Abstracts (first author)
Sexual selection more than parasitism explains functional variation of the MHC across mammals
Diversity at the major histocompatibility complex (MHC), vitally important for vertebrate immune defense, varies widely across species. Parasites have been identified as a major evolutionary force driving MHC polymorphisms across species, but sexual selection and disassortative mating is another likely mechanism. MHC-based mating preferences have been observed for multiple species including humans, but the generality of mate choice as a driver of MHC polymorphism in the wild is debated. In reality, both parasite-mediated selection and sexual selection may act in concert in wild populations. To investigate potential contributions of parasitism and sexual selection in explaining among-species variation in MHC diversity, we used comparative methods to examine measures of MHC diversity across 115 mammal species, including carnivores, chiroptera, primates, rodents, and ungulates. Specifically, we tested whether parasite species richness and relative testes size (as an indicator of sexual selection) were correlated with two measures of MHC class II DRB diversity: allelic richness and nucleotide diversity. Controlling for mammal phylogeny, neutral mutation rate and confounding ecological variables (i.e. population size, body mass, and sampling effort), we found that parasite species richness was positively correlated with MHC nucleotide diversity for bats and ungulates, and negatively correlated for carnivores. In contrast, relative testes size was positively correlated with MHC nucleotide diversity for carnivores, rodents, and ungulates, and for all taxa combined. Mammal taxonomic group was the strongest predictor of MHC allelic richness, with ungulates having lower diversity in general. This study provides support for both parasite-mediated selection and sexual selection in shaping variation in functional MHC polymorphism across a broad suite of mammals, and importantly, suggests that sexual selection may be more ubiquitous than previously thought.