Candidate ionotropic taste receptors in the Drosophila larva

Significance The coding of taste is based on the activity of taste receptors. We examine in Drosophila the expression of a group of ∼35 ionotropic receptors (IRs), the IR20a clade, about which remarkably little is known. We find evidence that 11 are expressed in the larva. Most are expressed in neurons of the pharynx, a taste organ. Others are expressed in body-wall neurons that may be chemosensory. The organization of IR gene expression differs from the organization of expression of Gustatory receptor taste receptor genes. Among neurons expressing IR20a receptors in the pharynx, two projection patterns in the CNS can be distinguished, perhaps representing cues with different valence. Our results suggest that the IR20a clade encodes a class of larval taste receptors. We examine in Drosophila a group of ∼35 ionotropic receptors (IRs), the IR20a clade, about which remarkably little is known. Of 28 genes analyzed, GAL4 drivers representing 11 showed expression in the larva. Eight drivers labeled neurons of the pharynx, a taste organ, and three labeled neurons of the body wall that may be chemosensory. Expression was not observed in neurons of one taste organ, the terminal organ, although these neurons express many drivers of the Gr ( Gustatory receptor ) family. For most drivers of the IR20a clade, we observed expression in a single pair of cells in the animal, with limited coexpression, and only a fraction of pharyngeal neurons are labeled. The organization of IR20a clade expression thus appears different from the organization of the Gr family or the Odor receptor ( Or ) family in the larva. A remarkable feature of the larval pharynx is that some of its organs are incorporated into the adult pharynx, and several drivers of this clade are expressed in the pharynx of both larvae and adults. Different IR drivers show different developmental dynamics across the larval stages, either increasing or decreasing. Among neurons expressing drivers in the pharynx, two projection patterns can be distinguished in the CNS. Neurons exhibiting these two kinds of projection patterns may activate different circuits, possibly signaling the presence of cues with different valence. Taken together, the simplest interpretation of our results is that the IR20a clade encodes a class of larval taste receptors.