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 symb ionts of many species of woody dicotyledonous plants belonging to eigh t families. Several strains isolated from diverse actinorhizal plants growing in different geographical areas were used in this study. The p hylogenetic relationships of these organisms and uncharacterized micro symbionts that are recalcitrant to isolation in pure culture were dete rmined by comparing complete 16S ribosomal DNA sequences. The resultin g phylogenetic tree revealed that there was greater diversity among th e Alnus-infective strains than among the strains that infect other hos t plants. The four main subdivisions of the genus Frankia revealed by this phylogenetic analysis are (i) a very large group comprising Frank ia alni and related organisms (including Alnus rugosa Sp+ microsymbion ts that are seldom isolated in pure culture), to which Casuarina-infec tive strains, a Myrica nagi microsymbiont, and other effective Alnus i nfective 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-defin ed, coherent Frankia cluster are the genera Geodermatophilus, ''Blasto coccus,'' Sporichthya, Acidothermus, and Actinoplanes. However, the tw o 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.