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SUMM.'iRYRhizobia 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. Sequences of the small subunit ribosomal RNA (SSU or 16S rRNA) support the well-established subdivision of rhizobia into three genera: Rhizobium, Bradyrhizobium and Azorhizobium. These all lie within the alpha subdivision of the Proteohacteria, hut on quite distinct branches, each of which also includes many bacterial species that are not rhizobia. Rhizobium, by this definition, is still broad and poh'phyletic, so there have recently been suggestions that this genus should be split into four genera. SSU sequences may be tbe best phylogenetic tool we have, hut they are not an infallible guide to evolutionary relationships, particularly among closely related species: slow evolution, recombination, 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 were transferred hetween phylogenetically distinct bacteria, so that the 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.

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