Sustained immune activation is associated with susceptibility to the amphibian chytrid fungus

The disease chytridiomycosis caused by the fungus Bd has devastated amphibian populations worldwide. Functional genomic contributions to host susceptibility remain enigmatic and vary between species and populations. We conducted experimental Bd infections in Rana yavapaiensis, a species with intraspecific variation in chytridiomycosis susceptibility, to assess the skin and spleen transcriptomic response to infection over time. We predicted that increased immune gene expression would be associated with a positive disease outcome, but we instead found that surviving frogs had significantly reduced immune gene expression compared to susceptible frogs and to uninfected controls. MHC class II gene expression was also significantly higher in susceptible frogs compared to surviving frogs. Furthermore, susceptible frogs expressed a significantly larger number of distinct class II alleles, demonstrating a negative correlation between class II expression, functional diversity, and survival. Ex...