The Frontier of Ecological Speciation: Investigating Western Populations of Rhagoletis pomonella

As scientists, we don't often have the opportunity to observe the early stages of a biological introduction and even more rare to witness the beginnings of a speciation event. However, the discovery of the apple maggot, Rhagoletis pomonella, in the western US offers a unique opportunity to study both of these events in an agriculturally and economically important system that is well known as the model for the study of incipient sympatric speciation. Its recently observed introduction to the western United States allows for the testing of hypotheses regarding the origins of western populations. In addition, discovery of its attack on native black hawthorns (Crataegus douglasii) and invasive ornamental hawthorns (C. monogyna) are relevant to studies on sympatric host shifts of phytophagous insects as a result of divergent selection.

The goal of my dissertation was to test for evidence supporting the hypothesis that populations of R. pomonella in the West and Mountain West in the United States are the result of a recent introduction and are not native, to test for evidence of recent host shifting from apple to black and ornamental hawthorn in the West that has resulted in ecological host race formation, and discuss a novel method for the control of invasive species. We tested for evidence of an introduction by genotyping populations from the western range of R. pomonella and made comparisons between western populations and known native populations from the East and the South. We tested for evidence of host race formation by performing behavior assays to test for differences in behavior and by identifying differences in allele frequencies and loci under divergence between different host-associated populations in the West. Lastly, we suggest the concept of 'invasivory,' meaning to eat invasive species, as a novel method for control of invasive species populations.

Our results show that there is genetic evidence that populations from the West in Washington State are likely the result of an apple fly introduction from the East and populations from the Mountain West in Utah and Colorado are likely the result of a red hawthorn fly introduction from the East. In addition, behavior studies show attraction to natal host-odor blends and avoidance of non-natal host-odor blends by all host-associated populations of R. pomonella. Lastly, there is genetic evidence for host race formation in the West through the identification of loci showing divergence in pairwise comparisons. The loci under divergent selection in the West are the same loci under divergent selection in the East, which suggests that the same traits and underlying genes are under selection. In addition, there is evidence for gene flow between different host-associated populations at sympatric sites, which indicates that selection must be strong for divergence to still be occurring. Lastly, though 'invasivory' is an interesting experiment, the act of eating invasive species is unlikely to result in changes in invasive species populations. However, through public outreach, it is possible that 'invasivory' as a concept can be used as an educational tool to bring attention to the threat of invasive species and thereby result in the prevention invasive species establishment through vigilance and changes in public policy.

The work throughout my dissertation makes contributions to our knowledge of invasive species, ecological divergence, and host race formation in sympatry. This thesis has revealed a new component to the natural and evolutionary history of the Rhagoletis pomonella system in the context of invasion and speciation biology.