Multiple gene genealogical analyses reveal both common and distinct population genetic patterns among replicons in the nitrogen-fixing bacterium Sinorhizobium meliloti

Sinorhizobium meliloti is a Gram-negative alpha-proteobacterium that can form symbiotic relationships with alfalfa and fix atmospheric nitrogen. The complete genome of a laboratory strain, Rm1021, was published in 2001, and the genome of this strain is arranged in three replicons: a chromosome of 3.65 million base pairs (Mb), and two megaplasmids, pSymA (1.35 Mb) and pSymB (1.68 Mb). However, the potential difference in genetic variation among the three replicons in natural strains remains poorly understood. In this study, a total of 16 gene fragments were sequenced, four from pSymA and six each from the chromosome and pSymB, for 49 natural S. meliloti strains. The analyses identified significant differences in divergence among genes, with the mean Hasegawa-Kishino-Yano-1985 (HKY85) distance ranging from 0.00157 to 0.04109 between pairs of strains. Overall, genes on pSymA showed the highest mean HKY85 distance, followed by those on pSymB and the chromosome. Although evidence for recombination was found, the authors' population genetic analyses revealed overall significant linkage disequilibria among genes within both pSymA and the chromosome. However, genes on pSymB were in overall linkage equilibrium, consistent with frequent recombination among genes on this replicon. Furthermore, the genealogical comparisons among the three replicons identified significant incongruence, indicating reassortment among the three replicons in natural populations. The results suggest both shared and distinct patterns of molecular evolution among the three replicons in the genomes of natural strains of S. meliloti.