Conserved Evolution of MHC Supertypes among Japanese Frogs Suggests Selection for Bd Resistance

SIMPLE SUMMARY: This study explored the major histocompatibility complex (MHC) of a variety of Japanese frog species, to further understand why they may have resistance to the deadly chytrid fungus Batrachochytrium dendrobatidis (Bd), which causes mortality and decline in many other amphibians. MHC supertyping analysis showed that all examined East Asian frogs contained at least one MHC-IIb allele belonging to supertype ST-1, indicating that functional properties in the peptide binding sites of MHC-II are conserved among East Asian frogs and some other anurans across the world. Preliminary analysis also suggests that MHC-IIb supertypes ST-1 and ST-2 have higher overall peptide binding ability, regardless of where the peptides are derived from (both Bd and non-Bd). The findings support the hypothesis that MHC-IIb among East Asian frogs may have co-evolved under similar selective pressures, possibly due to the presence of Bd in the region, contributing to their resistance to the disease. Our study helps to further elucidate the complex relationships between anuran MHC and Bd pathogen. ABSTRACT: The chytrid fungus Batrachochytrium dendrobatidis (Bd) is a major threat to amphibians, yet there are no reports of major disease impacts in East Asian frogs. Genetic variation of the major histocompatibility complex (MHC) has been associated with resistance to Bd in frogs from East Asia and worldwide. Using transcriptomic data collated from 11 Japanese frog species (one individual per species), we isolated MHC class I and IIb sequences and validated using molecular cloning. We then compared MHC from Japanese frogs and other species worldwide, with varying Bd susceptibility. Supertyping analysis, which groups MHC alleles based on physicochemical properties of peptide binding sites, identified that all examined East Asian frogs contained at least one MHC-IIb allele belonging to supertype ST-1. This indicates that, despite the large divergence times between some Japanese frogs (up to 145 million years), particular functional properties in the peptide binding sites of MHC-II are conserved among East Asian frogs. Furthermore, preliminary analysis using NetMHCIIpan-4.0, which predicts potential Bd-peptide binding ability, suggests that MHC-IIb ST-1 and ST-2 have higher overall peptide binding ability than other supertypes, irrespective of whether the peptides are derived from Bd, other fungi, or bacteria. Our findings suggest that MHC-IIb among East Asian frogs may have co-evolved under the same selective pressure. Given that Bd originated in this region, it may be a major driver of MHC evolution in East Asian frogs.