The vomeronasal system
The accessory olfactory system, or vomeronasal system, is present only in tetrapods and lacking in fish. Lungfish, the closest living relatives of tetrapods, have been shown to possess vomeronasal primordia, but a distinct vomeronasal system first appeared in amphibians. The vomeronasal organ first appeared in amphibians. The accessory olfactory system is mainly involved in the detection of pheromones, chemosensory stimuli that signal information among members of the same species. Pheromonal communication is widespread in amphibians, and the vomeronasal system of amphibians appears well suited to study the nature and evolution of vertebrate pheromonal communication. The vomeronasal organ of Xenopus laevis, already present in its larval stages, is anatomically segregated from the main olfactory epithelium. Its receptor neurons project their axons to the accessory olfactory bulb, a structure situated laterally to the main olfactory bulb.
Vomeronasal receptor neurons of larval Xenopus laevis express type 2 vomeronasal receptors (V2Rs) like the mammalian vomeronasal organ, but in contrast to mammals they do not express type 1 vomeronasal receptors (V1Rs) that are instead expressed in the main olfactory epithelium (see Gliem et al., 2013). These features show that also the expression of olfactory receptor gene expression in Xenopus laevis is in a transitional stage.
The physiological purpose of the accessory olfactory system in Xenopus laevis, and especially of its larvae, is unknown. Molecules acting as pheromones are also not known. We set out to find suitable pheromones and to identify the physiological purpose of the accessory olfactory system in larval and adult Xenopus laevis.