A taste of the Drosophila gustatory receptors

Abstract

Insects such as the fruit fly, Drosophila melanogaster, rely on contact chemosensation to detect nutrient-rich foods, to avoid consuming toxic chemicals, and to select mates and hospitable zones to deposit eggs. Flies sense tastants and nonvolatile pheromones through gustatory bristles and pegs distributed on multiple body parts including the proboscis, wing margins, legs, and ovipositor. The sensilla house gustatory receptor neurons, which express members of the family of 68 gustatory receptors (GRs). In contrast to mammalian chemosensation or Drosophila olfaction, which are initiated by receptors composed of dimers of one or two receptor types, the functional Drosophila GRs may include three or more subunits. Several GRs appear to be expressed in multiple cell types that are not associated with contact chemosensation raising the possibility that these proteins may have roles that extend beyond the detection of tastants and pheromones.

Figure 1

Taste sensilla. (A) Taste sensilla are distributed on the labellum, pharynx, legs and wing margins. (B) A labellum showing l-, s- and i-type sensilla. (C) Structure of a sensillum with four dendrites extending above the surface of the cuticle. The sensillum has a terminal pore.

Figure 2

Drosophila gustatory receptors. (A) A dendrogram of the Drosophila GR subfamily adapted from Robertson et al. [17]. The GRs that have been characterized functionally are indicated. Green and red lettering highlight GRs that are required for the responses to sugars and bitter compounds respectively. The blue boxes indicate GRs that function in male courtship towards females or in avoiding courtship with other males. The two GRs that are required for sensing CO₂ gas in ORNs are indicated. (B) Tastants or pheromones detected by GRs shown to function in contact chemosensation. (C) The Gr64 cluster. The six genes appear to include a single promoter and one polyadenylation site. The rectangles indicate exons. The genes that have been subjected to functional analyses (Gr64a and Gr64f) are highlighted in green.

Figure 3

Assaying the gustatory response using behavior and electrophysiology. (A) Two-way choice assay. Flies are starved overnight and placed in a microtiter dish containing two types of tastants mixed with either red or blue food coloring (e.g. sugar versus sugar plus a bitter tastants). The colors of the abdomens are assessed by visual inspection. Wild-type flies will choose the sugar without the bitter compound. The dyes are switched to make sure they do not influence the preferences for the tastants. The preference index (PI) is calculated as follows: PI= (N^Blue + 0.5^Mix)/(N^Red + N^Blue + N^Mix) or (N^Red + 0.5^Mix)/(N^Red + N^Blue + N^Mix). (B) Tip recording. The photograph show an electrode placed over a wild-type l-type sensillum on the labellum. Representative traces showing action potentials induced in wild-type and ΔGr64 mutant flies (missing the entire Gr64 locus; Figure 2C) in response to 50 mM sucrose.