Evolution of resistance to chytridiomycosis is associated with a robust early immune response

Potentiating the evolution of immunity is a promising strategy for addressing biodiversity diseases. Assisted selection for infection resistance may enable the recovery and persistence of amphibians threatened by chytridiomycosis, a devastating fungal skin disease threatening hundreds of species globally. However, knowledge of the mechanisms involved in the natural evolution of immunity to chytridiomycosis is limited. Understanding the mechanisms of such resistance may help speed-assisted selection. Using a transcriptomics approach, we examined gene expression responses of endangered alpine tree frogs (Litoria verreauxii alpina) to subclinical infection, comparing two long-exposed populations with a naïve population. We performed a blinded, randomized and controlled exposure experiment, collecting skin, liver and spleen tissues at 4, 8 and 14 days postexposure from 51 wild-caught captively reared infection-naïve adult frogs for transcriptome assembly and differential gene expression analyses. We analysed our results in conjunction with infection intensity data, and the results of a large clinical survival experiment run concurrently with individuals from the same clutches. Here, we show that frogs from an evolutionarily long-exposed and phenotypically more resistant population of the highly susceptible alpine tree frog demonstrate a more robust innate and adaptive immune response at the critical early subclinical stage of infection when compared with two more susceptible populations. These results are consistent with the occurrence of evolution of resistance against chytridiomycosis, help to explain underlying resistance mechanisms, and provide genes of potential interest and sequence data for future research. We recommend further investigation of cell-mediated immunity pathways, the role of interferons and mechanisms of lymphocyte suppression.