Smells like evolution: the role of chemoreceptor evolution in behavioral change

In contrast to physiology and morphology, our understanding of how behaviors evolve is limited. This is a challenging task, as it involves the identification of both the underlying genetic basis and the resultant physiological changes that lead to behavioral divergence. In this review, we focus on chemosensory systems, mostly in Drosophila, as they are one of the best-characterized components of the nervous system in model organisms, and evolve rapidly between species. We examine the hypothesis that changes at the level of chemosensory systems contribute to the diversification of behaviors. In particular, we review recent progress in understanding how genetic changes between species affect chemosensory systems and translate into divergent behaviors. A major evolutionary trend is the rapid diversification of the chemoreceptor repertoire among species. We focus mostly on functional comparative studies involving model species, highlighting examples where changes in chemoreceptor identity and expression are sufficient to provoke changes in neural circuit activity and thus behavior. We conclude that while we are beginning to understand the role that the peripheral nervous system (PNS) plays in behavioral evolution, how the central nervous system (CNS) evolves to produce behavioral changes is largely unknown, and we advocate the need to expand functional comparative studies to address these questions.