Investigation of the localization of olfactory receptor-expressing neurons in the olfactory organ of lungfish

Lungfish, an extant lobe-finned fish, is the phylogenetically closest to terrestrial tetrapods. The olfactory organ of lungfish, which consists of a series of folds, has two distinct sensory epithelia: the lamellar olfactory epithelium, which covers the folds, and the recess epithelium, which is located at the base of the folds and the caudal posterior end of the olfactory organ. Previous studies have shown that the lamellar olfactory epithelium corresponds to the olfactory epithelium in the olfactory organs of aquatic fishes, and the recess epithelium corresponds to the vomeronasal epithelium in the olfactory organs of terrestrial tetrapods, based on its ultrastructural and histological characteristics. However, the expression pattern of olfactory receptors in the olfactory organ of lungfish is still unclear, and the detailed differences in olfactory functions between the lamellar olfactory epithelium and the recess epithelium remain unclear. Therefore, the olfactory receptor genes in lungfish were identified by RNA-seq analysis using total RNA extracted from lungfish olfactory organs, and the expression patterns of these genes in the two types of sensory epithelia were observed using histological methods.