Spatially restricted expression of candidate taste receptors in the Drosophila gustatory system

Background: Taste is an important sensory modality in most animals. In Dros ophila, taste is perceived by gustatory neurons located in sensilla distrib uted on several different appendages throughout the body of the animal. Her e we show that the gustatory receptors are encoded by a family of at least 54 genes (GF genes), most of which are expressed exclusively in a small sub set of taste sensilla located in narrowly defined regions of the fly's body . Results: BLAST searches with the predicted amino acid sequences of 6 7-tran smembrane-receptor genes of unknown function and 20 previously identified, putative gustatory receptor genes led to the identification of a large gene family comprising at least 54 genes, We investigated the expression of eig ht genes by using a Gal4 reporter gene assay and found that five of them we re expressed in the gustatory system of the fly, Four genes were expressed in 1%-4% of taste sensilla, located in well-defined regions of the probosci s, the legs, or both, The fifth gene was expressed in about 20% of taste se nsilla in all major gustatory organs, including the taste bristles on the a nterior wing margin. Axon-tracing experiments demonstrated that neurons exp ressing a given Gr gene project their axons to a spatially restricted domai n of the subesophageal ganglion in the fly brain. Conclusions: Our findings suggest that each taste sensillum represents a di screte, functional unit expressing at least one Gr receptor and that most G r genes are expressed in spatially restricted domains of the gustatory syst em. These observations imply the potential for high taste discrimination of the Drosophila brain.