Multilocus sequence analysis reveals multiple symbiovars within Mesorhizobium species

The genus Mesorhizobium includes species nodulating several legumes, such as chickpea, which has a high agronomic importance. Chickpea rhizobia were originally described as either Mesorhizobium ciceri or M. mediterraneum. However, rhizobia able to nodulate chickpea have been shown to belong to several different species within the genus Mesorhizobium. The present study used a multilocus sequence analysis approach to infer a high resolution phylogeny of the genus Mesorhizobium and to confirm the existence of a new chickpea nodulating genospecies. The phylogenetic structure of the Mesorhizobium clade was evaluated by sequence analysis of the 16S rRNA gene, ITS region and the five core genes atpD, dnaJ, glnA, gyrB, and recA. Phylogenies obtained with the different genes are in overall good agreement and a well-supported, almost fully resolved, phylogenetic tree was obtained using the combined data. Our phylogenetic analyses of core genes sequences and their comparison with the symbiosis gene nodC, corroborate the existence of one new chickpea Mesorhizobium genospecies and one new symbiovar, M. opportunistum sv. ciceri. Furthermore, our results show that symbiovar ciceri spreads over six species of mesorhizobia. To our knowledge this study shows the most complete Mesorhizobium multilocus phylogeny to date and contributes to the understanding of how a symbiovar may be present in different species.