Local adaptation of the MHC class IIβ gene influences the susceptibility to a common parasite in wood frogs (Lithobates sylvaticus)

The major histocompatibility (MHC) genes code for membrane-bound proteins that are involved in parasite/pathogen recognition. The link between the MHC and immunity makes these genes an important genetic marker to evaluate in systems where infectious disease is associated with population declines. Amphibians are an ideal model to test the role of the MHC in infectious disease resistance, as parasites and pathogens currently threaten populations worldwide. We evaluated the importance of the MHC in resistance to trematodes, a common amphibian parasite. We characterized the diversity of MHC class IIβ and 14 microsatellite loci, average trematode loads, and average tolerance to pesticides in 14 populations of wood frogs (Lithobates sylvaticus) in northwestern Pennsylvania. To explore local adaptation in the MHC IIβ, we quantified genetic differentiation of MHC IIβ and microsatellite markers (Fst). In addition, we evaluated the associations between MHC IIβ diversity and trematode prevalence or tolerance to pesticides. Populations where individuals shared more alleles were associated with higher trematode loads than populations with an even distribution of alleles. This suggests that individuals possessing common MHC IIβ alleles are more susceptible to trematode infection. In addition, we noted a positive association between MHC IIβ diversity and tolerance to pesticides, suggesting that processes which influence the evolution of pesticide tolerance (e.g., proximity to anthropogenic environments) may similarly affect immune-related traits. As such, future research should investigate the mechanisms through which anthropogenic and natural factors shape the evolution of the MHC, such as the prevalence of other parasites in the environment.