Microbial diversification is driven by geographic and ecological factors, but how the relative importance of these factors varies among species, geographic scales, and habitats remains unclear. Streptomyces, a genus of antibiotic-producing, spore-forming, and widespread bacteria, offers a robust model for identifying the processes underlying population differentiation. We examined the population structure of 37 Streptomyces olivaceus strains isolated from various sources, showing that they diverged into two habitat-associated (free-living and insect-associated) and geographically disparate lineages. More frequent gene flow within than between the lineages confirmed genetic isolation in S. olivaceus. Geographic isolation could not explain the genetic isolation; instead, habitat type was a strong predictor of genetic distance when controlling for geographic distance. The identification of habitat-specific genetic variations, including genes involved in regulation, resource use, and secondary metabolism, suggested a significant role of habitat adaptation in the diversification process. Physiological assays revealed fitness trade-offs under different environmental conditions in the two lineages. Notably, insect-associated isolates could outcompete free-living isolates in a free-iron-deficient environment. Furthermore, substrate (e.g., sialic acid and glycogen) utilization but not thermal traits differentiated the two lineages. Overall, our results argue that adaptive processes drove ecological divergence among closely related streptomycetes, eventually leading to dispersal limitation and gene flow barriers between the lineages. S. olivaceus may best be considered a species complex consisting of two cryptic species. IMPORTANCE Both isolation by distance and isolation by environment occur in bacteria, and different diversification patterns may apply to different species. Streptomyces species, typified by producing useful natural products, are widespread in nature and possess high genetic diversity. However, the ecological processes and evolutionary mechanisms that shape their distribution are not well understood. Here, we show that the population structure of a ubiquitous Streptomyces species complex matches its habitat distribution and can be defined by gene flow discontinuities. Using comparative genomics and physiological assays, we reveal that gains and losses of specific genomic traits play a significant role in the transition between free-living and host-associated lifestyles, driving speciation of the species. These results provide new insights into the evolutionary trajectory of Streptomyces and the notion of species.
Keywords: Streptomyces; gene flow; habitat adaptation; lifestyle; population genetics; speciation.