Abstract
ABSTRACTWe show that Streptomyces biogeography in soils across North America is influenced by the regional diversification of microorganisms due to dispersal limitation and genetic drift. Streptomyces spp. form desiccation-resistant spores, which can be dispersed on the wind, allowing for a strong test of whether dispersal limitation governs patterns of terrestrial microbial diversity. We employed an approach that has high sensitivity for determining the effects of genetic drift. Specifically, we examined the genetic diversity and phylogeography of physiologically similar Streptomyces strains isolated from geographically distributed yet ecologically similar habitats. We found that Streptomyces beta diversity scales with geographic distance and both beta diversity and phylogenetic diversity manifest in a latitudinal diversity gradient. This pattern of Streptomyces biogeography resembles patterns seen for diverse species of plants and animals, and we therefore evaluated these data in the context of ecological and evolutionary hypotheses proposed to explain latitudinal diversity gradients. The data are consistent with the hypothesis that niche conservatism limits dispersal, and historical patterns of glaciation have limited the time for speciation in higher-latitude sites. Most notably, higher-latitude sites have lower phylogenetic diversity, higher phylogenetic clustering, and evidence of range expansion from lower latitudes. In addition, patterns of beta diversity partition with respect to the glacial history of sites. Hence, the data support the hypothesis that extant patterns of Streptomyces biogeography have been driven by historical patterns of glaciation and are the result of demographic range expansion, dispersal limitation, and regional diversification due to drift.
Highlights
We show that Streptomyces biogeography in soils across North America is influenced by the regional diversification of microorganisms due to dispersal limitation and genetic drift
If the same forces govern the diversification of both macro- and microorganisms, it will be possible to bring the full theoretical framework which underpins evolutionary ecology to bear in evaluating the causes and consequences of microbial diversity
The analysis of physiologically similar strains from ecologically similar sites improves our ability to detect biogeographical patterns that result from drift by minimizing the importance of selection
Summary
A total of 924 Streptomyces strains were isolated and characterized from 15 sites spanning the United States (see Table S1 in the supplemental material). The isolated strains encompassed 208 unique rpoB sequences, which were classified into 107 OTUs with clusters defined at a patristic distance of 0.01 (OTUrpoB) (see Fig. S1 in the supplemental material). This distance has previously been observed to roughly correlate with species boundaries for Streptomyces [28]. Good’s coverage was 0.88 for unique rpoB sequences and 0.95 for OTUrpoB, indicating high coverage of Streptomyces taxonomic diversity as captured under our isolation conditions (see Fig. S2 in the supplemental material). Identical rpoB sequences were observed in sites separated by more than 5,000 km (sites MS and AK2), indicating the potential for longrange dispersal. The null model of random taxon assortment between sites was rejected, indicating that taxon
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