Adaptive Variation in Desiccation Resistance in Rhagoletis

Despite the ever-present challenges associated with invasive species, many environmental barriers exist that limit the spread of exotics. However, there is a growing number of examples of species overcoming these constraints via adaptive evolution years or decades after their initial introduction. The necessary genetic variation stems either from hybridization with a closely related species, or from shifting allele frequencies from standing variation in the population. Since its introduction to the Pacific Northwest, the apple maggot fly, Rhagoletis pomonella (Walsh 1867), has invaded all of coastal Washington, but has only small, isolated populations in the central and eastern parts of the state. The Cascade Mountains form a rain shadow that restricts the amount of precipitation in these regions, making it much drier than the western parts of Washington. I investigated aridity as an environmental constraint for the spread of R. pomonella, as well as potential sources of genetic variation for desiccation resistance in sympatric populations. First, I tested the potential for dry conditions, like those in the interior of Washington, to influence fitness in Rhagoletis flies, and act as a factor limiting their distribution. I found that individuals from a wetter part of Washington did not survive as well in dry conditions, but that individuals from a drier location were unaffected by desiccation treatment. The percent of weight that each pupa had remaining after treatment was the best predictor of survival. Second, I examined the variation in desiccation resistance in R. pomonellaʼs native sister-species, R. zephyria across a finer environmental gradient, to look at possible adaptive variation within the species. I found that R. zephyria pupae from west of the Cascade Range show less desiccation resistance than those east of the Range, and that this pattern is indicative of local adaptation. Average annual precipitation and elevation of each transect site were the best predictors of how much weight each pupa would retain after desiccation treatment. Finally, I measured desiccation resistance between apple-infesting R. pomonella, and hawthorn-infesting R. pomonella to begin to gauge the possible standing variation present in the speciesʼ genome. I found that the hawthorn host-race shows significantly more desiccation resistance than the apple host-race. This could be because the hawthorn hostrace must endure a longer pre-winter diapause period, when conditions would be less favorable for water-balance strategies. The factors limiting the spread of R. pomonella are complex, but variation exists in both a native sister species and a sympatric host race. Determining whether there is a genetic factor associated with resistance would allow us to begin to gauge the relative importance of introgression and standing variation in the invasion of R. pomonella into the Pacific Northwest.