Male moths are endowed with odorant receptors (ORs) to detect species-specific sex pheromones with remarkable sensitivity and selectivity. We serendipitously discovered that an endogenous OR in the fruit fly, Drosophila melanogaster, is highly sensitive to the sex pheromone of the silkworm moth, bombykol. Intriguingly, the fruit fly detectors are more sensitive than the receptors of the silkworm moth, although its ecological significance is unknown. By expression in the "empty neuron" system, we identified the fruit fly bombykol-sensitive OR as DmelOR7a (= DmOR7a). The profiles of this receptor in response to bombykol in the native sensilla (ab4) or expressed in the empty neuron system (ab3 sensilla) are indistinguishable. Both WT and transgenic flies responded with high sensitivity, in a dose-dependent manner, and with rapid signal termination. In contrast, the same empty neuron expressing the moth bombykol receptor, BmorOR1, demonstrated low sensitivity and slow signal inactivation. When expressed in the trichoid sensilla T1 of the fruit fly, the neuron housing BmorOR1 responded with sensitivity comparable to that of the native trichoid sensilla in the silkworm moth. By challenging the native bombykol receptor in the fruit fly with high doses of another odorant to which the receptor responds with the highest sensitivity, we demonstrate that slow signal termination is induced by overdose of a stimulus. As opposed to the empty neuron system in the basiconic sensilla, the structural, biochemical, and/or biophysical features of the sensilla make the T1 trichoid system of the fly a better surrogate for the moth receptor.S ince the identification of bombykol (IUPAC name (10E,12Z)-hexadeca-10,12-dien-1-ol) as a sex pheromone for the silkworm moth, Bombyx mori (1), hundreds of sex pheromones have been identified from lepidopteran species (2). In general, femaleproduced sex pheromones in moths are complex mixtures of straight-chain acetates, aldehydes, and alcohols, with 10-12 carbon atoms and up to three unsaturations (3). This fatty acid-derived semiochemistry (4, 5) is unique to chemical communication in moths, although (7Z)-dodec-7-en-1-yl acetate also has been identified as a sex pheromone for the Asian elephant, Elephas maximus (6). Male moths detect these sex pheromones with such high selectivity and sensitivity that minimal structural modifications render them inactive (7), whereas a single molecule of bombykol is estimated to be sufficient to activate olfactory receptor neurons (ORNs) in male antennae of the silkworm moth (8). These species-specific communication channels seem to have evolved under a strong evolutionary pressure into a sophisticated signal receptor system. In contrast, we have serendipitously identified an ORN housed in the ab4 basiconic sensilla in the antennae of the fruit fly, Drosophila melanogaster, which responds to bombykol with high sensitivity (9), although the ecological significance of this bombykol detector in the fruit fly remains enigmatic. The OR housed in the ab4a neuron was pr...