All-Russia Research Institute for Agricultural Microbiology, Russian Academyof Agricultural Sciences, Podbelsky Sh. 3, St. Petersburg, 189620, Russia

Abstract

The data are reviewed on the population structure and evolutionary dynamics of the nodule bacteria(rhizobia) which are among the most intensively studied microorganisms. High level of the populationpolymorphism was demonstrated for the rhizobia populations using the enzyme electrophoresis (MLEEprofiles). The average value of Nei's coefficient of heterogeneity were: 0, 590for rhizobia (Rhizohium, Bradyrhizohium), 0, 368 for enterobacteria (Escherichia, Salmonella, Shigella)and 0, 452 for pathogenic bacteria (Bordelella, Borrelia, Eryispelotrix, Haemophilus, Heliohacter, Listeria, Mycobacterium, Neisseria, Staphylococcus) populations. In spite of being devoid of the effective systemsfor the gene conjugative transfer, many rhizobia populations possess an essentially panmictic structure. However, the enterobacteria populations in which the gene transfer may be facilitated due to theconjugative F- and R-factors, usually display the clonal population structure. The legume host plant isproved to be a key factor that determines the high levels of polymorphism and of panmixia as well as highevolutionary rates of the symbiotic bacteria populations. The host may ensure: a) an increase in mutationand gene transfer frequencies; b) stimulation of the competitive (selective) processes in both symbiotic andfree-living rhizobia populations. A "cyclic" model of the rhyzobia microevolution is presented whichallows to assess the inputs the interstrain competition for the saprophytic growth and for the hostnodulation into evolution of a plant-associated rhizobia population. The nodulation competitiveness in therhizobia populations is responsible for the frequency-dependent selection of the rare genotypes which mayarise in the soil bacterial communities as a result of the transfer of symbiotic (sym) genes from virulentrhizobia strains to either avirulent rhizobia or to the other (saprophytic, phytopathogenic) bacteria. Therefore, the nodulation competitiveness may ensure: a) panmictic structure of the natural rhizobiapopulations; b) high taxonomic diversity of rhizobia which was apparently caused by a broad sym geneexpansion in the soil bacterial communities. The kin selection models are presented which explainevolution of the "altruistic" (essential for the host plant, but not for the bacteria themselves) symbiotictraits (e.g., the ability for symbiotic nitrogen fixation and for differentiation into non-viable bacteroids) inthe rhizobia populations. These models are based on preferential multiplication of the nitrogen-fixingclones either in plania (due to an elevated supply of the nitrogen-fixing nodules with photosynthates) or explania (due to a release of the rhizopines from the nitrogen-fixing nodules). Speaking generally, interactions with the host plants provide a range of mechanisms increasing a genetic heterogeneity and anevolutionary potential in the associated rhizobia populations.