Understanding drivers of inter-population variation in the gut microbiomes of sister species of dung beetles

Despite its importance in host physiology, how the microbiome varies within and among populations of hosts is not well understood. However, differential abiotic and biotic selection pressures across a species’ range likely lead to variation in the microbiome. In addition, symbiotic microbiota may differ more between closely-related species in sympatry than in allopatry if selection favors the reduction of interspecific competition. We investigated variation in the maternally-transmitted, beneficial gut microbiomes of Phanaeus vindex and P. difformis, sister species of dung beetle that compete for the same resources in sympatry and occur across a wide range of climatic conditions that may affect their gut microbiota. We sampled and sequenced bacterial/archaeal 16S rDNA from guts of P. difformis and P. vindex collected across 17 sympatric and allopatric sites, exploring how climatic data, soil microbial diversity, distance between sites, and sympatry or allopatry predicted the observed patterns of gut microbial variation. Gut microbial communities were best predicted by spatial relationships among sampling locations, the abundance of cattle in the sampling area, and temperature and precipitation. Contrary to our hypotheses, we did not find that the gut microbial communities of P. vindex and P. difformis differed more in sympatry than in allopatry, nor that P. vindex exhibits greater turnover in the gut microbiome among populations. However, we found that the gut microbiome communities of P. vindex and P. difformis both shift between allopatry and sympatry, and that the gut microbiome of P. vindex likely experiences a greater shift. While more research is needed, it is possible that differences in their gut microbiomes allow P. vindex and P. difformis to more effectively partition their niches in sympatry. Our work argues for further exploration of the gut microbiome’s potential role in niche partitioning and local adaptation.