HMN 2026: What is the neuron pair in fruit flies that makes life or death decisions

For the fruit fly, a sense of taste is critical to whether it thrives or dies. The little winged creature has taste organs in its mouthpiece as well as throughout its body, including its legs, abdomen and wing margins. When a fruit fly lands on a ripe or rotting fruit, it instantly receives information about whether the fruit is bitter or sweet. Sweetness indicates a caloric payday that cues the fly to feed; bitterness prompts the fly to move on from the potentially toxic substance.

Researchers in the lab of Brown University professor Gilad Barnea have identified the pair of neurons that make this critical choice. The insights on how flies navigate this complex decision-making process, a process not previously clear to scientists, are published online in Nature Communications.

“If a fly makes just one mistake about what to eat, it may die,” said Barnea, a professor of neuroscience and director of the Center for the Neurobiology of Cells and Circuits at Brown’s Carney Institute for Brain Science. “So the decision is super important. 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 said. This could indicate that the decision-making mechanism is conserved across species, including humans. Once identified, such human neurons could be attractive targets for pharmaceutical intervention.

Barnea and his research team are known for creating the trans-Tango toolkit, a suite of genetically encoded tools that can map neural circuitry in the fruit fly and other organisms. Lead study author Doruk Sava?, who earned his Ph.D. from Brown and is now a researcher at Harvard University, had the initial impulse to investigate taste in the fruit fly when he first joined the Barnea lab and saw, via trans-Tango, surprising results.

“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,” Sava? said. “But what I was seeing was that there’s a neuron that is ‘listening’ to both.”

That neuron is called subesophageal LK, or SELK. Sava? found that bitter- and sweet-sensing neural populations send messages to the SELK neurons at different strengths. The bitter-sensing population sends a stronger signal, while the sweet-sensing population sends a weaker one. The SELK neuron interprets these signals and uses one of two different chemical messengers to tell the fly what to do next. If the substance is sweet, the SELK neuron releases a neurotransmitter that directs the fly to eat; if it is bitter, the SELK neuron secretes a neuropeptide to stop the feeding.

“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,” said Barnea. “If I may give Mother Nature a compliment, it’s a very elegant solution.”

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