The symbiosis between legume plants and root-nodule bacteria is important to human welfare and to natural ecosystems, because it provides nitrogen compounds that are often a key limiting resource. Despite more than a century of intensive study of the phenomenon, biologists did not discover until recently that certain legume symbionts belong to a group known as the beta-Proteobacteria. Curiously, these symbionts are not related to any previously known nodule bacteria. At least two separate lineages in the beta-Proteobacteria have evolved to become legume symbionts (including strains of the genera Burkholderia and Cupriavidus). However, this trait is restricted to only a few of the species in these genera, and is highly atypical in the beta-Proteobacteria as a whole. Thus, the evolution of symbiotic ability is a recent innovation in this group, and was likely due to acquisition of symbiotic genes from distantly related bacteria. Such cases of naturally occurring lateral gene transfer between distantly related organisms have the potential to greatly affect the ecological relationships and evolution of bacteria in natural environments. However, biologists have only a limited understanding of these processes. The aim of this project is to analyze DNA sequence variation of both symbiotic beta-Proteobacteria and their legume hosts to reconstruct the biogeography and history of this relationship. The research will focus on symbiotic bacteria from all species of Mimosa plants native to the continental United States in order to determine whether beta-Proteobacterial symbionts have spread out of tropical biomes into temperate regions of North America, and whether the evolution of new legume species in this group has been associated with parallel evolution of new types of bacterial symbionts. Inoculation experiments will also be performed to test whether lateral transfer of symbiosis-related genes from older lineages of nodule bacteria has influenced the amount of specialization exhibited by beta-Proteobacteria toward various plant hosts.
The scientific value of this research will be to provide a broader understanding of the evolutionary mechanisms, including lateral gene transfer, that contribute to diversification of microbial symbioses. In addition, species related to both Burkholderia and Cupriavidus nodule symbionts are important human or plant pathogens, and some of the legume species that host beta-Proteobacterial symbionts are invasive weeds of global concern. Therefore, this research will also help to provide a better understanding of the basic ecology of these bacteria and the frequency of major ecological transitions (e.g., from parasitism to mutualism), in order to better evaluate their impact on the environment and on human health. The project will also foster integration of research and science education by holding workshops for middle school and high school life science teachers to improve teaching practices concerning how scientists analyze organism relationships and phylogenetic history.
