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

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Nathan D. Roueche, Weston J. Staubus, Jennifer L. Hill, Christa Kohnert and Dietmar Schwarz, Biology Department, Western Washington University, Bellingham, WA, Jeffrey L. Feder, Biological Sciences, University of Notre Dame, Notre Dame, IN

Background/Question/Methods
Species with widespread distributions across environmental gradients often exhibit localized adaptation to abiotic conditions. This is particularly true for poikilotherms. The lack of local adaption is often a constraint on the successful establishment and expansion of invasive alien species. Hybridization with locally adapted endemic species has been a frequent occurrence in successful biological invasions, therefore understanding the variation within native populations capable of hybridizing with regionally occurring invasive species is an important and understudied aspect of invasive species management. 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 found throughout the region associated with its nearly ubiquitous host plant Symphoricarpos albus (common snowberry) and is known to hybridize with the invasive orchard pest R. pomonella. R. pomonella range expansion in the region is surmised to have been curtailed at least in part by atmospheric desiccation of emergent larvae in drier regions. Desiccation stress was administered shortly after larval emergence from host fruit.

Results/Conclusions
Using regional bioclimate norms for the period 1981-2010 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 rapidly changing climatic regimes in the region, as well as illustrating the potential for R. pomonella to experience release from bioclimatic constraints via hybridization with native R. zephyria.