Abstract: Adnan SYED

Ancestral amphibian V2Rs are expressed in the main olfactory epithelium

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Adnan Syed, Sigrun I. Korsching

Institute of Genetics, University of Cologne, Zülpicher Strasse 47a, 50674 Cologne, Germany

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The sense of smell helps animal species to evade predators, localize prey and recognize viable mates. In mammals olfactory receptor families are segregated into different olfactory organs, main olfactory epithelium (MOE) and vomeronasal organ (VNO). In contrast, teleost fish olfactory receptor families are intermingled in a single sensory surface. To what extent such differences influence the coding and discrimination abilities of the respective olfactory systems is unclear, and the evolutionary path toward such segregation is unknown. The analysis of amphibians, which are early diverging tetrapods compared with mammals, may shed light on this transition from shared sensory surface to segregated subsystems.
For this study, we focused on the V2R (vomeronasal type 2 receptor) gene family, by thorough datamining we showed the Xenopus V2R family to encompass nearly 500 genes in total. A phylogenetic analysis led to the identification of three distinct subdivisions in the largest group of V2R genes (A1, A2, A3). We used this sequence information to clone several Xenopus laevis V2R gene representatives of above mentioned subdivisions. Xenopus laevis (African clawed frog), is the system of choice for physiological studies of the amphibian olfactory system and furthermore, a close relative of Xenopus tropicalis. We report here that to our surprise, several V2R genes were expressed exclusively in the MOE, and not in the VNO. These genes occupied basal positions in the phylogenetic tree, whereas late diverging V2R genes were exclusively expressed in the VNO. Moreover, within the MOE V2R genes are expressed in a basal zone, partially overlapping, but clearly distinct from an apical zone of OMP and odorant receptor-expressing cells. The unique bimodal V2R expression pattern in main and accessory olfactory system of amphibians presents an excellent opportunity to study the transition of V2R gene expression during evolution of higher vertebrates.