Physiological Mechanisms of Desiccation Resistance in Fruit-Parasitic Rhagoletis Flies

Understanding drought adaptation in fruit-parasitic Rhagoletis flies is essential for evaluating the potential for eastward spread of the invasive apple maggot fly, R. pomonella, from coastal Washington into arid central Washington, which poses a threat to the largest crop of U.S. apples. A closely related native species, R. zephyria, provides an opportunity to study existing drought adaptation in the region as it is locally adapted to drought conditions in central Washington. Here, I aim to elucidate physiological mechanisms underlying desiccation resistance in R. pomonella and R. zephyria, as well as determine if the trait is plastic or canalized in R. zephyria. Pupal diapause could be an advantageous state under drought stress because metabolisms are suppressed, limiting active water loss. To test diapause regulation as a mechanism contributing to desiccation resistance, I observed the proportions of diapausers (vs. direct developers) under high (drought) and low (non-drought) vapor pressure deficits in three host races of invasive R. pomonella and two populations of native R. zephyria to determine if 1) there was past selection on diapause regulation that led to higher proportions of diapausers in drought resistant populations and 2) drought stress affected diapause regulation. R. zephyria lacked direct development completely so diapause regulation cannot account for greater desiccation resistance in populations from arid vs. humid regions in Washington. The proportions of diapausers in R. pomonella were greatest among black hawthorn infesting flies (high desiccation resistance) and similar between apple infesting flies (low desiccation resistance) and ornamental hawthorn infesting flies (intermediate desiccation resistance), and not affected by drought treatment, suggesting diapause regulation is not the primary mechanism contributing to desiccation resistance in the invasive species. Next, I conducted a differential gene expression experiment to explore additional mechanisms and to categorize canalized versus plastic transcriptional responses to drought stress. Gene expression in newly egressed R. zephyria larvae was largely canalized in drought resistant and susceptible populations, though drought resistant larvae responded more to low humidity conditions (relative to humid conditions) than drought susceptible larvae, suggesting that local drought adaptation in R. zephyria is impacted by a genotype x environment interaction. Annotation of differentially expressed genes suggest differences in cuticular hydrocarbon profiles could underlie variable desiccation resistance and highlighted potential differences in development speeds between populations. In conclusion, desiccation resistance in R. zephyria and R. pomonella is likely multi-faceted and the primary mechanism that accounts for variation in desiccation resistance among populations is yet to be identified. Furthermore, desiccation resistance appears to be adaptive to local climates in R. zephyria and potentially constrained by host related fitness tradeoffs in R. pomonella.