c-fos expression in the olfactory epithelium of the East African cichlid (Haplochromis chilotes) in response to odorant exposure.

Fishes use olfaction to gain varied information vital for survival and communication. To understand biodiversity in fishes, it is important to identify what receptors individual fish use to detect specific chemical compounds. However, studies of fish olfactory receptors and their ligands are still limited to a few model organisms represented primarily by zebrafish. Here, we tested the c-fos expression of olfactory sensory neurons (OSNs) in an East African cichlid, the most diversified teleost lineage, by in situ hybridization with a c-fos riboprobe. We confirmed that microvillous neurons contributed the most to the detection of amino acids, as in other fishes. Conversely, we found that ciliated neurons contributed the most to the detection of conjugated steroids, known as pheromone candidates. We also found that V2Rs, the major receptor type in microvillous neurons, exhibited differential responsiveness to amino acids, and further suggest that the cichlid-specific duplication of V2R led to ligand differentiation by demonstrating a differential response to arginine. Finally, we established a non-lethal method to collect cichlid urine and showed how various OSNs, including V1R+ neurons, respond to male urine. This study provides an experimental basis for understanding how cichlids encode natural odours, which ultimately provides insight into how olfaction has contributed to the diversification of cichlids.