Genetic variation in local populations of Sinorhizobium meliloti

Restriction fragment length polymorphism (RFLP) analysis was used to assess genetic variation in samples of two field populations of Sinorhizobium meliloti (from sites 2km apart) that had been characterized on the basis of phage sensitivity. Restricted genomic DNAs from 120 isolates were hybridized to each of four DNA probes from the S. meliloti chromosome, two from the symbiotic megaplasmid (pnod) and two from the nonsymbiotic megaplasmid (pexo). Data for these RFLPs showed that there were totals of 22 chromosomal types, 33 pnod types and 18 pexo types. Analysis of data for chromosomal RFLP's revealed a high proportion of genotypes common to the two field sites, a very low coefficient of genetic differentiation and moderate, but similar mean genetic diversity values for the S. meliloti isolates from each of the sites (H=0.24 and 0.25, respectively), suggesting that these isolates do not represent distinct populations. The population at one field site exhibited a low, but significant level of chromosomal linkage disequilibrium whereas the population at the other site exhibited a panmictic structure (linkage equilibrium). Dendrograms (UPGMA) revealed differences in the grouping of isolates based on megaplasmid (pnod and pexo) genotype relative to the same isolates defined by chromosomal genotype. The distribution of pnod megaplasmid types across chromosomal backgrounds was almost random and contrasted with the apparent non-random distribution of pexo megaplasmid types. Collectively, our data suggest that genetic recombination of chromosomal as well as megaplasmid sequences had occurred in these populations of S. meliloti.