The genetic structure of Rhizobium populations

Allelic profiles for 14 loci were determined for 48 strains of Rhizobium meliloti and 43 strains of R. leguminosarum bv. viciae using multi-locus enzyme electrophoresis. From these results, and using data from published studies, estimates of multi-locus linkage disequilibrium were obtained for a number of rhizobial species. The magnitude of linkage disequilibrium estimates suggest that the genetic structure of rhizobial populations spans the range from strictly clonal to essentially panmictic. R. meliloti appears to consist of two ecologically isolated subpopulations of strains, where recombination appears to be frequent within populations but not between populations. Symbiotically-effective populations of R. etli appear to maintain a clonal population structure, while non-symbiotically-effective strains of this species, those lacking the symbiotic plasmid, exhibit a panmictic population structure. No linkage disequilibrium was detected among strains of R. leguminosarum bv. viciae, suggesting frequent recombination, whereas recombination appears to be less frequent among strains of R. leguminosarum bv. trifolii.