For the fruit fly, taste is critical to whether it thrives or dies. With taste organs in its mouth piece and throughout its body, including its legs, abdomen, and wing margins, the little creature can sense whether a fruit is bitter or sweet as soon as it lands on it. Sweetness indicates a caloric payday that cues it to feed; bitterness prompts it to move on from a potentially toxic substance.

Researchers in the lab of Gilad Barnea, PhD, have identified a pair of neurons that make this critical choice. Their insights were “If a fly makes just one mistake about what to eat, it may die—so the decision is super important,” says Barnea, the Sidney A. Fox and Dorothea Doctors Fox Professor of Ophthalmology, Visual Science, and Neuroscience. “This newly discovered mechanism illustrates the impressive level of computation that a single neuron can do.”

Recent work has reported a similar finding in the mouse brain, Barnea says. This could indicate that the decision-making mechanism is conserved across species, including humans. Once identified, such human neurons could be attractive drug targets.

Lead study author Doruk Savaş PhD’24, now a researcher at Harvard, had the initial impulse to investigate taste in the fruit fly when he first joined the Barnea lab as a graduate student.

“The mainstream understanding in the field was that sweet-sensing neural populations only talk to a certain subset of neurons, and bitter-sensing populations only talk to a different subset, and there’s really no interplay between them,” Savaş says. “But what I was seeing was that there’s a neuron that is listening to both.”

That neuron is called subesophageal LK, or SELK. Savaş found that bitter- and sweet-sensing neural populations send messages to the SELK neurons at different strengths. The SELK neuron interprets these signals and uses one of two different chemical messengers to tell the fly whether to eat or not.

“We knew there were many ways for a circuit to make a decision, but we didn’t know before this discovery that a single neuron could be the one to do it,” Barnea says. “If I may give Mother Nature a compliment, it’s a very elegant solution.”