The Chemosensory Receptor Repertoire of a True Shark Is Dominated by a Single Olfactory Receptor Family

Throughout the animal kingdom chemical senses are one of the primary means by which organisms make sense of theirenvironment. To achieve perception of complex chemosensory stimuli large repertoires of olfactory and gustatory receptorsare employed in bony vertebrates, which are characterized by high evolutionary dynamics in receptor repertoire size andcomposition. However, little is known about their evolution in earlier diverging vertebrates such as cartilaginous fish, whichinclude sharks, skates, rays, and chimeras. Recently, the olfactory repertoire of a chimera, elephant shark, was found to becuriously reduced in odorant receptor number. Elephant sharks rely heavily on electroreception to localize prey; thus, it isunclear how representative their chemosensory receptor repertoire sizes would be for cartilaginous fishes in general. Here, wehave mined the genome of a true shark,Scyliorhinus canicula(catshark) for olfactory and gustatory receptors, and haveperformed a thorough phylogenetic study to shed light on the evolution of chemosensory receptors in cartilaginous fish. Wereport the presence of several gustatory receptors of the TAS1R family in catshark and elephant shark, whereas TAS2Rreceptors are absent. The catshark olfactory repertoire is dominated by V2R receptors, with 5–8 receptors in the other threefamilies (OR, ORA, TAAR). Species-specific expansions are mostly limited to the V2R family. Overall, the catshark chemo-sensory receptor repertoires are generally similar in size to those of elephant shark, if somewhat larger, showing similarevolutionary tendencies across over 400 Myr of separate evolution between catshark and elephant shark.