DIVERSITY AND PHYLOGENY OF RHIZOBIA

Rhizobia are the bacteria that form nitrogen-fixing nodules on legumes . The current list of four rhizobium genera and 17 species is reviewed , with some comments on likely future developments in the taxonomy. Se quences of the small subunit ribosomal RNA (SSU or 16S rRNA) support t he well-established subdivision of rhizobia into three genera: Rhizobi um, Bradyrhizobium and Azorhizobium. These all lie within the alpha su bdivision of the Proteobacteria, but on quite distinct branches, each of which also includes many bacterial species that are not rhizobia. R hizobium, by this definition, is still broad and polyphyletic, so ther e have recently been suggestions that this genus should be split into four genera. SSU sequences may be the best phylogenetic tool we have, but they are not an infallible guide to evolutionary relationships, pa rticularly among closely related species: slow evolution, recombinatio n, intraspecific variation and even intragenomic heterogeneity are all limitations that can be illustrated by examples from the rhizobia. It seems likely that the ability to form legume nodules was not present in the common ancestor of all rhizobia but that the nodulation genes w ere transferred between phylogenetically distinct bacteria, so that th e phylogeny of nodulation genes will probably differ from that of the bacteria that carry them. Nitrogen fixation genes are often linked to nodulation genes, but they need not have the same evolutionary history .