In this international collaborative project researchers from U.S. and Canada will perform experiments to study the role of cuticular hydrocarbons (CHC) in species recognition and the maintenance of boundaries between two closely related species of mushroom-feeding flies, Drosophila subquinaria and Drosophila recens. Male contact pheromones consist of a suite of CHCs, and preliminary data suggests that these differ between the two species. Three specific aims will be pursued to test the hypothesis that male CHCs are the key signal traits that are used by females during mate choice and that underlie species discrimination, and that these traits and the female preferences for them have diverged among populations in response to reinforcing selection generated by the presence of the other species. The results of these studies will lay the groundwork for a long-term collaborative research program that will shed light on how behaviors diverge to generate reproductive isolation and ultimately speciation.
Knowledge of the male signals and female preferences will inform hypotheses about whether the genetic basis of within vs. between species female discrimination is the same, and how simple vs. complex we expect these traits to be at the genetic level. The genetic basis of female discrimination and evolutionary genomics of species differentiation in these fly species might be more generally applicable to an understanding of the behavior, ecology, and genetics of reproductive isolation. The data resources will be made available to the community. This project will provide the opportunity to develop a long-term collaboration between early career researchers in the U.S. and Canada and will involve training of students in both laboratories, including women and ethnic minorities. Involvement of junior researchers, especially from groups underrepresented in science, in collaborative international activities is a major goal of OISE.
How do new species arise? The processes and mechanisms by which a single species splits into two is a fundamental question in biology. The long-term goal of this research is to understand the selective processes and behavioral mechanisms that are key to various stages of the speciation process. As a foundation for achieving this goal, this project investigated how females recognize and choose among males with which to mate. Specifically, it identified the key male signal traits used by females for mate choice and species discrimination, and tested whether these traits and female preferences for them have diverged among populations. This study utilized the closely related North American fly species Drosophila subquinaria and D. recens. D. subquinaria females from populations where D. recens is also found will not mate with D. recens, whereas D. subquinaria females from outside the region of overlap will mate with D. recens males. Moreover, these choosy D. subquinaria females also discriminate against males from their own species from distant populations, suggesting the early stages of speciation within D. subquinaria. D. recens shows neither of these patterns. This pattern of behavioral isolation is consistent with a history of strong selection on D. subquinaria females to avoid mating with D. recens males, likely as a result of low hybrid fitness. The central outcome of this research was to show that in these species male pheromones are key signal traits used by females during mate choice and that underlie species discrimination, and that these traits have diverged among populations in manner consistent with selection generated by the presence of the other species. We accomplished this using several lines of evidence. First, using sensory ablation techniques, we demonstrated that male olfactory cues are essential for mating by D. subquinaria females. Second, we showed that male pheromones are a target of sexual selection in both species, consistent with female mate preferences for these traits. Third, we manipulated male pheromones, and showed that this alters patterns of female mate discrimination in D. subquinaria, both with respect to D. recens males and their own males. Fourth, we showed that a common bacterial infection in D. recens is not a cue in the discrimination of D. subquinaria females against D. recens males. Fifth, we chemically identified the pheromones and showed that both species share the same suite of compounds. Furthermore, D. recens and D. subquinaria males differed extensively in relative concentrations of many of these compounds, whereas none differed between the females of the two species, suggestive of differences in sexual selection on males. Finally, we examined variation in pheromone composition among multiple populations of each species, and showed that populations differ in these traits in a pattern consistent with mate preferences, strongly suggesting pheromones as a target of selection. Several broader impacts resulted from this funding. First, this funding enabled the development of an international collaboration between research groups at the University of Georgia and the University of Ottawa. Second, more than a dozen high school, undergraduate, and graduate students received training and mentoring in all stages of the scientific process, including experimental design, behavioral and statistical analyses, and writing and oral presentation methods. Third, these students also gained experience in mentoring younger researchers, and in working collaboratively, including communication, organization, and writing skills. Finally, several of these undergraduate students were from groups underrepresented in science, and nearly all matriculated into graduate programs.