Thermal breakdown in host-parasitoid interactions is driven by a viral symbiont

Understanding how high temperature events impact interactions between insects and their natural enemies is a critical challenge in the era of Anthropogenic climate change. Parasitoids are important top-down controllers of insect herbivores, but recent work shows they generally have lower thermal tolerances than their hosts. The reason for this phenomenon is unclear. As a case study, we tested the physiological and transcriptomic causes of thermal breakdown in ecological interactions between a host (Manduca sexta) and parasitoid (Cotesia congregata), and found evidence that the parasitoid’s endosymbiotic virus is the limiting factor in the tri-partite ecological interaction. Immunosuppression of the host insect by the parasitoid’s endosymbiont is necessary for successful parasitism, but prior exposure to a 24 hour heat shock disrupted this function, resulting in partial recovery of host immune responses. At the transcriptomic level, the virus displayed patterns of altered gene expression mediated by its circular genomic structure. This work adds to a growing body of literature suggesting symbionts are vital determinants of insect thermal tolerance, and provides a framework for understanding how genetic and molecular processes give rise to organismal and ecological outcomes in response to high temperature events caused by climate change.