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The major histocompatibility complex in sexual selection concerns how major histocompatibility complex (MHC) molecules allow for immune system surveillance of the population of protein molecules in a host cells. In 1976, Yamazaki et al demonstrated a sexual selection mate choice by male mice for females of a different MHC. Major histocompatibility complex genes, which control the immune response and effective resistance against pathogens, have been able to maintain an extremely high level of allelic diversity throughout time and throughout different populations. In addition to its role in immune function, studies suggest that the MHC is also involved in mate choice for many vertebrates through olfactory cues. There are several proposed hypotheses that address how MHC-associated mating preferences could be adaptive and how the MHC has maintained its enormous allelic diversity.〔 The vast source of genetic variation affecting an organism’s fitness stems from the co-evolutionary arms race between hosts and parasites. There are two nonmutually exclusive hypotheses for explaining this. One is that there is selection for the maintenance of a highly diverse set of MHC genes if MHC heterozygotes are more resistant to parasites than homozygotes - this is called ''heterozygote advantage''. The second is that there is selection that undergoes a frequency-dependent cycle - and is called the ''Red Queen hypothesis''. ==Hypotheses== In the first hypothesis, if individuals heterozygous at the MHC are more resistant to parasites than those that are homozygous, then it is beneficial for females to choose mates with MHC genes different from their own, and would result in MHC-heterozygous offspring - this is known as disassortative mating. Individuals with a heterozygous MHC would be capable of recognizing a wider range of pathogens and therefore of inciting a specific immune response against a greater number of pathogens—thus having an immunity advantage. Unfortunately, the MHC-heterozygote advantage hypothesis has not been adequately tested.〔 The second hypothesis for the maintenance of MHC diversity by parasites is the Red Queen hypothesis. If individuals’ MHC alleles render different resistances to a particular parasite, then the allele with the highest resistance is favored, selected for, and consequently spread throughout the population. Recombination and mutation cause generation of new variants among offspring, which may facilitate a quick response to rapidly evolving parasites or pathogens with much shorter generation times. However, if this particular allele becomes common, selection pressure on parasites to avoid recognition by this common allele increases. An advantageous characteristic that allows a parasite to escape recognition spreads, and causes selection against what was formerly a resistant allele. This enables the parasite to escape this cycle of frequency-dependent selection, and such a cycle eventually leads to a co-evolutionary arms race that may support the maintenance of MHC diversity.〔 The inbreeding avoidance hypothesis has less to do with host-parasite relationships than does the heterozygote advantage hypothesis or the Red Queen hypothesis. The extreme diversity in the MHC would cause individuals sharing MHC alleles to be more likely to be related. As a result, one function of MHC-disassortative mating would be to avoid mating with family members and any harmful genetic consequences that could occur as a result. Mating with relatives, or inbreeding, increases the amount of overall homozygosity—not just locally in the MHC. An increase in genetic homozygosity may be accompanied not only by the expression of recessive diseases and mutations, but by the loss of any potential heterozygote advantage as well.〔 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. Furthermore, one would presume that said difference in genes would impart a difference in fitness as well, which could potentially be chosen or selected for. 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. If this is the case, either through the heterozygote advantage nypothesis or the Red Queen hypothesis, then selection also favors mating practices that are MHC-dependent. Therefore, mate choice—with respect to the MHC—has probably evolved so that females choose males either based on diverse genes (heterozygote advantage and inbreeding avoidance hypotheses) or “good” genes. The fact that females choose is naturally selected, as it would be an advantageous trait for females to be able to choose a male that provided either an indirect or direct benefit. As a result of female choice, sexual selection is imposed on males. This is evidenced by genetic “advertisement” - an example of this would be the existence of exaggerated traits, such as the elaborate tail-feathers of male peacocks. However, in humans both sexes exert mate choice. Accordingly, the maintenance of allelic diversity in the MHC would not be due to sexual selection. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Major histocompatibility complex and sexual selection」の詳細全文を読む スポンサード リンク
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