Identification of major histocompatibility complex genotypes associated with resistance to an amphibian emerging infectious disease.

Abstract

Amphibian chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd), emerged from Asia and spread globally. By comparing functional MHC IIß1 alleles from an Asian Bd-resistant anuran species (Bufo gargarizans) with those of an Australasian Bd-susceptible species (Litoria caerulea), we identified MHC genotypes with a glycine deletion (G90β1) and adjacent motifs in the deepest pathogen-derived peptide-binding groove associated with Bd resistance. Every Bd-resistant individual, but no susceptible individuals, possessed at least one allele encoding the variant. We detected trans-species polymorphism at the end of the MHC IIβ1 sequences in both species. The G90β1 deletion was encoded by different alleles in the two species, suggesting it may have evolved independently in the two species rather than having been derived from a common ancestor. It is possible that MHC adaptations to chytridiomycosis evolved independently among amphibian species. These results are consistent with a scenario by which MHC adaptations that confer resistance to the pathogen have evolved by convergent evolution. Immunogenetic studies such as this are critical to ongoing conservation efforts.