As a result of both deliberate and accidental transport across regions, many species of legumes have become invasive weeds that have greatly altered the ecology of plant communities throughout the world. The disproportionate ecological impact of legumes is strongly related to their symbiosis with root nodule bacteria (rhizobia). This symbiosis provides legumes with an abundant supply of usable nitrogen, an essential nutrient that is often a primary limiting factor for plant growth. However, the influence of this symbiosis on invasion by legumes is poorly understood. Legumes must acquire rhizobia from their immediate environment following seed germination, because bacteria are not carried internally within seeds. As a result, introduced legumes will often arrive at new habitats where well-adapted mutualist partners from their ancestral range are nonexistent. Even if legume colonists have sufficiently unspecialized nodulation behavior to allow them to use bacterial strains obtained from other legume taxa indigenous to a site, there is no reason to expect that such strains will be optimally adapted to the novel host. Invasion success may thus depend on whether colonist plants can select for more beneficial symbiotic partners over time. To date, there has been no research to test whether evolutionary changes in symbiotic compatibility are an important factor in legume range expansion. This project will use the highly invasive legume Scotch broom (Cytisus scoparius) as a model system to answer several fundamental questions about demographic and evolutionary processes affecting biological invasion by legumes. Scotch broom is native to Europe, but is an aggressive invader on four other continents. The specific objectives of the project are: 1) to use DNA sequencing of ribosomal genes to identify the geographic origin of rhizobial symbionts for invasive Scotch broom plants in two separate regions of the U.S. These bacteria will be compared to samples from a broad range of other legumes species native to both Europe and North America. 2) to test whether adaptation of nodule bacteria to invading legumes has involved horizontal transfer of bacterial genes encoding symbiotic traits. DNA sequencing will also be performed on two bacterial genes essential to the nitrogen-fixation symbiosis, to test whether the genealogical relationships of different DNA regions have been altered by transfer of genetic material between bacterial strains; 3) to use field inoculation experiments to determine whether the demographic performance of legume colonists is limited by a scarcity of compatible rhizobia. The growth of plants in natural environments with and without exposure to supplemental root-nodule bacteria will be compared; and 4) to evaluate whether the rhizobia available to legume invaders are of inferior quality, by comparing how plants grow when inoculated with bacteria from the plant's ancestral range (Europe) vs. bacteria from newly colonized North American sites.
One of the most serious types of human environmental impact on natural ecosystems involves activities that have caused species to invade new geographic regions. By providing insights about the role of microbial symbionts in legume range expansion, this project will contribute to a better understanding of biological invasions, and will also generate concepts and methods relevant to specific programs for managing a variety of invasive legume weeds.
