Evolution and diversity of Frankia
Résumé
Frankia is an actinobacterium that induces symbiotic nodules on the root of 25 genera of woody dicotyledonous plants (Benson and Silvester 1993). It took a century of unsuccessful efforts before it could be isolated in pure culture and its taxonomic status thus remained disputed for a long time. Pure cultures can be grouped according to host infectivity into (1) strains infective on Alnus, (2) strains infective on Casuarina, (3) strains infective on Elaeagnaceae, (4) unisolated strains and (5) uninfective isolates. These strains have been grouped into genospecies and there are between 3-7 Alnus-infective, 1 Casuarina-infective and 8-12 Elaeagnus-infective species. The phylogeny of these species has been reconstructed using rrs sequences and Frankia is now ascribed to the Frankiaceae, one of the six families of the suborder Frankinae. Genus Frankia is subdivided into phylogenetic clusters: #1-Frankia alni and related (contains Casuarina-infective and most Alnus-infective strains), #2-Unisolated strains present on Rosaceous, Datisca, Coriaria and Ceanothus, #3-Elaeagnaceae-infective strains and #4-a large group of "atypical" strains.