Type 2 vomeronasal receptor expression in the olfactory organ of African lungfish, Protopterus annectens.

The olfactory organ of tetrapods, with few exceptions, comprises the main and accessory organs: olfactory epithelium (OE) and vomeronasal organ (VNO). Unlike tetrapods, teleost fish lack a VNO. However, lungfish, a type of sarcopterygian fish closely related to tetrapods, possesses a lamellar OE similar to the OE of teleosts and a recess epithelium (RecE) resembling the amphibian VNO. The RecE has been hypothesized as a primordial VNO. Olfactory receptors in tetrapods are distinctively expressed in the OE and VNO. For instance, type 2 vomeronasal receptors (V2Rs) in Xenopus are categorized into those exclusively expressed in the OE and those solely expressed in the VNO. It remains unclear whether V2Rs are differentially expressed between the lamellar OE and RecE in lungfish. This study investigated V2R expression in the lamellar OE and RecE of the African lungfish, Protopterus annectens. P. annectens V2Rs were categorized into three groups: those exclusively expressed in the lamellar OE, those exclusively expressed in the RecE, and those expressed in both the lamellar OE and RecE. V2Rs exclusively expressed in the RecE and those expressed in both the lamellar OE and RecE formed a distinct clade in the phylogenetic tree, whereas others were solely expressed in the lamellar OE. These findings suggest that lungfish V2R expression represents an intermediate stage toward complete segregation between V2Rs expressed in the OE and those expressed in the VNO.