Cascading adaptation to abiotic environmental variation across two trophic levels of parasites

Specialist parasites are intimately adapted to the biotic environment created by their hosts. However, many parasites also have free-living life stages that are exposed to the abiotic conditions outside their host organism. As host and parasite co-occur in the same habitat they are exposed to the same abiotic stressors. In order to track the their host across climatic gradients in space parasites must be able to adapt to abiotic environmental variation. The environment experienced by the host may therefore lead to “cascading” adaptations in specialist parasites that may extend across multiple trophic levels. Snowberries are an ubiquitous plant in the Pacific Northwest and occur an both humid coastal areas and arid rain shadows. Its berries are parasitized by the larvae of the snowberry maggot fly, Rhagoletis zephyria, that is in turn host to larval braconid parasitoids. We collected R. zephyria larvae from >40 geographic locations spanning several climatic gradients in the Pacific Northwest and exposed both un-parasitized and parasitized larvae to desiccation treatment in the lab, targeting the vulnerable stage between leaving the host fruit and pupariation in the soil in late summer. We found that climatic factors explained >57% of variation in fly desiccation resistance with flies from more arid locations showing greater drought resistance. Fly desiccation resistance was a strong predictor of parasitoid desiccation resistance (R²>71%). These phenotypic data are consistent with the abiotic variation across the geographic range driving cascading adaptation to drought across two trophic levels of snowberry parasites.