates which harbours immune gene that encode for proteins involved in antigen presentation

4
Q

What does the MHC play important biological functions in

A

resistance to infectious,

individual odours, mating preferences, kin recognition and reproductive success

5
Q

Where are the most polymorphic genes in the vertebrate genome

A

MHC

6
Q

What is the main characteristic of the MHC

A

strong linkage disequilibrium

7
Q

What is linkage disequilibrium

A

Linkage disequilibrium refers to the non-random association of alleles at two or more loci in a general population. When alleles are in linkage disequilibrium, haplotypes do not occur at the expected frequencies.

8
Q

What two types of balancing selection have been proposed to explain the MHC

A
Overdominance model (heterozygous superiority)
Negative frequency dependent selection model
9
Q

What are the 3 main question surrounding the MHC

A

How can selection maintain high levels of polymorphism in small populations?
How can balancing selection maintain MHC alleles over long evolutionary time (trans-species polymorphism)?
Why is the MHC is linked to so many diseases?

10
Q

How does the MHC present itself in small, bottlenecked populations

A

MHC has a high level of diversity which neutral genetic marker loci have very low diversity

11
Q

What is trans-species polymorphism?

A

is the occurrence of similar alleles in related

species

12
Q

What dos polymorphic mean?

A

discontinuous genetic variation

13
Q

What pairs share identical MHC alleles

A

Guppies and P. picta and humans and chimps

14
Q

Why is it strange that identical MHC sequences have been found in distantly-related species

A

because host-parasite coevolution and Red Queen arms race predict a fast turnover of alleles

15
Q

What pathlogies are associated with the MHC in humans

A

psoriasis, rheumatoid arthritis, insulin-dependent diabetes, autoimmune disorders

16
Q

What can you conclude from the observation that ‘MHC has a strong haplotype block structure’

A

Epistasis must be strong
Positive epistasis between immune genes that operate well together
Negative epistasis between bad mutations

17
Q

What can you conclude from the observation that ‘many disease mutations in the MHC

A

Faster mutation rate
More important genes that can cause disease
Less efficient selection against mutations

18
Q

What could cause the less efficient selection against mutations in the MHC

A

There must be benefits from those mutations (antagonistic pleiotropy)
Mutations do not get often expressed (are rare and recessive, which mean they are ‘sheltered’ by a healthy wild type allele)

19
Q

What is Muller’s ratchet and what does it cause

A

the accumulation of mutations in asexuals. Mutations can become fixed because without recombination, purifying selection is inefficient.

20
Q

What does genetic hitchhiking do to genetic variation

A

reduces genetic variation because the ‘fate’ of polymorphisms depends on selection on nearby genes.

21
Q

What is epitasis

A

The masking of the phenotypic effect of alleles at one gene by alleles of another gene e.g albinism

22
Q

What dos ABC stand for

A

Associated Balanced Complexes

23
Q

What are ABCs

A

Recessive deleterious mutations accumulate in linkage blocks in haplotypes

24
Q

In ABC theory, what type of selection is occuring on the MHC

A

Selection on the MHC is overdominance (on the immune genes) and purifying selection (on the linked mutations)

25
Q

What do the current models say about the MHC

A

Current models (overdominance, frequency dependence) suggest that balancing selection acts on a single gene only

26
Q

Key features of ABC evolution

A

Accumulation of recessive deleterious mutations in MHC haplotype blocks in a process similar to Muller’s ratchet
These mutations are potentially disease causing
They furthermore define the haplotype block structure by epistatic selection against recombinants
Selection against mutations is strong when haplotype block becomes common, which retains homogeneous allele frequency (and polymorphism) in the population
This selection can also work without selection on the actual immune gene

27
Q

What was Aguilar et al 2004 theory about MHC diversity

A

They argued the MHC diversity is maintained by balancing selection, with MHC loci having a selection coefficient greater than 0.5

28
Q

what did the original over dominance model state

A

overdominance model states that polymorphism is

maintained because heterozygous individuals are able to recognize a wider variety of parasites

29
Q

What was originally thought to maintain the high level of polymorphsim in the MHC

A

Selection by parasites and sexual

selection

30
Q

what does negative frequency-dependent

selection model assumes that

A

The negative frequency-dependent
selection model assumes that, because selection favours parasites that can avoid recognition by the most common MHC variants, rare MHC alleles offer better parasite
resistance

31
Q

When is the MHC polymorphism prone to erosion

A

when there is little or no parasite selection, MHC polymorphism is prone to erosion by random genetic drift, particularly in small
populations.

32
Q

What is the issue with traditional model of balancing selection on the MHC when assuming parasites are the only agent of selection

A

often require unrealistically large selection coefficients

(SR0.2) to explain the observed MHC polymorphism in small populations

33
Q

what does the fact that the MHC’s islands of high linkage disequilibria (LD) interspersed by recombination hot spots suggest is happening

A

epistasis and genetic hitch-hiking

34
Q

what does the traditional balancing selection models not consider

A

ignores the potential role of linkage and epistatic

gene–gene interactions.

35
Q

What did the ABC model propose

A

recessive disease-causing mutations can accumulate in the MHC in a process similar to Muller’s ratchet
Natural selection fails to purge these mutations because
this so-called ‘sheltered load’ only rarely becomes expressed, given the large number of MHC alleles and high gene diversity

36
Q

What reduces the effective rate of recombination in the MHC

A
functional epistasis   and
haplotype block (haploblock) structure
37
Q

How does Muller’s ratchet affect the MHC

A

Because there is reduced recombination, mutations are less effectively removed

38
Q

What reduces the selection effectiveness at removing mutation from the MHC

A

the reduced effective rate of recombination and the typically has a high gene diversity and large number of
alleles which makes it rare for bad alleles to be shown in the homozygous form