Biological Invaders often undergo adaptive evolution to colonize a new environment. To adapt, invaders must draw on standing variation within the population or acquire new alleles via introgression from related species. Native relatives may be particularly good sources of adaptive variation as they evolved in the invaded range. The invasive apple maggot fly has rapidly spread through the coastal Pacific Northwest, but progress into the arid interior is slow. The species infests apples and hawthorns on the coast, yet only small populations on black hawthorn exist east of the Cascades. In contrast, native snowberry maggots thrive on snowberries in both the coastal and arid climate. We hypothesize that apple maggots on apple have limited desiccation resistance, whereas the native snowberry maggot has drought adaptations that may introgress into the apple maggot via previously observed gene flow. We tested for standing variation within the apple maggot by comparing host associated populations from apple and two hawthorns. We collected individuals of both species and multiple hosts from coastal and arid Washington State and measured pupal water loss at different humidities at day 4 and 8 after leaving the host fruit. After treatment pupae were transferred to a common overwintering environment.
Results/Conclusions
In a transect of native snowberry flies from the coast to the Columbia basin, pupae west of the Cascade crest lost significantly more water under dry conditions than pupae east of the crest. Initial water loss of coastal pupae translates into significantly lower survival after overwintering. This pattern is consistent with local adaptation as pupae from all sites were reared in the same common lab conditions after leaving the moist host fruit. Apple maggots from apple and late fruiting English haw – both found only coastally – showed similar or more severe water loss than coastal snowberry flies suggesting that lower desiccation resistance may hinder colonization of the arid interior. Apple maggots from black hawthorn showed the same desiccation resistance as interior snowberry maggots in both coastal and arid Washington. These findings suggest that both standing variation within the apple maggot itself and local adaptation of a hybridizing sibling species may serve as sources of adaptive variation for the invasive. Genetic architecture of desiccation resistance may constrain drought adaptation in apple flies due to tradeoffs with potentially correlated traits that distinguish apple maggot flies on apple and black hawthorn (life cycle timing) and snowberries (larval performance in fruit).