Following up on my post from a few days ago about the unexpected results looking at MHC polymorphism and mating system- It seems to me that I should make explicit the alternative hypotheses…
So basically there are 4, not-strictly-mutually-exclusive ones that I will test.
- MHC polymorphism is related to sexual behavior, i.e. mating system. Basically saying that relative to monogamous species, non-monogamous species are at greater risk for contracting infectious disease secondary to frequency and diversity of sexual contacts. It is this risk that nudges up “optimal’” MHC polymorphism and therefore likely enhances polymorphism.
- MHC polymorphism is related to social behavior. Here we recognize that animals that are frequently in close contact with conspecifics are are at risk for infectious disease as a direct result of tis prolonged social contact. If social behavior is important in generating selection for MHC polymorphism, then more social species are the guys that have greater polymorphism.
- The environment dictates polymorphism. This is the “old hat” hypothesis. I explicitly test it though using a common garden type of experiment.
- That all these adaptive hypotheses are junk- and that teh factor most related to current polymorphism is evolutionary history- i.e. phylogeny.
So let me back up another bit and tell you that there are differences in general patterns of social behavior in the animals included in that preliminary analysis. Peromyscus californicus (monogamous) spends a significant portion of its life in close contact with it’s mate, and their offspring.
P. maniculatus however, is relatively asocial and usually only contacts other animals within the context of mating.
So given this, and focusing in on the sociality hypothesis, which species will you predict to have the greater polymorphism??
I hope to be able to provide mechanistic support for this finding via collection of ectoparasites from the various species. Greater numbers of ectoparasites on social species might be considered support for the sociality hypothesis. As of right now- I don’t have enough info to say…
So maybe it is that although we all think of sex as being a significant contributor to infectious disease risk- that prolonged non-sexual contact is much more risky…..
2 responses so far ↓
1 Orion // Oct 27, 2007 at 12:27 pm
From a population genetics point of view, I’d expect more polymorphism in a monogamous species for the following reasons:
- Under monogamy less-random mating (more full-siblings than half-siblings) slows the allele fixation/extinction process. More mixing per generation in non-monogamous species drives novel alleles to fixation or extinction faster.
- Under monogamy mate choice is a more important decision, hence the push for (substructure-driving) assortative mating is stronger. One confound for this is that at least in humans, Wedekind’s T-shirt study seems to indicate mates with dissimilar MHC are rated more attractive (disassortative mating).
- Under monogamy mate choice is a longer-term hence riskier decision, so the “safe” choice is a more similar mate (to avoid genetic incompatibility leading to unviable offspring). Fewer resources are on the line with one promiscuous mating event, so more “exotic” (well-mixed) mate choices are more likely.
A related question is: how often does MHC incompatibility cause a disastrous autoimmune response in the womb or later in life? Because deaths from autoimmune disorders drive selection just like deaths from disease…
(Disclaimer: everything I think I know about genetics, I learned from lurking at Gene Expression).
2 Meirav Rath // Oct 31, 2007 at 10:48 pm
That’s an interesting question. From birds I know that the polymorph males are very bad fathers (ah, but what of fish?)
You’d think an a-social specie would want to wear out definitions of its sexes, so that when two individuals meet they’ll know exactly what the other wants and avoid misunderstanding and needless conflict - like why felines are so expressive - but felines themselves show little polymorphisms (compared to, say, birds) accept for the lions who ARE social so…
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