The role of diapause and host fruit odor preference in sympatric race formation of Rhagoletis pomonella.

The Rhagoletis pomonella species complex is a paradigm for sympatric speciation. Sympatric speciation hypothesizes that adaptation to alternative environments, without geographical barriers, is an initial trigger for population divergence. Walsh (1867) proposed that races of R. pomonella infesting the ancestral host hawthorn (Crataegus spp.) and the introduced apple (Malus pumila) were an example of sympatric speciation in action. The goal of my dissertation was to determine the roles that diapause and host odor identification, both crucial adaptive differences between host races and sibling species, play in R. pomonella race formation and speciation. Diapause traits act as an allochronic premating barrier to gene flow because populations eclosing at different times cannot freely interbreed. Previous studies hypothesized that warm prewinter conditions exert strong selection pressures on apple and southern populations to avoid developing a non-diapause generation and that eclosion order in nature is not predicted by allozymes. These hypotheses were tested by raising flies under standardized conditions in the laboratory. The results showed that the apple and southern populations were less prone to non-diapause development and adult emergence did parallel eclosion order in nature. Host odor recognition is significant because Rhagoletis flies mate exclusively on their host plants fruit. Host choice, therefore, translates directly into mate choice, forming a premating barrier to gene flow. Studies have shown that flies prefer their natal volatiles. Here, hybrid generations between host races and sibling species were raised and tested for their preference to fruit volatiles. Surprisingly F1 hybrids showed limited responses to fruit volatiles, suggesting that the hybrids lack of behavioral response to fruit odor may be a post-mating barrier to gene flow. Parental responses to fruit odor were reconstituted in a proportion of F2 and backcross progeny (30-50%). In conclusion, the results add another layer of complexity to understanding speciation in Rhagoletis. The evidence suggests the genetic architecture of diapause and fruit odor discrimination is complicated but resolves that both traits serve as pre- and post-mating barriers to gene flow. Characterization of the traits responsible for reproductive isolation and speciation is therefore possible in R. pomonella.