THE GENETIC-STRUCTURE OF RHIZOBIUM POPULATIONS

Allelic profiles for 14 loci were determined for 48 strains of Rhizobi um meliloti and 43 strains of R. leguminosarum by, viciae using multi- locus enzyme electrophoresis. From these results, and using data from published studies, estimates of multi-locus linkage disequilibrium wer e obtained for a number of rhizobial species. The magnitude of linkage disequilibrium estimates suggest that the genetic structure of rhizob ial populations spans the range from strictly clonal to essentially pa nmictic. R. meliloti appears to consist of two ecologically isolated s ubpopulations of strains, where recombination appears to be frequent w ithin populations but not between populations. Symbiotically-effective populations of R. etli appear to maintain a clonal population structu re, while non-symbiotically-effective strains of this species, those l acking the symbiotic plasmid, exhibit a panmictic population structure . No linkage disequilibrium was detected among strains of R. leguminos arum bv. viciae, suggesting frequent recombination, whereas recombinat ion appears to be less frequent among strains of R. leguminosarum by. trifolii.