Evolution of Mechanisms that Control Mating in Drosophila Males
Courtship in Drosophila comprises a complex series of species-specific behaviors controlled by genetic and neural substrates. These behaviors, as well as their underlying mechanisms, can evolve quickly to facilitate reproductive isolation – the reduction of gene flow between populations. This makes Drosophila courtship an excellent model for studying how genetic changes can generate divergent behavioral programs. Members of the chemosensory receptor gene families are required to regulate different aspects of Drosophila courtship. For instance, chemoreceptors allow male flies to discern potential mates in order to restrict courtship towards receptive, conspecific females. In Chapter 1, I review recent studies on the chemosensory control of courtship in D. melanogaster, and highlight key studies on the evolution of courtship behaviors.
In Chapter 2, we identify genetic and neural pathways that prevent D. melanogaster males from courting females of other fly species. The chemoreceptor Gr32a recognizes nonvolatile aversive cues present on heterospecific females and is required to inhibit interspecies courtship. In addition, activity of Gr32a neurons is necessary and sufficient to inhibit this behavior. We extended our work to non-model species of Drosophila to explore how chemosensory pathways that regulate courtship may evolve. In Chapter 3, we show that two closely related fly species use distinct mechanisms to inhibit interspecies mating. In both D. simulans and D. melanogaster, Gr32a is expressed in the male foreleg tarsi, and it is essential for sensing the bitter tastant quinine. However, Gr32a is not required for inhibiting interspecies courtship in D. simulans as it is in D. melanogaster. Although chemoreceptor mechanisms inhibiting interspecies courtship have differentiated, we find that a similar chemosensory pathway promotes courtship in both species.
Many questions remain about the evolution of mechanisms that preclude interspecies courtship. In Chapter 4, I discuss experiments that address these questions. Preliminary results show that chemosensory pathways that detect bitter and sweet tastants may also control mating success, suggesting that such pathways may be appropriated to regulate seemingly disparate behaviors. In summary, comparative genetic and neural studies in closely related Drosophila species can contribute to a greater understanding of how courtship and other innate behaviors evolve.