Despite its importance in host physiology, how the microbiome varies within and among populations of hosts is not well understood. However, differential selection pressures across a species’ range likely lead to corresponding variation in the microbiome. In addition, character traits, such as the microbiome, should differ more between ecologically similar species in sympatry than in allopatry because selection favors the reduction of interspecific competition. We investigated variation in the maternally-transmitted, adaptive gut microbiomes of Phanaeus vindex and P. difformis, sister species of dung beetle that exhibit niche partitioning in sympatry and other potential factors affecting microbiome variation across their ranges. We hypothesized that 1) the gut microbiomes of P. vindex and P. difformis would be more different in sympatry than in allopatry, reflecting sympatric niche partitioning, and 2) that the gut microbiome of broadly-distributed, cosmopolitan P. vindex would show greater beta diversity than that of narrowly-distributed, sand-specializing P. difformis. To test these hypotheses, we assessed the gut bacterial and archaeal diversity of P. vindex and P. difformis sampled in sympatry and in allopatry. We explored how climatic data, soil texture and soil microbial diversity, distance between sites, and sympatry or allopatry predicted observed patterns of gut microbial variation.
Results/Conclusions
Among allopatric and among sympatric populations of both Phanaeus species, the turnover in gut microbial communities was best predicted by distance among sampling locations, followed by differences in climatic variables. Because the gut microbiome is under a degree of host genetic control, greater host genetic differences across increased distances may explain this result. We also found evidence supporting both hypotheses. First, the gut microbiome of P. vindex varied more among populations than did that of P. difformis. Across its range, we observed P. vindex on more varied soil habitats than P. difformis, and greater variation in microhabitat selection may translate to greater variation in the gut microbiome. Second, we found that the gut microbial communities of P. vindex and P. difformis differed more in sympatry than in allopatry, following other researchers’ observations that P. vindex is found less on sand, the preferred soil type of P. difformis, when the two species co-occur. 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.