Trehalose sensitivity of the gustatory receptor neurons expressing wild-type, mutant and ectopic Gr5a in Drosophila.

Among ~70 candidate gustatory receptor genes identified from the Drosophila genome, Gr5a is the only gene that is allelic to a known gene, Tre, which controls taste sensitivity. Tre was discovered as an X-linked genetic polymorphism (Tre+ and Tre01) among wild populations and laboratory strains (Tanimura et al., 1982). Recent molecular studies on Gr5a and other gustatory receptor genes supported the finding that Tre encodes a functional gustatory sugar receptor and have provided novel information on the sugar sensitivity in the gustatory receptor neurons. Gr5a was found to be one of the candidate gustatory receptor genes discovered from the Drosophila genome database (Clyne et al., 2000), with the locus in close proximity to Tre (Berkeley Drosophila Genome Project; http:// flybase.net). Mutations were obtained using a P-element insertion near the Tre locus (Isono et al., 1998). Subsequent molecular analysis of the mutations provided key information to prove that Tre is identical to Gr5a (Dahanukar et al., 2001; Ueno et al., 2001). The induced mutations and the spontaneous mutation Tre01 provide a clue to the understanding of how a specific chemosensory receptor protein contributes to the sensitivity of the receptor neurons and the feeding response. In this paper we present physiological and behavioral data for the gustatory receptor TRE encoded by Gr5a in wild-type, mutant and transformant flies and discuss the sugar sensitivity of the gustatory receptor neurons.