Characteristics of Sibling Species Sinorhizobium meliloti and S. medicae: Analysis of Local Population of Medic Symbionts

Abstract Comparative genetic analysis of Sinorhizobium (Ensifer) meliloti and S. medicae, which form local population of the symbionts of black medic, growing in a limited area of sod-podzolic soil in Pushkinsky region of St. Petersburg was performed. Nucleotide polymorphism analysis showed that interspecific differences for p-distance considerably exceeded intraspecific variation, with the divergence between S. meliloti and S. medicae for the housekeeping genes being more pronounced than for the symbiotically specialized genes. These rhizobia species are similar in the genome composition, containing a chromosome, two chromids, and up to five plasmids. The superiority of S. meliloti over S. medicae in the size of chromids and in their proportion in the genome, amounting to 8–10%, was revealed only at the low significance level (0.05 < P0 < 0.10). A more statistically significant (P0 < 0.01) difference between the species was revealed in the local pangenome composition. In particular, S. meliloti was superior to S. medicae in the sizes of pangenome core and accessory parts, but was inferior to it in the size of the strain-specific part of pangenome and the total number of genes. The studied rhizobia species did not differ in the efficiency of symbiosis with the black medic under the conditions of microvegetation tube test. Phenotypic similarity of the genetically diverged species of S. meliloti and S. medicae, stably coexisting in the soil-plant ecosystem, makes it possible to consider them as sibling species, which it is suggested to unite into the superspecies S. melmed.