Molecular Phylogeny of the Genus Frankia and Related Genera and Emendation of the Family Frankiaceae

The members of the actinomycete genus Frankia are nitrogen-fixing symbionts of may 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 Alnus-infective strains than among the strains that infect other host plants. The four main subdivisions of the genus Frankia revealed by this phylogenetic analysis are (i) a very large group comprising Frankia alni and related organisms (including Alnus rugosa Sp+ microsymbionts that are seldom isolated in pure culture), to which Casuarina-infective strains, a Myrica nagi microsymbiont, and other effective Alnus-infective strains are related; (ii) unisolated microsymbionts of Dryas, Coriaria, and Datisca species; (iii) Elaeagnus-infective strains; and (iv) "atypical" strains (a group which includes an Alnus-infective, non-nitrogen-fixing strain). Taxa that are related to this well-defined, coherent Frankia cluster are the genera Geodermatophilus, "Blastococcus," Sporichthya, Acidothermus, and Actinoplanes. However, the two genera whose members have multilocular sporangia (the genera Frankia and Geodermatophilus) did not form a coherent group. For this reason, we propose that the family Frankiaceae should be emended so that the genera Geodermatophilus and "Blastococcus" are excluded and only the genus Frankia is retained.