The intellectual and scientific focus of the research centers on how new species of Rhagoletis pomonella fruit flies form. Speciation occurs as genetic barriers to gene flow evolve between populations that cause them to become reproductively isolated from one another. Traditionally, speciation has been thought to require that populations be completely geographically isolated by physical barriers to movement (e.g., mountain ranges, large rivers, oceans). By precluding individuals from moving between populations, geographic isolation results in genetic changes occurring independently between populations which, over time, can accumulate to the point that the populations evolve into reproductively isolated species. The apple maggot fly, Rhagoletis pomonella, has been argued to be a possible exception, however. As early as 1864, Benjamin Walsh hypothesized that certain insects that specialize on attacking specific plants could speciate without geographic isolation ("in sympatry") in the process of them shifting and ecologically adapting to new host plants. In particular, Walsh cited the shift of Rhagoletis pomonella from its native host plant hawthorn (Crataegus) to introduced, domesticated apple trees in the United States - an event that occurred in the 1850's - as a real time example of his idea of sympatric speciation in action. Subsequent genetic studies have confirmed Walsh's hypothesis that the apple and hawthorn-infesting forms of Rhagoletis pomonella are partially reproductively isolated "host races", potentially in the first stages of diverging into new species. An important factor responsible for the divergence of the apple and hawthorn flies is that apple and hawthorn trees fruit at different times of the year, resulting in the one-generation per year life cycles of apple and hawthorn flies having to be offset in order for them to be able to attack apple vs. hawthorn trees. Recently, an interesting twist to the Rhagoletis story has been discovered. Some of the genes responsible for the difference in the life history of apple and hawthorn flies in the United States may have originated over a million years ago in an isolated population of hawthorn-infesting flies in Mexico. These genes were introduced into the U.S. in the distant past during a period when the Mexican and US hawthorn-fly populations came into contact (perhaps 500,000 years ago), and very recently these genes helped facilitate the shift to domesticated apple following the planting of apples in the US by European colonists. Thus, geographic isolation and gene flow may act in conjunction with ecological specialization to help trigger "sympatric" speciation events (i.e., the introduction of apples into the U.S. provided hawthorn flies with a new ecological niche - a temporal resource island - that they were able to take advantage of, in part, because of the genetic variation they possessed due to past gene flow from Mexico). The current grant proposal will test this biogeographic hypothesis thorough an extensive analysis of DNA sequences of genes located throughout the genome of Rhagoletis flies. The research will involve a survey of flies collected throughout Mexico to determine the extent of the geographic range of Rhagoletis flies in the country and pinpoint possible sources of gene flow into the U.S. in the past. In addition, the genetic analysis will indicated whether certain genes and genomic regions have characteristics that made them more or less prone to have moved into the US fly population (e.g., whether or not the genes reside within inversions - stretches of chromosomes that have different linear orders of genes along them - that have been hypothesized to retard gene flow between species). In this manner, the research will resolve the genetic history underlying the recent shift and formation of the new form of apple-infesting Rhagoletis pomonella.
The broader impacts and societal benefits of the research are multifaceted. The project will integrate science training and educational activities at the local, community, University, and international levels. The study will involve the participation of a Post-Doctoral associate and graduate, undergraduate, and high school students in various aspects of the work ranging from field and laboratory research to class modules designed to give high school students and educators hands on experience with molecular genetic techniques and analysis. The funding provided by the current proposal will also expand and strengthen the Population Biology program at the University of Notre Dame at a critical stage of its development. In addition, the proposed research represents an international collaboration between researchers at Notre Dame and two distinguished Mexican colleagues, Dr. Martin Aluja and Dr. Juan Rull, at the Ecological Institute in Xalapa, Veracruz. Drs. Aluja and Rull have received parallel support from CONACyT to fund the collaboration. Finally, the proposed Rhagoletis research has significant applied, as well as basic, scientific implications. Rhagoletis pomonella is a major economic pest of apples, and related species attack cherries, blueberries, and several other crops. The question of the extent and host range of Rhagoletis pomonella in Mexico has important implications concerning the import and export of fruit between the two countries, as well as how new populations of pest species form and can be controlled. The impact of the proposed research is therefore ranges from new basic and applied scientific findings, to educational and societal benefits.
