Wild cyclic voles maintain high neutral and MHC diversity without strong evidence for parasite-mediated selection

The major histocompatibility complex (MHC) is an important component of vertebrate immune defense involved with self/nonself recognition and disease susceptibility. The high variability of genes of the MHC is thought to arise from both parasite-mediated and sexual selection. An outstanding question involves the degree to which balancing selection can oppose genetic drift to maintain high MHC diversity in the face of population bottlenecks. To address this question we examined genetic diversity and population structure at neutral (microsatellite) and MHC genes in montane voles Microtus montanus (Peale, 1848)] subject to high amplitude population fluctuations, and compared these to measures of infection by common gastrointestinal parasites. We found high neutral and MHC allelic variability, indicating low impacts of genetic drift despite large fluctuations in population size. Greater MHC diversity did not predict lower parasite richness or infection by the two most common endoparasites (cestodes and coccidian protozoa), as might be expected if genotypic composition confers resistance to infection. One specific MHC allele predicted lower cestode intensity, but we found no other associations between MHC and infection measures. Neutral heterozygosity was positively associated with total parasite richness, possibly owing to greater parasite tolerance among heterozygous relative to more inbred hosts. Overall, these results suggest that factors beyond the parasites examined here, such as high inter-patch migration, mate choice, gene conversion or other infectious agents, are likely maintaining the high levels of MHC diversity observed in wild montane voles.