Geographic variation in desiccation resistance of Rhagoletis zephyria in the Pacific Northwest

Species with widespread distributions across environmental gradients often exhibit localized adaptation to abiotic conditions. The lack of local adaption is often a constraint on the successful establishment and expansion of invasive alien species. Here we use a common garden experimental design to assess desiccation resistance in Rhagoletis zephyria in the Pacific Northwest. R. zephyria is a locally adapted endemic known to hybridize with the invasive orchard pest R. pomonella. Desiccation stress was administered shortly after larval emergence from host fruit (Symphoricarpos albus, common snowberry), the life stage wherein the fly is most susceptible to atmospheric desiccation.

Using regional bioclimate norms we were able to model the geographic variation of R. zephyria desiccation resistance in the study region as well as determine the influence of seasonal temperature and precipitation regimes on a dynamic phenotypic landscape. While the overall pattern of populations of R. zephyria occurring in the xeric regions east of the Cascade Range expressing increased desiccation resistance when compared to those populations from the highly mesic coastal regions west of the Cascades, variation was also shown to exist within these regions in correspondence with localized climates such as rain shadows and precipitation catchments. This variation in response to abiotic conditions suggests that R. zephyria populations in the Pacific Northwest possess the genetic means of adaptation in the face of changing regional climatic regimes, as well as illustrating the potential for R. pomonella to experience release from bioclimatic constraints via hybridization with native R. zephyria.