Remarkable diversity of vomeronasal type 2 receptor ( OlfC ) genes of basal ray-finned fish and its evolutionary trajectory in jawed vertebrates

The vomeronasal type 2 receptor ( V2R , also called OlfC ) multigene family is found in a broad range of jawed vertebrates from cartilaginous fish to tetrapods. V2R s encode receptors for food-related amino acids in teleost fish, whereas for peptide pheromones in mammals. In addition, V2R s of teleost fish are phylogenetically distinct from those of tetrapods, implying a drastic change in the V2R repertoire during terrestrial adaptation. To understand the process of diversification of V2R s in vertebrates from “fish-type” to “tetrapod-type”, we conducted an exhaustive search for V2R s in cartilaginous fish (chimeras, sharks, and skates) and basal ray-finned fish (reedfish, sterlet, and spotted gar), and compared them with those of teleost, coelacanth, and tetrapods. Phylogenetic and synteny analyses on 1897 V2R s revealed that basal ray-finned fish possess unexpectedly higher number of V2R s compared with cartilaginous fish, implying that V2R gene repertoires expanded in the common ancestor of Osteichthyes. Furthermore, reedfish and sterlet possessed various V2R s that belonged to both “fish-type” and “tetrapod-type”, suggesting that the common ancestor of Osteichthyes possess “tetrapod-type” V2R s although they inhabited underwater environments. Thus, the unexpected diversity of V2R s in basal ray-finned fish may provide insight into how the olfaction of osteichthyan ancestors adapt from water to land.