Abstract: Microorganisms, such as bacteria, were long thought to have cosmopolitan distributions globally, however, a growing body of evidence suggests that free-living microbial communities are governed by the same processes that influence plant and animal community similarity and assembly. Few studies have examined the spatial variation in host-associated bacteria communities, which could follow different patterns due to host specificity and the inability of some microbes to persist outside of the host environment. In this study, we examine the spatial variation in gut bacteria community composition of four bee genera in geographically isolated high-elevation meadows in the San Madrean Sky Islands of Arizona and New Mexico. The chosen bee genera differ in traits, such as sociality and size, to capture a spectrum of microbial transmission routes and limitations. We expect a stronger decline in microbial similarity with increasing distance between meadows in smaller and solitary bees due to the increased importance of dispersal limitation and environmental transmission, respectively. We used 16s rRNA gene amplicon sequencing to determine gut bacteria composition, evaluated Bray-Curtis dissimilarity between each sample, and used Mantel tests to determine the correlation between bacterial community dissimilarity and geographic distance. We also determine how much microbial dissimilarity is due to host species turnover between sites. We found that across all the bee genera analyzed, the bacterial microbiomes of bees in meadows that were geographically closer together were more similar. However, the steepness of the decay in similarity with distance varied between genera. The composition of bacterial communities differed significantly among host genera and showed site-specific differences for certain bacterial taxa. In conclusion, this study provides evidence of the distance decay of similarity relationship in host-associated microbiome communities among phylogenetically and functionally distinct hosts.
