Amphibians have been affected globally by the disease chytridiomycosis, caused by the fungus
Batrachochytrium dendrobatidis
(
Bd
), and we are just now beginning to understand how immunogenetic variability contributes to disease susceptibility. Lineages of an expressed major histocompatibility complex (MHC) class II locus involved in acquired immunity are associated with chytridiomycosis susceptibility in controlled laboratory challenge assays. Here, we extend these findings to natural populations that vary both in exposure and response to
Bd
. We find that MHC alleles and supertypes associated with
Bd
survival in the field show a molecular signal of positive selection, while those associated with susceptibility do not, supporting the hypothesis that heritable
Bd
tolerance is rapidly evolving. We compare MHC supertypes to neutral loci to demonstrate where selection versus demography is shaping MHC variability. One population with
Bd
tolerance in nature shows a significant signal of directional selection for the same allele (allele Q) that was significantly associated with survival in an earlier laboratory study. Our findings indicate that selective pressure for
Bd
survival drives rapid immunogenetic adaptation in some natural populations, despite differences in environment and demography. Our field-based analysis of immunogenetic variation confirms that natural amphibian populations have the evolutionary potential to adapt to chytridiomycosis.
Adaptive tolerance to a pathogenic fungus drives major histocompatibility complex evolution in natural amphibian populations
