We investigate how the molecular and cellular maps of the Drosophila olfactory system are integrated. A correspondence is established between individual odor receptors, neurons, and odors. We describe the expression of the Or22a and Or22b receptor genes, show localization to dendritic membranes, and find sexual dimorphism. Or22a maps to the ab3A neuron, which responds to ethyl butyrate. Analysis of a deletion mutant lacking Or22a, along with transgenic rescue experiments, confirms the mapping and demonstrates that an Or gene is required for olfactory function in vivo. Ectopic expression of Or47a in a mutant cell identifies the neuron from which it derives and its odor ligands. Ectopic expression in a wild-type cell shows that two receptors can function in a single cell. The ab3A neuron does not depend on normal odor receptor gene expression to navigate to its target in the CNS.

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Odorant-binding proteins (OBPs) are a major constituent of the aqueous perireceptor compartment in vertebrates and in insects. Although different in primary structure, they are supposed to serve similar functions in both animal groups: (i) OBPs may act as solubilizers and carriers of the lipophilic odorants in the aqueous mucus or sensillum lymph; (ii) OBPs may act in addition as peripheral filters in odor discrimination by selectively binding certain classes of odorants; (iii) OBPs may present the stimulus molecule in a particular way to the receptor proteins to facilitate signal transduction; (iv) OBPs may clean the perireceptor space from unwanted and toxic compounds; (v) OBPs may rapidly deactivate odorants after stimulation of the receptors. Experimental evidence in favor of this multiple role of OBPs is reviewed.

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Deciphering the genome of the fruitfly, Drosophila melanogaster, has revealed 39 genes coding for putative odorant-binding proteins (OBPs), more than are known at present for any other insect species. Using specific antibodies, the expression mosaic of five such OBPs (OS-E, OS-F, LUSH, PBPRP2, PBPRP5) on the antenna and maxillary palp has been mapped in the electron microscope. It was found that (1) OBP expression does correlate with morphological sensillum types and subtypes, (2) several OBPs may be co-localized in the same sensillum, and (3) OBP localization is not restricted to olfactory sensilla. The expression of PBPRP2 in antennal epidermis sheds some light on the possible evolution of OBPs.

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