P. Normand et al., MOLECULAR PHYLOGENY OF THE GENUS FRANKIA AND RELATED GENERA AND EMENDATION OF THE FAMILY FRANKIACEAE, International journal of systematic bacteriology, 46(1), 1996, pp. 1-9
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.