Insight into the evolution of symbiosis in the Cupriavidus genus: high conserved symbiotic island and a patchy phylogenetic distribution.
Currently, there are three recognized rhizobial genera belonging to the beta branch of the proteobacteria; Trinickia, Paraburkholderia, and Cupriavidus. These beta-rhizobia have been found associated with legume species mainly within the Mimosoideae and Papillonoideae. Most diversity, evolutionary, and functional studies have focused on Paraburkholderia, whereas few have addressed the diversity and evolution of symbiosis in the Cupriavidus genus. The present work aimed to provide an actual view of the symbiotic Cupriavidus diversity and to analyse the origin and evolution of their symbiotic genes. Using whole-genome information for phylogenetic reconstruction, we showed that the described symbiotic Cupriavidus strains belong to five distinct lineages, although they are intermixed with non-symbiotic species. The high synteny and sequence conservation of symbiotic genes suggest a common origin of acquisition for all rhizobial Cupriavidus described so far. However, we observed very low sequence conservation among (mega)plasmids carrying the symbiotic island, excluding the existence of a conserved symbiotic plasmid within beta-rhizobia. We can conclude that up to now there are five rhizobial species within the Cupriavidus genus, and we predict the description of new symbiotic species in the near future.