Variation in Disease Susceptibility Is Associated With Immunogenetics in a Threatened Amphibian

Emerging infectious diseases affect wildlife populations around the world, making effective conservation strategies essential to the long-term persistence of threatened species. Understanding the drivers of protective host responses to pathogens can help guide conservation efforts. Species that vary intraspecifically in their susceptibility to pathogens are often best for identifying significant drivers, as this reduces potentially confounding effects of highly divergent genetic and adaptive backgrounds. Boreal toads (Anaxyrus boreas boreas) exhibit such intraspecific variation in disease susceptibility. This species ranges across western North America and declines due to the fungal pathogen Batrachochytrium dendrobatidis (Bd) have varied in severity across populations. Previous experimental work shows that some populations of boreal toads are better able to tolerate Bd than others, though the mechanisms are currently unclear. We sequenced the immune locus MHC IIβ exon 2 across tolerant and susceptible populations of boreal toads to determine if host genetics is correlated with Bd susceptibility in this system. We found that tolerant populations differ from susceptible populations at this locus. Specifically, tolerant populations had several distinct MHC alleles, higher observed heterozygosity, and a higher proportion of alleles belonging to one of two functional supertypes. Using these data, we predicted the Bd response of additional boreal toad populations with unknown susceptibility. Boreal toad conservation efforts will benefit from the ability to identify at-risk populations and individuals with genetic signatures of Bd tolerance for use in captive breeding and reintroduction efforts.