The members of the actinomycete genus are nitrogen-fixing symbionts of many species of woody dicotyledonous plants belonging to eight families. Several strains isolated from diverse actinorhizal plants growing in different geographical areas were used in this study. The phylogenetic relationships of these organisms and uncharacterized microsymbionts that are recalcitrant to isolation in pure culture were determined by comparing complete 16S ribosomal DNA sequences. The resulting phylogenetic tree revealed that there was greater diversity among the -infective strains than among the strains that infect other host plants. The four main subdivisions of the genus revealed by this phylogenetic analysis are (i) a very large group comprising and related organisms (including Sp+ microsymbionts that are seldom isolated in pure culture), to which -infective strains, a microsymbiont, and other effective -infective strains are related; (ii) unisolated microsymbionts of , and species; (iii) -infective strains; and (iv) “atypical” strains (a group which includes an -infective, non-nitrogen-fixing strain). Taxa that are related to this well-defined, coherent cluster are the genera , and However, the two genera whose members have multilocular sporangia (the genera and ) did not form a coherent group. For this reason, we propose that the family should be emended so that the genera and “” are excluded and only the genus is retained.


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