Deoxyribonucleic acid homologies were determined among 27 strains of , 4 strains of , and 4 strains of . Results from related strains indicated that deoxyribonucleic acid homologies correlate with serological relationships and that the ability to form nodules on legume roots can be lost without a detectable change in homology with an independent reference strain. All rhizobia which effectively nodulated , and formed one population with an average relatedness of 70% (range, 49 to 94%) and a Δ of 0.0 to 11.8° C with respect to reference strains capable of nodulating the first two clover species. Two strains from African species and two strains from a northern Asiatic species were less closely related. The average relatedness of strains from to clover rhizobia was 46% (range, 37 to 50%), and the was 6.5 to 11.8° C. Taxonomic revisions consistent with these observations are discussed. It is proposed that and should be combined under the name which has priority, Frank. Within this species various biovars should be designated according to plant specificity. should be retained at present as a separate species and examined in more detail. The results are discussed in relation to proposed genetic bases for plant specificity.


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