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).

BACKGROUND: The olfactory organ of turtles consists of an upper chamber epithelium (UCE) associated with glands and a lower chamber epithelium (LCE) lacking glands. The odorant receptors (ORs) are a family of olfactory receptors generally categorized into classes I and II based on sequence similarity. In amniotes, class I and II ORs are thought to preferentially detect hydrophilic and hydrophobic molecules, respectively. The relative abundance of OR classes vary among turtles: while class I ORs predominate in aquatic turtles, class II ORs are more abundant in terrestrial turtles. However, it remains unclear where these receptors are expressed in the olfactory organ of turtles.

RESULTS: In this study, we identified eight class I ORs and 61 class II ORs and examined the distribution of OR-expressing cells in the olfactory organ of the African spurred tortoise (Centrochelys sulcata). In situ hybridization analysis revealed that all representative class I ORs examined in C. sulcata are expressed in both the UCE and the LCE, whereas all representative class II ORs examined are restricted to the UCE. These findings are in marked contrast to our previous observations in the olfactory organ of the sea turtle Chelonia mydas. Also, double-labeling experiments revealed that class I ORs are not co-expressed with the transcription factor B-cell lymphoma/leukemia 11b (Bcl11b), suggesting that Bcl11b-mediated repression of class I OR expression is conserved in C. sulcata.

CONCLUSION: Present findings suggest that ecological adaptation of turtles may be correlated with variation in the expression site of ORs within the olfactory organ.