Major histocompatibility complex and sexual selection
The vast source of genetic variation affecting an organism's fitness stems from the co-evolutionary arms race between hosts and parasites.
There are several proposed hypotheses that address how MHC-associated mating preferences could be adaptive and how an unusually large amount of allelic diversity has been maintained in the MHC.
[9] This finding has been replicated in another study in mice and again in fish[10][11] In some cases, excess heterozygosity can lead to decreased fitness.
[12] The optimality hypothesis states too much variability in the MHC can result in a failure of T-cells to distinguish themselves non-selves, and thereby increase the risk of autoimmune disease.
[19] In the course of searching for potential mates, it would benefit females to be able to discriminate against "bad" genes in order to increase the health and viability of their offspring.
If female mate choice occurs for "good" genes, then it is implied that genetic variation exists among males.
Generally, the extreme polymorphism of MHC genes is selected for by host-parasite arms races (the Red Queen hypothesis); however, disassortative mate choice may maintain genetic diversity in some species.
Depending on how parasites alter selection on MHC alleles, MHC-dependent mate-choice may increase the fitness of the offspring by enhancing its immunity, as mentioned earlier.
MHC-based sexual selection is known to involve olfactory mechanisms in such vertebrate taxa as fish, mice, humans, primates, birds, and reptiles.
[1] At its simplest level, humans have long been acquainted with the sense of olfaction for its use in determining the pleasantness or the unpleasantness of one's resources, food, etc.
At a deeper level, it has been predicted that olfaction serves to personally identify individuals based upon the genes of the MHC.
Humans can not only detect, but also assess, and respond to environmental (chemical) olfactory cues—especially those used to evoke behavioral and sexual responses from other individuals, also known as pheromones.
Each MHC protein binds to a specific peptide sequence, yielding a set of uniquely bound peptide-MHC complexes for each individual.
During cellular turnover, the MHC-peptide complex is shed from the cell surface and the fragments are dispensed in bodily fluids such as blood serum, saliva, and urine.
Scientists believe that commensal microflora, microorganisms that line epithelial surfaces open to the external environment such as the gastrointestinal tract and vagina, further degrade these fragments, which are made volatile by this process.
[1][20][21] MHC similarity in humans has been studied in three broad ways: odor, facial attractiveness, and actual mate choice.
Upon smelling the shirts, it was found that men and women who were reminded of their own mate or ex-mate had dramatically fewer MHC alleles in common with the wearer than would be expected by chance.
Furthermore, when the degree of similarity between the wearer and the smeller was statistically accounted for, there was no longer a significant influence of MHC on odor preference.
[27] In another fish, the three-spined stickleback, it has been shown that females desire MHC diversity in their offspring, which affects their mate choice.
Similarly, MHC diversity in house sparrows, Passer domesticus, suggests that MHC-disassortative mate choice occurs.
Females preferred to associate with odor samples obtained from males more distantly related at the MHC I loci.
In the scarlet rosefinch Carpocus erythrinus, females engaged in extra-pair paternity much less frequently when their mates were MHC-heterozygous.
The Atlantic salmon, Salmo salar, is one species in which sperm competition is influenced by the variation in the major histocompatibility complex, specifically that of the Class I alleles.
Atlantic salmon males have higher rates of successful fertilization when competing for eggs from females genetically similar at the class I genes of the MHC.
It is proposed that there is a chemo-attraction system responsible for the egg itself being able to discriminate and selectively choose between MHC-heterozygous and MHC-homozygous males.
