Hagfish olfactory repertoire illuminates lineage-specific diversification of olfaction in basal vertebrates

Chemosensory receptors for olfaction of vertebrates have undergone remarkable diversification, but the ancestral repertoire remains incompletely understood. Here, we present a comprehensive genome-wide survey and gene expression analyses of the olfactory repertoire in hagfish, a cyclostome group that typically inhabits the deep sea with a well-developed olfactory system. We identified 48 olfactory receptors (ORs), two vomeronasal type 1 receptors (V1Rs), 135 typical vomeronasal type 2 receptors (V2Rs), and no trace amine-associated receptors (TAARs) in the inshore hagfish, Eptatretus burgeri. Expression analysis confirmed that most of these genes are expressed predominantly in the olfactory organ, supporting that these receptors are functional. Our findings suggest that typical olfactory V2Rs were present in the common ancestor of all vertebrates. This study highlights the evolution of vertebrate olfaction in a lineage-specific manner, underscoring the importance of hagfish as a key animal for reconstructing the evolutionary history of early vertebrates.