The snowberry fly, Rhagoletis zephyria, inhabits a wide geographic range in Washington state traversing the Cascade mountain range; consequently, the habitats of flies in the interior of the state are more arid than habitats in coastal regions and flies are locally adapted to drought conditions. Adaptive variation in desiccation resistance in the snowberry fly may confer greater desiccation resistance to the drought susceptible, invasive apple maggot via gene flow and promote the invasive fly’s expansion into the arid interior of the state. However, the underlying genetic and physiological mechanisms of desiccation resistance in Rhagoletisare currently unclear. Furthermore, weight loss data suggests that snowberry flies locally adapted to arid environments exhibit greater plasticity when placed under desiccation stress. Here, we sought to test for evolved vs. plastic responses to desiccation stress and elucidate mechanisms associated with desiccation resistance by exposing snowberry larvae from populations east or west of the Cascades to ~85% or ~43% relative humidity, then used high thoughput sequencing of mRNA on the Illumina Hi-seq platform to examine differential gene expression between populations and humidity treatments. After identifying differentially expressed genes, we used DAVID to functionally annotate genes and perform enrichment analysis.
Overall, population differences were more numerous than treatment differences. Between populations, 495 genes were differentially expressed under high humidity and 860 genes under low humidity. Within populations, west side flies had similar expression with only 25 genes differentially expressed between treatments while east side flies had 215 genes different between treatments. Under both humidity treatments, east side flies down regulated ribosome associated proteins and RNAs compared to west side flies. In the low humidity treatment, east side flies up regulated oxidoreductases, including cytochrome p450s. The strong population effect on gene expression suggests that evolved differences in transcription underlie differences in drought adaptation between snowberry flies from either side of the Cascades. While humidity treatment did not have as strong of an effect on gene expression, east side flies reacted to humidity treatment while west side flies did not, suggesting that east side flies have evolved the ability to plasticly respond to desiccation stress. This response may relate to waterproofing properties of the cuticle, as certain p450s are involved in cuticular hydrocarbon synthesis. Evolved differences in gene expression among locally adapted snowberry flies could lead to increased desiccation resistance in invasive apple maggots via introgression.