Distinct Potentially Adaptive Accumulation of Truncation Mutations in Salmonella enterica serovar Typhi and Salmonella enterica serovar Paratyphi A

Gene inactivation through the accumulation of truncation (or premature stop codon) mutations is a common mode of evolution in bacteria. It is frequently believed to result from reductive evolutionary processes allowing purging of superfluous traits. However, several works have demonstrated that, similar to the occurrences of inactivating nonsynonymous (i.e., amino acid replacement) mutations under positive selection pressures, truncation mutations can also be adaptive where specific traits deleterious in particular environmental conditions need to be inactivated for survival. Here, we performed a comparative analysis of genome-wide accumulation of truncation mutations in Salmonella enterica serovar Typhi and Salmonella enterica serovar Paratyphi A. Considering the known convergent evolutionary trajectories in these two serovars, we expected a strong overlap of truncated genes in S. Typhi and S. Paratyphi A, emerging through either reductive or adaptive dynamics. However, we detected a distinct set of core truncated genes encoding different overrepresented functional clusters in each serovar. In 54% and 28% truncated genes in S. Typhi and S. Paratyphi A, respectively, inactivating mutations were acquired by only different subsets of isolates, instead of all isolates analyzed for that serovar. Importantly, 62% truncated genes (P < 0.001) in S. Typhi and S. Paratyphi A were also targeted by convergent amino acid mutations in different serovars, suggesting those genes to be under selection pressures. Our findings indicate significant presence of potentially adaptive truncation mutations in conjunction with the ones emerging due to reductive evolution. Further experimental and large-scale bioinformatic studies are necessary to better explore the impact of such adaptive footprints of truncation mutations in the evolution of bacterial virulence. IMPORTANCE Detecting the adaptive mutations leading to gene inactivation or loss of function is crucial for understanding their contribution in the evolution of bacterial virulence and antibiotic resistance. Such inactivating mutations, apart from being of nonsynonymous (i.e., amino acid replacement) nature, can also be truncation mutations, abruptly trimming the length of encoded proteins. Importantly, the notion of reductive evolutionary dynamics is primarily accepted toward the accumulation of truncation mutations. However, our case study on S. Typhi and S. Paratyphi A, two human-restricted systemically invasive pathogens exerting similar clinical manifestations, indicated that a significant proportion of truncation mutations emerge from positive selection pressures. The candidate genes from our study will enable directed functional assays for deciphering the adaptive role of truncation mutations in S. Typhi and S. Paratyphi A pathogenesis. Also, our genome-level analytical approach will pave the way to understand the contribution of truncation mutations in the adaptive evolution of other bacterial pathogens.