The evolutionary shift from aqueous to airborne olfaction: intermediate and novel properties in an amphibian, transitional species

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The vertebrate olfactory sense consists of several subsystems, which are anatomically separated in mammals. These subsystems traditionally were assumed to mediate segregated functions: the main olfactory epithelium containing ciliated olfactory receptor neurons was supposed to mediate food detection, whereas the vomeronasal organ containing two classes of microvillous receptor neurons was believed to mediate pheromone detection (note that the current picture is more complex). In fishes the corresponding cell types and homologous olfactory receptor families are present, however all are intermingled in a single olfactory epithelium. Fish olfactory receptor repertoires show marked similarities, but also distinctly different properties compared to their mammalian homologs. Large differences in function are expected and have been partially demonstrated. So far it is completely unclear, how the shift in organization and function of the olfactory system occurred. Amphibians represent an evolutionary intermediary between fishes and mammalian species with respect to life style (partially aquatic), anatomical specialisation (separate vomeronasal organ), and evolution of olfactory receptor gene families. We will combine advanced physiological and molecular biological approaches to deorphanize olfactory receptor neurons and olfactory receptor genes in the clawed frog, Xenopus lavis.