EVOLUTION OF SYMBIOTIC GENETIC SYSTEMS IN RHIZOBIA

Probable molecular and population-genetic mechanisms of microevolution of the genetic systems controlling interactions of root nodule bacter ia with legumes, as well as the main pathways of their macroevolution, are reviewed. It is suggested that synchronization of the rates of ba cterial and plant genome evolution was the condition necessary for coe volution of root nodule bacteria and legumes. This could have been ach ieved via an increase in recombination activity of the bacterial genom e during the formation of the virulence gene system. The latter proces s was associated with changes in the pattern of certain genes controll ing different metabolic functions in nonsymbiotic nitrogen-fixing orga nisms (origin of Bradyrhizobium and Azorhizobium, the ''primary'' root nodule bacteria capable of nitrogen fixation ex planta). Increased ge netic instability could have been to the transfer of main symbiotic ge nes into plasmids. This, in turn, resulted in the development of a com plex genetic population structure (origin of ''nonsymbiotic'' subpopul ations providing for a high frequency of horizontal transfer of symbio tic genes) and made possible the development of new rhizobia forms via symbiotic gene transfer to different soil bacteria (origin of Rhizobi um, the ''secondary'' root nodule bacteria incapable of nitrogen fixat ion ex planta). Comparing the patterns of genes controlling interactio ns with plants in Rhizobium and Agrobacterium showed that symbiotic an d parasitic traits of these related microorganisms developed independe ntly, although similar mechanisms could be responsible for evolution o f nodulation in the rhizobia-legume system and for evolution of phytop arasitic systems. It is suggested that both Rhizobium and Agrobacteriu m originated from saprophytic soil microorganisms capable of synthesiz ing certain cell wall molecules (polysaccharides, glucans, etc.) allow ing them to persist in tissues of higher plants.