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Amino acids (AAs) are essential dietary macronutrients that impact an organism's fitness in a concentration-dependent manner, but the mechanisms mediating AA detection to drive consumption are less clear. In Drosophila, we identified the repertoire of taste cells and receptors that are salient for feeding initiation when flies encounter a glutamate-rich AA peptide mixture, tryptone, using in vivo calcium imaging and the proboscis extension response. We found that tryptone attraction occurs through sweet cells, whereas feeding aversion is mediated through Ionotropic Receptor 94e (IR94e) cells and bitter cells, dependent on concentration. Further, our results corroborate previous findings that IR76b, IR51b, and IR94e detect AAs in specific cell types, even when exposed to a more complex peptide mixture. Additionally, we describe a new role for the appetitive IR56d receptor and bitter gustatory receptors in sensing tryptone. This work establishes a cellular and molecular framework salient for AA and peptide feeding initiation and highlights redundancy in aversive pathways that regulate AA feeding. 

Chemosensory cells across the body of Drosophila melanogaster evaluate the environment to prioritize certain behaviors. Previous mapping of gustatory receptor neurons (GRNs) on the fly labellum identified a set of neurons in L-type sensilla that express Ionotropic Receptor 94e (IR94e), but the impact of IR94e GRNs on behavior remains unclear. We used optogenetics and chemogenetics to activate IR94e neurons and found that they drive mild feeding suppression but enhance egg laying. In vivo calcium imaging revealed that IR94e GRNs respond strongly to certain amino acids, including glutamate, and that IR94e plus co-recep- tors IR25a and IR76b are required for amino acid detection. Furthermore, IR94e mutants show behavioral changes to solutions containing amino acids, including increased consumption and decreased egg laying. Overall, our results suggest that IR94e GRNs on the fly labellum discourage feeding and encourage egg laying as part of an important behavioral switch in response to certain chemical cues.