Almost all living things require mutualists to carry out basic biological functions. Plants need bacteria to grow, animal pollinators to reproduce, and insects to protect themselves from grazing herbivores. Aside from there being resource-exchange, reproductive, and protective mutualisms, there are other kinds of symbioses such as parasitism and commensalism. Sometimes these unique interactions can overlap and cause conflicts between the hosts and their symbionts. In the Wood lab, with the help of the National Science Foundation grant, we study the cross between parasitism and resource-exchange mutualism. Specifically, we observe the degree to which resource-exchange (legume-rhizobia) mutualisms are susceptible to parasitism. We presume that parasitic infection complicates the cooperation that exists between legumes and nitrogen-fixing rhizobia. To further observe this phenomenon, we use a tripartite system involving three model organisms: the Medicago truncatula legume (host), Ensifer meliloti rhizobia (mutualistic partner), and Meloidogyne hapla nematode (parasite). The rhizobia house themselves as nodules and the nematodes form galls on the roots of the plants. In order to quantify the degree to which mutualism and parasitism occurs on the legumes, we count the number of nodules and galls formed on the roots, as well as take biomass measurements of these plants’ features. Some members of our lab have performed elemental analyses to keep track of how much nitrogen the rhizobia are distributing to the plants and how much carbon the plants are giving back to the rhizobia. This helps us monitor how much the host and their symbionts are benefiting or losing from this three-way interaction. Using a reciprocal design with five rhizobia genotypes and two legume genotypes, where one experimental group received nematodes and rhizobia while the other only received rhizobia, it was found that parasites do cause changes to the legume-rhizobia mutualism. Therefore, the legumes’ susceptibility to parasitism can be considered a deciding feature of the mutualistic relationship between the legumes and rhizobia.