Molecular Population Genetics and Evolution of Rhizobia

Abstract

Studies of the natural genetic diversity of rhizobia have given us insights into the genetic structure and evolution of bacterial populations, and have also improved our understanding of the nitrogen-fixing symbiosis between legumes and rhizobia, which is important in many agricultural systems worldwide (I use “rhizobia” in the broad sense to cover all legume nodule symbionts). In legume cultivation, the farmer provides the plant partner, which is usually the product of an extensive breeding and selection process, and therefore genetically rather uniform. Sometimes the bacterial partner is added artificially too, in the form of an inoculant containing one or a few strains, but more often there is already a sufficient population of rhizobia living in the soil. Thus the cultivation of legumes is a partnership between a highly bred plant and a “wild” population of bacteria. The farmer has little control over the genetic composition of the indigenous population of rhizobia, and in fact there is abundant evidence that such populations are diverse, even in fields with a long agricultural history. In our laboratory during the last few years, we have tried to describe this diversity in ways that allow us to draw conclusions about the genetic interrelationships among the strains that make up these populations. Most of these studies have been on the species Rhizobium leguminosarum, but recently we have also been looking at the wider issue of the evolutionary relationships among rhizobia, in the broad sense, and between rhizobia and other groups of bacteria. In this contribution I shall summarize our progress to date in both these areas.