Organization and processing of gustatory information in the Drosophila brain

Taste is a sensory modality that is crucial to animal survival. It informs the animal whether the presented food is safe for consumption and whether the food meets the nutritional requirements under some circumstances. In Drosophila melanogaster, the vinegar fly, taste is also involved in other complex behaviors such as courtship and oviposition. Due to the multifaceted roles of taste in the fly, it is difficult to deduce unifying sensory coding principles in the fly brain. I utilized the recently developed fly whole-brain connectome to examine taste coding in the early steps of sensory processing. I found that the representation of taste quality is largely segregated at the local interneurons (LNs). Moreover, many taste projection neurons (TPNs) receive indirect inputs from the taste sensory neurons via a layer of cholinergic LNs, whose organization differs from the well-established fly olfactory circuits. Using functional imaging and morphological studies, my results suggest that each cholinergic LN serves as a node to convey feedforward information to a set of morphologically similar TPNs. Moreover, behavioral studies showed that not only taste quality representation but also circuits carrying short- and long-term reinforcement information are segregated in the early steps of taste circuits. Together, these results indicate that taste circuits diverge in early relays to conduct sensory processing in parallel, which ultimately leads to diversified functions from the limited number of taste qualities.