Molecular Biology and Evolution, Vol 10, 1303-1316, Copyright © 1993 by Society for Molecular Biology and Evolution
B Cournoyer, M Gouy and P Normand
Nucleotide sequences of approximately 213 bp of the nif H-D intergene and
the beginning of nifD were determined for symbiotic Frankia isolates from
the major host-infectivity groups. This region of the nif operon is
variable enough to classify most infective Frankia strains at the species
level. Phylogenetic inferences from these sequences are in agreement with
the 16S rRNA-derived phylogeny of the genus and, thus, are in favor of an
intrageneric evolution of nif genes by orthology. Phylogenetic lineages
derived from combined nifH-D intergene and partial nifD and 16S rRNA
sequences are supported for at least 93% of bootstrap replicates and are
useful for investigating evolutionary relationships of the genus and
symbiotic properties of this microorganism. The genus Frankia is divided
into two major phylogenetic clusters that match with the separation of
species according to the mechanism of infection of actinorhizal plants. One
cluster groups species strictly adapted to the mechanism of root hair
infection (RHI), and the other groups species adapted to the mechanism of
direct intercellular penetration. In the RHI cluster, the species infective
on Casuarina plants appears to have emerged from strains infective on
Alnus. The concordance between the symbiotic properties and the molecular
phylogeny of Frankia strains indicates a major role for the host plant in
the evolution and speciation of the genus Frankia.
ORIGINAL ARTICLE
Molecular phylogeny of the symbiotic actinomycetes of the genus Frankia matches host-plant infection processes
Laboratoire d'Ecologie Microbienne du Sol, Universite Claude-Bernard Lyon 1.
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