Yale University, Mol. Cell. Dev. Biology, P.O. Box 20 8103, Kline Biology Tower, New Haven, CT 06520-8103, USA.
Higher organisms can discriminate between many odors. The olfactory system of Drosophila melanogaster is well suited to study how many physiologically different olfactory receptor neurons (ORNs) are needed and what differentiates them. The availability of genetic and molecular techniques makes it an ideal experimental model. We characterized physiological responses from sensilla on the two olfactory organs of D. melanogaster. Extensive recordings have revealed at least 22 classes of neurons with different response spectra. They mainly respond to simple esters, alcohols and ketones but also to E2-hexenal, valerolactone, a sulfur compound, a terpene and a few aromatic compounds. Many of these chemicals occur in yeast and ripening fruit which are natural sources of food for D. melanogaster. Where examined carefully, dose-response curves are sigmoid with high sensitivity to one tested compound. However, CO2 sensitive neurons show a linear dose-response relation. ORNs not only differ in response spectra but also in response characteristics. Stimulation can result in inhibition or excitation with distinctly different temporal aspects. A gene that is partly responsible for endowing ORNs with specific identities is acj6. It is turned on early in the development of sensilla. Mutations in acj6 cause certain neurons to lose their responsiveness while others require new response characteristics. What are the genes that directly give an ORN it’s unique identity? A large family of putative odor receptor genes has recently been identified in the D. melanogaster genome. These genes are specifically expressed in subsets of ORNs and some are regulated by acj6. Identifying a large fraction of ORN classes has shown many of the coding possibilities of this fly’s nose and elucidated aspects of the genetic and molecular basis of olfaction.