Genomic Diversity and Evolution of Bacillus subtilis

Bacillus subtilis is the focus of both academic and industrial research. Previous studies have reported a number of sequence variations in different B. subtilis strains. To uncover the genetic variation and evolutionary pressure in B. subtilis strains, we performed whole genome sequencing of two B. subtilis isolates, KM and CGMCC63528. Comparative genomic analyses of these two strains with other B. subtilis strains identified high sequence variations including large insertions, deletions and SNPs. Most SNPs in genes were synonymous and the average frequency of synonymous mutations was significantly higher than that of the non-synonymous mutations. Pan-genome analysis of B. subtilis strains showed that the core genome had lower dN/dS values than the accessory genome. Whole genome comparisons of these two isolates with other B. subtilis strains showed that strains in different subspecies have similar dN/dS values. Nucleotide diversity analysis showed that spizizenii subspecies have higher nucleotide diversity than subtilis subspecies. Our results indicate that genes in B. subtilis strains are under high purifying selection pressure. The evolutionary pressure in different subspecies of B. subtilis is complex.