We have therefore isolated the complete repertoire of genes encoding the odorant receptors in Drosophila and used them to provide a molecular description of the organization of the peripheral olfactory system. Individual receptor gene families not only differ vastly in size across species but are often highly divergent with little or no sequence homology. Thus a common function of olfactory perception recruits different gene families of widely different size and sequence to accommodate the unique requirements of individual species. The diversity of receptor expression in individual sensory neurons has important implications for odor discrimination. In C. elegans it is estimated that chemosensory cells express up to 20 receptor genes, whereas in mammals, neurons express only 1 out of 1,000 receptor genes.
Discrimination requires that the brain determine which receptors have been activated by an odorant.
If a sensory neuron expresses only one receptor, then this problem reduces to the simpler requirement that the brain discern which neurons have been activated by a given odorant. We have performed two-color in situ hybridization experiments with receptor probes to demonstrate that individual sensory neurons in Drosophila express different complements of receptors. Multiple pairwise hybridization experiments for 10 DOR genes reveals that there is no overlap in the expression of any of these genes. These data provide strong support for a model reminiscent of the mammalian olfactory system in which individual sensory neurons express only a single receptor. These experiments indicate that there are 59 distinct neuronal cell types within the Drosophila olfactory organ.