RhizobiumPhylogenies and Bacterial Genetic Diversity

Abstract

The Leguminosae is one of the largest families of plants. It has a broad geographical distribution. The principal legume species have defined sites of origin and these coincide with the diversification centers for their “specific” symbiotic bacteria. These nitrogen-fixing bacteria, which form nodules in the roots or stems of the plants, belong to different bacterial lineages (Rhizobium, Bradyrhizobium, and Azorhizobium) related to other nonsymbiotic bacteria. A remarkable characteristic of these bacteria is their large genetic diversity. The genetic relationships among the different bacterial groups are being defined based mainly on the analysis of the sequences of the ribosomal genes. Recent results point out the need to have a broader genomic scope. Gene maps, genome sizes, and sequence of metabolic genes would serve to validate the present Rhizobium and Bradyrhizobium phylogenies. More realistic phylogenies should perhaps consider lateral transfer between clusters of bacteria. A compilation of records of bacterial genetic diversity, including enterobacteria and pathogens, is presented and compared with Rhizobium diversity. It is proposed that human activities are having important effects on microbe diversity.