Identifying genetic mutations that enhance fitness in Bacillus subtilis evolved under relaxed selection for sporulation

Little is known about how genetic variation at the nucleotide level contributes to competitive fitness within species. During a 6,000-generation study of B. subtilis evolved under relaxed selection for sporulation one strain emerged with a significantly different phenotype, notably a loss of sporulation and motility, and increased competitive fitness. To determine the genetic source of this advantage, OpGen optical mapping, whole genome 454 pyrosequencing and the CLC Genomics Workbench were used to identify single nucleotide polymorphisms (SNPs), insertions, and deletions that appeared between the evolved and ancestor strains. Of particular interest were those mutations that resulted in amino acid changes in coding sequences. Several mutations were identified in genes related to the observed phenotypic changes in sporulation, competitiveness, motility, heat shock, and acetoin biosythetic pathways. Since these pathways are under reduced selection in nutrient-rich shake culture, they are metabolically costly and reduce the competitiveness of the individual strains, rendering mutations negatively affecting these pathways selectively advantageous.