Genetic divergence and gene flow among Mesorhizobium strains nodulating the shrub legume Caragana

Although the biogeography of rhizobia has been investigated extensively, little is known about the adaptive molecular evolution of rhizobia influenced by soil environments and selected by legumes. In this study, microevolution of Mesorhizobium strains nodulating Caragana in a semi-fixing desert belt in northern China was investigated. Five core genes—atpD, glnII, gyrB, recA, and rpoB, six heat-shock factor genes—clpA, clpB, dnaK, dnaJ, grpE, and hlsU, and five nodulation genes—nodA, nodC, nodD, nodG, and nodP, of 72 representative mesorhizobia were studied in order to determine their genetic variations. A total of 21 genospecies were defined based on the average nucleotide identity (ANI) of concatenated core genes using a threshold of 96% similarity, and by the phylogenetic analyses of the core/heat-shock factor genes. Significant genetic divergence was observed among the genospecies in the semi-fixing desert belt (areas A–E) and Yunnan province (area F), which was closely related to the environmental conditions and geographic distance. Gene flow occurred more frequently among the genospecies in areas A–E, and three sites in area B, than between area F and the other five areas. Recombination occurred among strains more frequently for heat-shock factor genes than the other genes. The results conclusively showed that the Caragana-associated mesorhizobia had divergently evolved according to their geographic distribution, and have been selected not only by the environmental conditions but also by the host plants.