Impacts of Natural Selection on Evolution of Core and Symbiotically Specialized (sym) Genes in the Polytypic Species Neorhizobium galegae.

Nodule bacteria (rhizobia) represent a suitable model to address a range of fundamental genetic problems, including the impacts of natural selection on the evolution of symbiotic microorganisms. Rhizobia possess multipartite genomes in which symbiotically specialized (sym) genes differ from core genes in their natural histories. Diversification of sym genes is responsible for rhizobia microevolution, which depends on host-induced natural selection. By contrast, diversification of core genes is responsible for rhizobia speciation, which occurs under the impacts of still unknown selective factors. In this paper, we demonstrate that in goat's rue rhizobia (Neorhizobium galegae) populations collected at North Caucasus, representing two host-specific biovars orientalis and officianalis (N2-fixing symbionts of Galega orientalis and G. officinalis), the evolutionary mechanisms are different for core and sym genes. In both N. galegae biovars, core genes are more polymorphic than sym genes. In bv. orientalis, the evolution of core genes occurs under the impacts of driving selection (dN/dS > 1), while the evolution of sym genes is close to neutral (dN/dS ≈ 1). In bv. officinalis, the evolution of core genes is neutral, while for sym genes, it is dependent on purifying selection (dN/dS < 1). A marked phylogenetic congruence of core and sym genes revealed using ANI analysis may be due to a low intensity of gene transfer within and between N. galegae biovars. Polymorphism in both gene groups and the impacts of driving selection on core gene evolution are more pronounced in bv. orientalis than in bv. officianalis, reflecting the diversities of their respective host plant species. In bv. orientalis, a highly significant (P0 < 0.001) positive correlation is revealed between the p-distance and dN/dS values for core genes, while in bv. officinalis, this correlation is of low significance (0.05 < P0 < 0.10). For sym genes, the correlation between p-distance and dN/dS values is negative in bv. officinalis but is not revealed in bv. orientalis. These data, along with the functional annotation of core genes implemented using Gene Ontology tools, suggest that the evolution of bv. officinalis is based mostly on adaptation for in planta niches while in bv. orientalis, evolution presumably depends on adaptation for soil niches. New insights into the tradeoff between natural selection and genetic diversity are presented, suggesting that gene nucleotide polymorphism may be extended by driving selection only in ecologically versatile organisms capable of supporting a broad spectrum of gene alleles in their gene pools.

Genetic relationship of Sinorhizobium meliloti and Sinorhizobium medicae strains isolated from Caucasian region.

Sinorhizobium meliloti and Sinorhizobium medicae are two closely related species of the genus Sinorhizobium showing a similar host range, nodulating leguminous species of the genera Medicago, Melilotus and Trigonella, but their phylogenic relationship has not been elucidated yet. In this paper we report the application of three different molecular markers, (i) RFLP of nodD genes, (ii) 16S-23S rDNA intergenic gene spacer fingerprinting and (iii) amplification fragment length polymorphism to S. meliloti and S. medicae strains isolated from the Caucasian area, which is the region of origin of the host plant Medicago. The analysis of data could suggest the origin of S. medicae strains from an ancestral S. meliloti population.