Invasive and native apple maggot sibling species differ in desiccation resistance

Background/Question/Methods

The invasive apple maggot fly (Rhagoletis pomonella) is largely absent from the arid regions of the Pacific Northwest, while its native sibling, the snowberry fly (R. zephyria), is common throughout the region. Because these two species are so closely related, hybridization, leading to genetic introgression through fertile offspring, is not uncommon. This system gives us the opportunity to test whether an invasive insect pest could gain alleles from a native relative that would enable it to become better adapted to its invaded environment. We propose that one of the factors limiting the apple fly to the west of Washington is the much drier climate in the interior of the state, and that this is a factor more easily tolerated by snowberry flies. We have placed newly formed pupae of both species in dry conditions akin to those in the interior of Washington during the normal fruiting season, before allowing them to complete diapause. We have compared the rates at which each population lost water weight, as well as the relative rates of emergence of each population. 

Results/Conclusions

When exposed to dry conditions, R. zephyria individuals collected from western Washington have a greater resistance to desiccation than do their R. pomonella counterparts. The snowberry flies tend to lose water at a slower rate than apple flies, and retained a higher proportion of their weight after treatment, despite having a less favorable surface area/volume ratio. In addition, R. zephyria samples from central Washington show little desiccation at all when exposed to the same conditions, suggesting that these native flies have become locally adapted to the drastically different climates east and west of the Cascade Mountains. These experiments are part of a genome association study to identify regions of the genome associated with desiccation resistance and introgression between native and invasive fly species.