MHC

Question 1

what does conventional sexual selection assume?

Answer 1

the ‘best’ mate is the same for everyone. this gives additive benefits of mating with someone.

Question 2

why may there be non additive benefits of mate choice?

Answer 2

if the value of a mate depends on your own genotype. eg MHC

Question 3

what is MHC

Answer 3

Major histocompatibility complex.
set of vertebrate genes crucial to immunity. highly polymorphic, >100 alleles per locus, all individuals unique.
it is ancient, all mammals have diff combinations of the same alleles.
influences the bacterial flora on skin so influences odour.

Question 4

what roles does MHC have in the immune system?

Answer 4

1. development of self /non self recognition.
2. activates many components of the immune system. eg cytotoxic T lymphocytes, helper t cells, macrophages, antibody secreting B cells.

Question 5

how does MHC affect mate choice in mice?

Answer 5

demonstrated by Yamazaki et al - can discriminate individuals on one MHC gene difference. prefer MHC dissimilar mates, based on odour.
cross fostering experiments show familial imprinting - preference for MHC dissimilar mates to siblings in litter.

Question 6

how does MHC affect mate choice in humans?

Answer 6

• Wedekind et al 1995 - Women prefer odour of MHC dissimilar males but, if on pill, preference shifts to MHC similar
• Ober et al 1997- Hutterites’ marriages are more ‘MHC-different’ than expect by chance
  not all studies find this pattern

Question 7

what is the benefit to different MHC mates?

Answer 7

inbreeding avoidance

Two concern parasite-mediated sexual selection

Question 8

how does choosing MHC different mates avoid inbreeding?

Answer 8

Potts 1994 - study of mice in semi-natural enclosures
Heterozygosity at the MHC loci was NOT related to fitness, it was merely a marker for overall heterozygosity, which DID affect fitness.
-However, very small pop (also in hutterites study), and already quite inbred, with little parasites.

Question 9

how does MHC relate to plants?

Answer 9

Seen parallel with angiosperms - incompatibility systems evolved from immune defence type mechanism. high polymorphism is essential.

Question 10

how is heterozygosity an advantage?

Answer 10

At the MHC locus: if allele A was better for parasite X, and allele B better for parasite Y, then better to be AB.
not all studies agree.

Question 11

which studies don’t agree that there is a heterozygote advantage?

Answer 11

1. Potts 1994 - in mice, fitness and MHC heterozygosity not related
2. Paterson et al 1998 - Soay sheep, on St Kilda, one particular MHC genotype is better for nematode resistance, rather than heterozygosity.
3. Hill et al 1991 - large scale study of malaria resistance in humans. MHC genes (not sickle cell.)

Question 12

Why may having rare alleles be advantageous?

Answer 12

Red Queen hypothesis
Hamilton-Zuk hypothesis
Better to have rare genotype because current parasites adapted to attack currently common genotypes.
Better to have offspring with different MHC to you if you risk passing on diseases to offspring.
Wedekind et l 1996 - accidental hepatitis putbreak in lab mouse colony. females produced more heterozygous offspring even thought they had the same mates. perhaps because of spontaneous abortion, under disease risk hetero zygotes more likely to live.
alternatiely - either sperm selection by the egg, or a biased 2nd meiotic division (this occurs after fertilisation, so sperm genotype recognised)

Question 13

other than sex, why would you want to recognise relatedness through MHC?

Answer 13

kin recognition, nothing to do with inbreeding avoidance.

mice prefer to nest with close kin, using odour as a cue. Atlantic char grow faster when with kin.