Nor climate nor human impact factors: Chytrid infection shapes the skin bacterial communities of an endemic amphibian in a biodiversity hotspot.

The bacterial communities of the amphibian skin (i.e., the bacteriome) are critical to the host's innate immune system. However, it is unclear how different drivers can alter this function by modulating the bacteriome's structure. Our aim was to assess the extent to which different host attributes and extrinsic factors influence the structure of the bacterial communities of the skin. Skin bacterial diversity was examined in 148 individuals of the four-eyed frog ( Pleurodema thaul ) from 16 localities spanning almost 1800 km in latitude. The richness and beta diversity of bacterial families and the richness and abundance of Bd -inhibitory bacterial genera were used to describe their structure. Predictors associated with the host (developmental stage, genetic lineage, individual Batrachochytrium dendrobatidis [ Bd ] infection status) and the landscape (current climate, degree of anthropogenic disturbance) were used in the statistical modeling in an information theoretical approach. Bd infection and host developmental stage were the only predictors affecting bacteriome richness, with Bd + individuals and postmetamorphic stages (adults and juveniles) having higher richness than Bd - ones and tadpoles. High diversity in Bd + individuals is not driven by bacterial genera with known anti- Bd properties. Beta diversity was not affected by Bd infection and was mainly a consequence of bacterial family turnover rather than nestedness. Finally, for those bacterial genera known to have inhibitory effects on chytrid , Bd+ individuals had a slightly higher diversity than Bd - ones . Our study confirms an association between Bd infection and the host developmental stage with the bacterial communities of the skin of P. thaul. Unexpectedly, macroclimate and human impact factors do not seem to play a role in shaping the amphibian skin microbiome. Our study exemplifies that focusing on a single host-parasite system over a large geographic scale can provide essential insights into the factors driving host-parasite-bacteriome interactions.