In situ hybridization analysis of the expression of class I and II odorant receptors in the olfactory organ of the African spurred tortoise (Centrochelys sulcata)

Abstract

Abstract In this study, in situ hybridization was used to determine the localization of odorant receptor (OR)-expressing cells in the olfactory organ of the African spurred tortoise (Centrochelys sulcata). The olfactory organ of turtles generally consists of an upper chamber epithelium (UCE) containing glands and a lower chamber epithelium (LCE) devoid of glands. The UCE is considered an air-nose that is receptive to airborne odorants, whereas the LCE is considered a water-nose that is receptive to waterborne odorants. The ORs are a family of olfactory receptors categorized into classes I and II based on their sequence similarities. Class I ORs are thought to be receptive to water-soluble odorants, and class II ORs to volatile odorants. However, where these receptors are expressed within the olfactory organ in turtles remains unclear. This study was performed to analyze the distribution of OR-expressing cells in the olfactory organ of C. sulcata. Class II ORs were revealed to be expressed in the UCE, implying that the UCE of C. sulcata functions as an air-nose. In contrast, class I ORs were expressed in both the UCE and LCE, implying that both of these epithelia function as a water-nose in the olfactory organ of C. sulcata. Furthermore, double-labeling experiments indicated that class I ORs are not co-expressed with the transcription factor B-cell lymphoma/leukemia 11b (Bcl11b), implying that the regulatory mechanism by which Bcl11b suppresses class I OR expression is conserved in C. sulcata. Further studies are needed to clarify the functional significance of the expression of class I ORs in the entire olfactory organ and to determine whether this is a trait shared among terrestrial turtles or found only in one species of tortoise that inhabits arid regions.