The Structure and Evolution of Beta-Rhizobial Symbiotic Genes Deducedfrom Their Complete Genomes

Abstract Nitrogen-fixing Rhizobia were discovered more than 100 years ago. They are classified into two clades, Alphaand Beta-rhizobia. Their symbiotic function is remarkable, but its origin and evolution has been confusing from a phylogenetic perspective. In this study, we make use of 33 publicly available complete genome sequences downloaded from NCBI, which consist of bacteria and archaea, and focus on 10 strains, constructing symbiotic structural maps for them based on their genomes and previous gene annotations. Phylogenies of the symbiosisessential genes nodA and nifH were examined. Although large incongruities with some hypotheses from previous studies were detected by the present study, we support the general concept that Beta-rhizobia were the original symbionts of legumes, but that their symbotic genes originated from a common ancestor to the Alpha-rhizobia. We also confirm that the spread and maintenance of symbiotic genes occurred mainly through vertical transmission, with lateral transfer playing a significant, albeit supporting, role.