Largest vertebrate vomeronasal type 1 receptor gene repertoire in the semiaquatic platypus

Vertebrate vomeronasal chemoreception plays important roles in many aspects of an organism's daily life, such as mating, territoriality, and foraging. Vomeronasal type I receptors (VIRs) and vomeronasal type 2 receptors (V2Rs), 2 large families of G protein-coupled receptors, serve as vomeronasal receptors to bind to various pheromones and odorants. Contrary to the previous observations of reduced olfaction in aquatic and semiaquatic mammals, we here report the surprising finding that the platypus, a semiaquatic monotreme, has the largest V1R repertoire and nearly largest combined repertoire of VIRs and V2Rs of all vertebrates surveyed, with 270 intact genes and 579 pseudogenes in the V1R family and 15 intact genes, 55 potentially intact genes, and 57 pseudogenes in the V2R family. Phylogenetic analysis shows a remarkable expansion of the VIR repertoire and a moderate expansion of the V2R repertoire in platypus since the separation of monotremes from placentals and marsupials. Our results challenge the view that olfaction is unimportant to aquatic mammals and call for further study into the role of vomeronasal reception in platypus physiology and behavior.