CAREER: Microbial extended phenotypes as drivers of host defense trait evolution

Many plants and animals rely on beneficial microbes for essential nutrients. As a result, these microbes are important for their host’s growth and survival. However, these microbes do not provide a free lunch: the host pays for microbial nutrients by providing resources like sugar back to the microbes. This project will test whether this cost also affects the host’s ability to defend itself. The researchers will address this question in legume plants. Legumes rely on a beneficial microbe for an essential nutrient (nitrogen), which they pay for with sugar. The researchers will test how this beneficial microbe affects the evolution of leaf defensive hairs, an important plant defense. These hairs prevent insects from eating the plant. If beneficial microbes affect leaf defensive hairs, then it means that they can influence whether their host will be able to evolve resistance to natural enemies like insects. This project will also train undergraduates in the fundamentals of field biology. The training will include teaching the next generation of field biologists to develop hypotheses, design experiments to test them, and analyze data. The overarching goal is to lower the barrier of entry to field biology for young scientists. This project will test the hypotheses that microbial symbionts affect defense trait evolution in their hosts, and that this effect is mediated by the resources that they provide and consume. The proposed work will close the existing gap between the hypothesized and realized contributions of microbial symbionts to host trait evolution by (1) determining how heritable variation in host traits arises from genetic variation in symbiont populations; (2) linking symbiont effects on host defense traits to the host resources they provide and consume; and (3) directly testing whether microbial symbionts contribute to host trait evolution in the wild. The resource-exchange mutualism between the legume Medicago lupulina and nitrogen-fixing Ensifer bacteria will be used to test these hypotheses. This proposal represents a novel integration of microbial symbioses into the evolutionary ecology of defense via the overlapping resource physiologies of symbiosis and defense. It bridges the historical gap between the study of mutualistic and antagonistic interactions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.