Population Genetics Of The Symbiotic Nitrogen-Fixing Bacteria Rhizobia

Symbiotic nitrogen-fixing Rhizobia are of global significance, both in terms of their ecological relationships and their importance as an environmentally benign source of nitrogen for crop plants. These bacteria are capable of forming mutualistic relationships with a variety of legume hosts, where they convert atmospheric nitrogen (N-2) to ammonia that is used to help meet the nitrogen needs of the host plant. In this chapter, we review our current understanding of the population genetics of this diverse group of bacteria. First we briefly describe the various types of genomic architectures that are found within rhizobial species. Next we outline the phylogenetic relationships among the recognized taxa. We then summarize the results of studies that have examined patterns of molecular genetic variation in rhizobial populations at local, regional, and global levels. The results of these studies have revealed several important insights, including the existence of extensive diversity within, as well as significant genetic differentiation between local and regional populations. The results also provide evidence for long-distance gene flow between continental populations. Several studies have also indicated the existence of low-to-intermediate levels of recombination within rhizobial populations. Fine-scale studies of specific genomic components (e.g. symbiotic plasmids) have also shown that certain genomic elements appear to be more prone to recombination than others. The results also showed that the different loci responsible for the development of the symbiosis appear to be under different forms of selection. Because most the population studies to date have focused on strains from root nodules, surprisingly little is known on the population genetics of the more numerous non-nodulating soil rhizobia. Future efforts to characterize these populations should significantly enhance our ability to manipulate rhizobial populations in agricultural ecosystems.