[Evolution and phylogeny of rhizobia].

Nitrogen fixation an ancient process that may is have originated in the archaean Eon under the primitive atmosphere anoxygenic conditions. Diazotrophy is an exclusive process of prokaryotes, only Euryarchaeota and 6 of 54 Bacteria phyla have diazotrophs lineages. Some of them coevolved with flowering plants for the establishment of molecular bases of a mutualistic symbiosis relationship. In rhizobia, the nitrogen fixation occurs inside the nodules, special structures on the roots or stems of legumes. Nodule organogenesis starts with the bacterial nodulation factors (Nod factors) codified in large plasmids or symbiotic islands in the bacterial genomes. Nodulation genes had more recent origin than the nitrogen fixation ones because the origin of the nod gene is associated with the origin of the hosts. The 16S rRNA phylogeny groups rhizobia in 7 genuses of the alpha-Proteobacteria: Bradyrhizobium, Mesorhizobium, Rhizobium, Sinorhizobium, Methylobacterium and Devosia, and two genuses recently described in f-Proteobacteria: Burkholderia and Wautersia. The phylogenies obtained with other chromosomal genes are similar at the genus level, but it is incongruent with the symbiotic gene (nif & nod) phylogeny, because horizontal gene transfer has allowed their evolution in function to the legume host fitness.