A transcriptomic reevaluation of the accessory olfactory organ in Bichir (Polypterus senegalus)

Fish possess one olfactory organ called the olfactory epithelium (OE), by which various chemical substances are detected. On the other hand, tetrapods possess two independent olfactory organs called the main olfactory epithelium (MOE) and vomeronasal organ (VNO), each of which mainly detects general odorants and pheromones, respectively. Traditionally, the VNO, so-called concentrations of vomeronasal neurons, was believed to have originated in tetrapods. However, recent studies have identified a primordial VNO in lungfish, implying that the origin of the VNO was earlier than traditionally expected. In this study, we examined the presence/absence of the VNO in the olfactory organ of bichir (Polypterus senegalus), which is the most ancestral group of extant bony vertebrates. In particular, we conducted a transcriptomic evaluation of the accessory olfactory organ (AOO), which is anatomically separated from the main olfactory organ (MOO) in bichir. As a result, several landmark genes specific to the VNO and MOE in tetrapods were both expressed in the MOO and AOO, suggesting that these organs were not functionally distinct in terms of pheromone and odorant detection. Instead, differentially expressed gene (DEG) analysis showed that DEGs in AOO were enriched in genes for cilia movement, implying its additional and specific function in efficient water uptake into the nasal cavity other than chemosensing. This transcriptomic study provides novel insight into the long-standing question of AOO function in bichir and suggests that VNO originated in the lineage of lobe-finned fish during vertebrate evolution.