Differential skin microbiomes and disease outcomes in sympatric hosts despite similar infection burdens

Abstract

Amphibians are declining worldwide, in large part due to chytridiomycosis: a disease of amphibian skin caused by the fungal pathogen, Batrachochytrium dendrobatidis (Bd). Resistance and tolerance to Bd infection can exhibit intra- and interspecific variation. This variation may depend, among other factors, on the host skin microbiome. In California's Sierra Nevada, endangered mountain yellow-legged frogs (MYL frogs, Rana sierrae and Rana muscosa) are highly susceptible to chytridiomycosis, while the co-occurring Pacific chorus frog (Pseudacris regilla) is less susceptible and may function as a Bd reservoir host. To investigate whether distinct Bd dynamics for these sympatric hosts relate to differences in skin microbial communities, we (1) compared Bd infection of MYL frogs and P. regilla across 23 Sierra Nevada sites, and (2) characterized the bacterial component of host skin microbiomes (or bacteriomes). Bd infection prevalence was approximately 80% in both frog species, and average infection intensities on P. regilla and MYL frogs did not differ. In contrast, bacteriome compositions were markedly distinct. P. regilla had a more diverse skin bacteriome than MYL frogs, and only MYL frog bacteriome diversity correlated with Bd infection intensity. Additionally, the P. regilla bacteriome contained a higher richness of putative antifungal bacteria than MYL frogs. Differences in the bacteriomes of these sympatric hosts may be due to differences in physiology or life histories. Further work will be important in determining whether the greater diversity and apparent stability of the P. regilla microbiome is a cause or consequence of the host's innate Bd tolerance.