High-throughput sequencing technology will be used to identify mutations that influence the fitness of rhizobia that are symbiotic partners of legumes, providing a window into the ways that rhizobia adapt to the symbiotic lifestyle. Use of experimentally evolved populations of rhizobia derived from the model rhizobium 'Sinorhizobium meliloti' will enable direct measurement of the extent of host-symbiont conflict by asking whether mutations that confer higher rhizobia fitness have positive or negative effects on host performance. Identification of mutants will lead to development of testable mechanistic hypotheses and macro-evolutionary predictions, connecting laboratory results to natural populations. The symbiosis between legumes and rhizobia, bacteria that form root nodules and fix atmospheric nitrogen, is among the best model systems for understanding coevolution. While the interaction is widely assumed to be mutually beneficial, there is ongoing debate over the extent to which rhizobia can cheat their host plants by receiving plant resources without providing fixed nitrogen in return. The rhizobia-legume symbiosis is a key component of sustainable agriculture, as legumes can grow without oil-derived fertilizers and enrich soil fertility over the long-term when used in crop rotations. Legumes include the economically important crop species soy, alfalfa, pea, bean, chickpea, and peanut that provide plant-based protein for human and animal nutrition. Thus, increased understanding of the rhizobia genes implicated in this study may prove useful to agriculture for designing more efficient legume-rhizobia symbioses.
