In Drosophila, sweet and bitter taste sensation is mediated mainly by gustatory receptors (Grs) expressed in gustatory receptor neurons (GRNs). GRNs are housed within taste sensilla that are distributed in different body parts, including the labellum, pharynx, distal segments of the legs (tarsi), wing margins, and ovipositor. Despite extensive studies that have described taste coding in the main taste organs (labellum and tarsi), little is known about the molecular and cellular basis of pharyngeal taste. Given that the pharynx lies in an anatomical position where it may serve as the last checkpoint for food ingestion, our aim is to explore pharyngeal taste coding for a better understanding how gustatory inputs from different taste organs are weighed and translated into appropriate feeding behaviors.
We first systematically analyzed the expression patterns of Gr-Gal4 lines representing all Grs via the Gal4/UAS system in three pharyngeal sense organs: labral sense organ, and ventral and dorsal cibarial sense organs.The Gr-to-neuron mapping results reveal distinct pharyngeal sensilla organization as compared to external taste organs.
To specifically study the function of pharyngeal GRNs in feeding behaviors, we took advantage of pox-neuro (poxn) mutants, in which the external taste bristles are transformed into mechanosensory bristles. Previously, we showed that poxn mutants have intact pharyngeal GRNs, arguing that poxn mutants serve as a good tool for dissecting the functional role of pharyngeal GRNs without other confounding taste inputs (e.g. labellar and tarsal gustatory inputs). Using binary choice assays, we found that poxn mutants retain similar feeding preferences for most sweet, bitter, salt, amino acid, and acid tastants as compared to wild-type flies. Moreover, we have identified classes of pharyngeal GRNs in mediating both food acceptance and avoidance behaviors, respectively by activating or silencing these pharyngeal GRNs in the poxn mutant background. Currently, we are investigating response profiles of pharyngeal GRNs to various tastants by Ca2+ imaging, which will allow comparisons of ligand specificity and sensitivity between taste organs.