Pathogen richness and abundance predict patterns of adaptive MHC variation in insular amphibians

The identification of the factors responsible for genetic variation and differentiation at adaptive loci can provide important insights into the evolutionary process, and is crucial for the effective management of threatened species. We studied the impact of environmental viral richness and abundance on functional diversity and differentiation of the MHC class Ia locus in populations of the black-spotted pond frog (Pelophylax nigromaculatus), an IUCN-listed species, on 24 land-bridge islands of the Zhoushan Archipelago and 3 nearby mainland sites. We found a high proportion of private MHC alleles in mainland and insular populations, corresponding to 32 distinct functional supertypes, and strong positive selection on MHC antigen-binding sites in all populations. Viral pathogen diversity and abundance was reduced at island sites relative to the mainland, and islands housed distinctive viral communities. Standardized MHC diversity at island sites exceeded that found at neutral microsatelli...