Geographic divergence of “Sulfolobus islandicus” strains assessed by genomic analyses including electronic DNA hybridization confirms they are geovars

Ten well-annotated genomes of “Sulfolobus islandicus” strains from different geographic locations have been released at the NCBI database. Whole genome based composition vector trees indicate that these strains show the same branching patterns as originally reported by multi-locus sequence analysis. To determine whether the ten strains meet the criteria for separate species, DNA–DNA hybridization (DDH) was performed in silico. DDH values of strains from the same geographic location, i.e., Iceland, Kamchatka and North America, ranged from 82.4 to 95.4 %, clearly qualifying them as members of the same species. The lowest DDH values found between locations ranged from 75.5 to 76.6 %, which exceed the 70 % DDH threshold for a species thereby indicating they are all members of the same species based on the currently accepted definition. The clear divergences of strains from the different geographic locations are sufficiently great to consider them as separate geovars. “S. islandicus” has not yet been validly named and a type strain has not been deposited in culture collections. We urgently recommend that those who study the organism fulfill the criteria of the International Code of Nomenclature of Bacteria in order to designate a type strain and to identify and deposit related strains of this species to make them available to the broader scientific community.