Mapping odor receptors to olfactory receptor neurons. A. Goldman¹, W. Van der Goes van Naters¹, D. Lessing¹, C. Warr², J.R. Carlson¹. 1) MCDB Dept., Yale University, New Haven, CT; 2) Biology Dept., Monash University, Melbourne, Australia.

   Olfactory receptor neurons (ORNs), located within sensilla on the surface of the antennae and maxillary palps, express odorant receptors (Ors) that determine odor specificity. It has been proposed that each ORN only expresses one of the ~60 Or genes. In the maxillary palp of Drosophila melanogaster there are 6 ORN classes, defined by their electrophysiological response to odors. These 6 classes adhere to a strict pairing rule, e.g. a pb1 sensillum contains a pb1A cell paired with a pb1B cell. In order to define the ligand specificity of individual Ors and to study the functional organization of the olfactory system, we have used multiple approaches to determine which Ors are expressed in each of the 6 ORN classes in the maxillary palp. 4 of these ORN classes are altered or missing in the acj6 mutant, in which Or gene expression is also altered. mRNA in situ hybridizations determined which Or genes are expressed in wild type and acj6 flies. We next labeled ORNs using Or promoters to drive the expression of GFP, and recorded electrophysiologically from GFP-labeled sensilla in live flies to determine their odor sensitivity, thus mapping the Or to a specific sensillar type. We then used the same promoters to drive the expression of the cell death gene reaper. Physiological analysis then determined which ORN of the two that are housed within a given sensillum is killed. The Or gene is thereby mapped to the ORN that is missing. Double labeling experiments with RNA probes for particular pairs of Or genes were found to label pairs of adjacent cells that cohabit specific types of sensilla. Taken together, this analysis has allowed us to map individual receptors to the neurons that express them. Some of the receptors appear narrowly tuned, and an odor can elicit a response from two neurons via different receptors. Interestingly, we also found one ORN class that co-expresses two Or genes, and preliminary data suggest that each of them has its own distinct odor sensitivity.