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

Abstract

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.

Keywords

• evolution
• hot spring
• cyanobacteria
• horizontal gene transfer