Deoxyribonucleic Acid Homology Among Strains of Rhizobium trifolii and Related Species

Deoxyribonucleic acid homologies were determined among 27 strains of Rhizobium trifolii, 4 strains of Rhizobium leguminosarum, and 4 strains of Rhizobium phaseoli. 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 Trifolium repens, Trifolium subterraneum, Trifolium ambiguum, and Vicia hirsuta formed one population with an average relatedness of 70% (range, 49 to 94%) and a ▵Tm(e) of 0.0 to 11.8°C with respect to reference strains capable of nodulating the first two clover species. Two strains from African Trifolium species and two strains from a northern Asiatic species were less closely related. The average relatedness of strains from Phaseolus vulgaris to clover rhizobia was 46% (range, 37 to 50%), and the Tm(e) was 6.5 to 11.8°C. Taxonomic revisions consistent with these observations are discussed. It is proposed that R. trifolii and R. leguminosarum should be combined under the name which has priority, Rhizobium leguminosarum Frank. Within this species various biovars should be designated according to plant specificity. R. phaseoli 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.