Genomic evolution of α-proteobacteria in the symbiotic system

One of the major strategies for evolution of bacteria, accompanied by pronounced changes in their genetic organization, is the formation of symbioses with eukaryotes. They provide their micropartners with nutrients and ecological niches, in which bacteria implement trophic or protective functions used by their hosts. Acquisition of the ability for symbiosis is associated with formation of specialized sym gene systems, which is often accompanied by a modification in bacterial genome structure. In nodule bacteria (rhizobia) – N2-fixing symbionts of leguminous plants, most of which belong to α-proteobacteria of order Hyphomicrobiales, the symbiogenic changes in genomes vary depending on the taxonomic position. In the evolutionarily primary rhizobia of family Bradyrhizobiaceae, which emerged directly from free-living N2-fixers, transition to symbiosis is accompanied by a significant (1.5–2 times) increase in the genome size. However, their genomes retain a unitary structure: in the majority of Bradyrhizobium strains, more than 95% of genes are located in chromosomes. In the secondary rhizobia of family Phyllobacteriaceae (Mesorhizobium, Phyllobacterium), which emerged by transfer of sym genes into soil bacteria, various stages were revealed for formation of multipartite genome harboring a significant part of genes in extrachromosomal elements (ECE) – plasmids and chromids. The most pronounced multicomponent genome structure is found in the Rhizobiaceae family (Rhizobium, Sinorhizobium, Neorhizobium), in which the total size of ECEs containing sym genes can exceed the size of chromosome. In these bacteria, transfer from tropical to temperate ecosystems was accompanied by narrowing the host specificity, however, modification of genome structure was revealed only in Sinorhizobium, in which the proportion of ECE increases up to 51% of genome. Such genomes were also revealed in associative (rhizospheric, endophytic) N2-fixers of genus Azospirillum, in which ECE comprise up to 60% of a genome. The irreversibly differentiated N2-fixing cellular forms, bacteroids formed by rhizobia are included into the temporary organelles, symbiosomes considered as precursors of N2 -fixing organelles which are revealed in some protists and are probably may be constructed for the crop plants.