Latitude delineates patterns of biogeography in terrestrial Streptomyces

We examined the biogeography of Streptomyces at regional spatial scales to identify factors that govern patterns of microbial diversity. Streptomyces are spore forming filamentous bacteria which are widespread in soil. Streptomyces strains were isolated from perennial grass habitats sampled across a spatial scale of more than 6,000 km. Previous analysis of this geographically explicit culture collection provided evidence for a latitudinal diversity gradient in Streptomyces species. Here we evaluate the hypothesis that this latitudinal diversity gradient is a result of evolutionary dynamics associated with historical demographic processes. Historical demographic phenomena have genetic consequences that can be evaluated through analysis of population genetics. We applied population genetic approaches to analyze population structure in six of the most numerically abundant and geographically widespread Streptomyces phylogroups from our culture collection. Streptomyces population structure varied at regional spatial scales and allelic diversity correlated with geographic distance. In addition, allelic diversity and gene flow are partitioned by latitude. Finally, we found that nucleotide diversity within phylogroups is negatively correlated with latitude. These results indicate that phylogroup diversification is constrained by dispersal limitation at regional spatial scales and they are consistent with the hypothesis that historical demographic processes have influenced the contemporary biogeography of Streptomyces.\n\nOriginality-Significance StatementWe provide the first population genetic evidence that patterns of Streptomyces biogeography, which manifest in geographically explicit patterns of gene flow and a latitudinal gradient of nucleotide diversity, result from dispersal limitation and regional diversification due to drift. This contribution elucidates evolutionary processes that underlie patterns of microbial biogeography.