Factors shaping the maintenance of variation in a symbiont-mediated host-enemy interaction

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In the battle against natural enemies, it has long been thought that animals are left to their own defenses. However, recent findings indicate that some rely on the defensive services of microbial symbionts. Pea aphids, for example, harbor maternally transmitted symbionts that defend them against parasitic wasps, which are used as biological control agents against these crop pests. When assessing variation in resistance to the wasp Aphidius ervi, it appears that inherited differences are driven by the presence of bacterial Hamiltonella defensa symbionts and the types of toxin-encoding bacteriophages (viruses) they harbor. Despite significant benefits conferred by these defensive elements, their presence is not universal within natural aphid populations, suggesting costs of infection and inefficient defense under some conditions. In this study, the researchers propose to explore the forces responsible for natural variation in the presence of defensive symbionts, examining the effects of temperature and levels of parasitism on symbiont prevalence. Predictions of reduced symbiont frequencies in response to high temperatures and infrequent parasitism will be tested through extensive field-based and molecular studies that quantify temperature and both symbiont and parasite prevalence over space and time. Similar approaches will be used to test the benefits of symbionts under natural conditions. Additionally, the investigators will elucidate the mechanisms of defense, focusing on the bacteriophage toxins that are expected to drive variation in resistance. Findings from this work will determine how these species interactions are expected to change with increasing global temperatures. They will also help in the design of more effective biological control programs, through identifying the mechanisms that crop pests employ to overcome parasitism. This research will result in the training of two graduate and several undergraduate students. Through publication and presentation at scientific meetings, and through integration into undergraduate coursework, the investigators will disseminate the findings through both traditional and novel approaches.