Study system and field sampling
The focal species for this study is Thoropa taophora, a cycloramphid frog with a unique tolerance for coastal habitat that allows a wide distribution across the coastal Atlantic Forest of São Paulo State (36). Adult T. taophora frogs (n=175 total) were sampled from each of ten study populations: seven island populations and three coastal mainland populations (Fig. 1, Table 1). Genetic diversity is lower in island T. taophora populations relative to coastal mainland populations, both at neutral (microsatellite) loci (35) as well as at the MHC IIB immunogenetic locus (17). To examine how host genetics impact skin microbiome diversity, skin swab samples were taken from the same individuals that were previously genotyped at MHC IIB (see Belasen et al. 2019). Frogs were thoroughly washed with sterile (autoclaved) distilled water and then swabbed on the ventral surface using standard protocols that minimize cross-contamination (37). DNA was extracted from swabs using a Qiagen DNeasy Blood and Tissue kit.