The CRISPR/Cas Machinery Evolution and Gene Flow in the Hot Spring Cyanobacterium Thermostichus

Drivers of the speciation in bacteria, including geographical isolation and horizontal gene transfer, are still poorly understood. Here, we characterized a new lineage within an anciently diverged thermophilic cyanobacteria. We sequenced the whole genome of a strain Thermostichus vulcanus isolated from the Rupite spring (Bulgaria), which is closely related to Theromstichus strains JA-2-3Aa and JA-2-3B′a(2-13). We performed phylogenetic inference, horizontal gene transfer estimation, and CRISPR/Cas system characterization. We found that the Rupite strain is a distinct species from strains JA-2-3Aa, and JA-2-3B′a(2-13). Furthermore, the horizontal gene transfer seemed to be more frequent among the geographically distant species than between the two species within the same hot spring. The CRISPR/Cas system had variable complexity among the species of Thermostichus in terms of both the number of spacers and genes. The Rupite strain had the highest, and JA-2-3Aa the lowest number of spacers among the analyzed strains, and the CRISPR spacers were only rarely shared among the strains. We conclude that the CRISPR/Cas system size varied among the lineages as well as the gene flow.