Pangenome Evolution in the Symbiotic Nitrogen Fixer Sinorhizobium meliloti

The high variability of genomes of strains belonging to the same bacterial species (pangenomes) questions the evolution of the species itself, in particular how the differentiation of strains depends on the role of the conserved versus the variable genomic fractions. Besides the presence of a chromosome, multiple large replicons, such as chromids and megaplasmids, add an additional level of complexity, as each replicon may have a different role in species evolution. The alphaproteobacterium Sinorhizobium meliloti , harboring a chromosome, a chromid, and a megaplasmid, is an ideal system to elucidate the mechanisms of species evolution. The genomic studies reviewed here (conducted on the genomes of various S. meliloti strains) suggest that each replicon has a specific role in the evolution of this species, with the pSymB chromid as the main player in strain differentiation and genus definition. Genes belonging to the pSymB chromid also appear to be more widespread in distant taxa compared to the other replicons, indicating that the origin of these replicons may also be different. In the evolution of a bacterial pangenome, such as in S. meliloti, not only core and dispensable genome fractions contribute to the species differentiation, but also each replicon has a different evolutionary role.