Hybridization between an invasive fruit fly and a variably plastic native sibling species

Introgressive hybridization can play an important role in the evolution of species ranges by introducing adaptive variation to populations at the margins. The apple maggot (Rhagoletis pomonella) is an introduced tephritid fly that has become abundant in the moist western counties of Washington State but remains scarce in the relatively arid central and eastern counties. In contrast, the snowberry maggot (R. zephyria), a native congener, is abundant in counties across the state. The difference in the distributions of the two species has been attributed to variation in their desiccation resistance because snowberry maggots, particularly those from central and eastern sites, have been shown to outperform apple maggots in low humidity conditions. Recent population genetic studies suggest that the two species are hybridizing, and that snowberry maggot alleles are asymmetrically introgressing into apple maggot populations. This has led to speculation that adaptive gene flow from snowberry maggot populations might facilitate the expansion of the apple maggot into new territories. In Chapter 1, I conduct laboratory crosses between apple maggots and snowberry maggots from western and central Washington, and contrast the performance of the two types of hybrid offspring with that of apple maggots in low humidity conditions. Both types of hybrids are found to be more resistant to desiccation and more likely to survive to adulthood than apple maggots. I do not observe a difference in the mean desiccation resistances of the two types of hybrids, but I do observe greater variation among hybrids sired by western Washington snowberry maggots than among those sired by central Washington snowberry maggots. These results imply that there is heritable variation in desiccation resistance between apple and snowberry maggots, and perhaps greater genetic variation within the western Washington snowberry maggot population than within the central Washington snowberry maggot population. The presence of such variation raises the possibility that introgressive hybridization with snowberry maggots from either region may lead to more desiccation-resistant apple maggot populations, but further research is needed to better understand the steps in between hybridization and potential adaptation. In Chapter 2, I investigate patterns of gene expression in the same populations of snowberry maggots that sired the hybrids in Chapter 1. Previous work has clearly established that snowberry maggots from the central Washington population lose less weight and are more likely to survive to adulthood than snowberry maggots from the western Washington population when they pupate in low humidity. However, those differences become insignificant in high humidity, suggesting that the biology of the two populations varies under different environmental conditions. I expose larvae from each population to either high or low humidity, and sequence their transcriptomes at the onset of barreling, a key point of transition between the larval and pupal stages. I identify individual genes and modules of genes which are differentially expressed between the populations and/or conditions, and determine whether those differences are environmentally canalized or arise via plasticity. I also discuss the expression patterns as they relate to patterns of desiccation resistance and survival in the two populations, and use Gene Ontology annotations to infer possible cellular responses to desiccation stress. I find that the majority of differences in gene expression between the two populations are attributable to differences in plasticity, and that plasticity is reduced overall in the central Washington population relative to the western Washington population. I also find that a wide range of biological processes are affected by differences in humidity, including protein metabolism, peroxisome activity, and development. My results add to a growing body of evidence that variation in plasticity is an important component of phenotypic variation, and suggest candidate mechanisms of desiccation resistance in snowberry maggots for further investigation.