Abstract
Rhizobium-legume symbioses serve as paradigmatic examples for the study of mutualism evolution. The genus Ensifer (syn. Sinorhizobium) contains diverse plant-associated bacteria, a subset of which can fix nitrogen in symbiosis with legumes. To gain insights into the evolution of symbiotic nitrogen fixation (SNF), and interkingdom mutualisms more generally, we performed extensive phenotypic, genomic, and phylogenetic analyses of the genus Ensifer. The data suggest that SNF likely emerged several times within the genus Ensifer through independent horizontal gene transfer events. Yet, the majority (105 of 106) of the Ensifer strains with the nodABC and nifHDK nodulation and nitrogen fixation genes were found within a single, monophyletic clade. Comparative genomics highlighted several differences between the "symbiotic" and "nonsymbiotic" clades, including divergences in their pangenome content. Additionally, strains of the symbiotic clade carried 325 fewer genes, on average, and appeared to have fewer rRNA operons than strains of the nonsymbiotic clade. Initial characterization of a subset of ten Ensifer strains identified several putative phenotypic differences between the clades. Tested strains of the nonsymbiotic clade could catabolize 25% more carbon sources, on average, than strains of the symbiotic clade, and they were better able to grow in LB medium and tolerate alkaline conditions. On the other hand, the tested strains of the symbiotic clade were better able to tolerate heat stress and acidic conditions. We suggest that these data support the division of the genus Ensifer into two main subgroups, as well as the hypothesis that pre-existing genetic features are required to facilitate the evolution of SNF in bacteria.