DNA Homology Between Species of the Rhizobia

DNA-DNA hybridizations were conducted among thirty-seven Bradyrhizobium and Rhizobium strains from diverse geographic origins. The data from thirty-eight hybridizations between fourteen fast- and slowgrowing strains indicated a distant genomic relationship exists. Seventy-five hybridizations were conducted among nineteen fast-growing strains. Three homology groups (F-I, F-II, and F-III) were evident with sixteen of the fast-growing strains being placed in one or another of these groups. Homology group F-I contains three Rhizobium leguminosarum biovar viceae strains and one R. leguminosarum biovar trifolii strain. Group F-II contains only R. meliloti strains. Group F-III contains five R. leguminosarum biovar viceae and two R. leguminosarum biovar trifolii strains. Three fast-growing strains appeared to be diverse from others and were placed in a heterogeneous group, F-IV. Fifty-seven hybridizations were conducted among eighteen slow-growing strains. Only nine strains could be placed into homology groups S-I, S-Ia (a subgroup of S-I), or S-II. Homology group S-I contains three Bradyrhizobium japonicum and two cowpea strains which had high homologies with both B. japonicum ATCC 10324T and cowpea strain 3G4b5. Two Bradyrhizobium sp. (Lupinus) strains compose S-Ia since they had above 70% DNA homology with B. japonicum ATCC 10324 but less than 53% DNA homology with cowpea strain 3G4b5. The remaining slow-growing strains demonstrated significant diversity and were placed in a heterogeneous group, S-III.