Convergent molecular evolution of genomic cores in Salmonella enterica and Escherichia coli.

One of the strongest signals of adaptive molecular evolution of proteins is the occurrence of convergent hot spot mutations: repeated changes in the same amino acid positions. We performed a comparative genome-wide analysis of mutation-driven evolution of core (omnipresent) genes in 17 strains of Salmonella enterica subspecies I and 22 strains of Escherichia coli. More than 20% of core genes in both Salmonella and E. coli accumulated hot spot mutations, with a predominance of identical changes having recent evolutionary origin. There is a significant overlap in the functional categories of the adaptively evolving genes in both species, although mostly via separate molecular mechanisms. As a strong evidence of the link between adaptive mutations and virulence in Salmonella, two human-restricted serovars, Typhi and Paratyphi A, shared the highest number of genes with serovar-specific hot spot mutations. Many of the core genes affected by Typhi/Paratyphi A-specific mutations have known virulence functions. For each species, a list of nonrecombinant core genes (and the hot spot mutations therein) under positive selection is provided.