Scientists Unveil the First-Ever Complete Map of an Adult Fruit Fly’s Brain, Captured in Stunning Detail

In 2013, neuroscientist Davi Bock and his team at the at the Howard Hughes Medical Institute’s Janelia Research Campus dipped an adult female fruit fly brain into a hardening chemical solution. By 2018, they had painstakingly sliced the solidified, poppy-seed-sized brain into 7,050 layers and taken 21 million pictures of the sections using electron microscopy.

They made the images public, and that data sparked a new idea: Fly neuroscientist Mala Murthy and computational neuroscientist Sebastian Seung at the Princeton Neuroscience Institute wondered if they could use the images to make a map of the fruit fly brain—down to each individual neuron and synapse.

“It seemed really ridiculous,” Murthy tells Science’s Rodrigo Pérez Ortega. “It was just too big a product. Nobody had made a map at that scale.”

They used artificial intelligence to reconstruct the fly’s brain cells based on the image data, but the model wasn’t perfect, and it made some mistakes. So, the researchers created the FlyWire consortium to recruit hundreds of volunteers to proofread and annotate the 3D brain, called a connectome, by hand.

At the end of this monumental collaborative effort, the team had produced the most complete brain map of any organism to date. It totaled nearly 140,000 neurons, 8,453 different types of neurons and more than 54.5 million synapses.

The team’s findings were published in a series of nine papers in the journal Nature on Wednesday. Unraveled end-to-end, the fruit fly brain’s neural wiring would stretch out to more than 490 feet, which is longer than four blue whales aligned nose-to-tail, per the New York Times’ Carl Zimmer.

“This is a huge deal,” Clay Reid, a neurobiologist at the Allen Institute for Brain Science who was not involved in the project, tells Nature News’ Sara Reardon. “It’s something that the world has been anxiously waiting for, for a long time.”

The FlyWire Connectome

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Previously, researchers had mapped parts of the much smaller brains of a larval marine worm (78 neurons), a larval sea squirt (177 neurons) and an adult roundworm (302 neurons). In a breakthrough last year, scientists published the first complete connectome of a larval fruit fly, featuring 3,000 neurons.

An adult fruit fly’s brain is much more complex, however—and most importantly, the small insects share 60 percent of human DNA, as well 75 percent of the genes that cause genetic diseases, per a statement. As such, understanding the fly’s brain in such detail could hold implications for connections in human brains—and the neural pathways that lead to certain behaviors. Fruit flies, like humans, can get drunk, sing and be kept awake with coffee, suggesting similarities in our brains.

“You might be asking why we should care about the brain of a fruit fly,” Seung tells the Guardian’s Ian Sample. “My simple answer is that if we can truly understand how any brain functions, it’s bound to tell us something about all brains.”

In one of the studies, scientists created a computer simulation of the fly’s brain to analyze the flow of sensory signals, per Nature News. They tested the modeled brain by exposing it to signals for sweet and bitter tastes. These stimuli triggered the brain to activate the motor neurons that in a real fly would move its proboscis, the insect equivalent of a tongue.

Perhaps unsurprisingly, the sweet trigger caused the simulation to send signals that would extend the proboscis, and the bitter one didn’t. Further study showed that the computer model was more than 90 percent accurate in predicting how a real fly’s brain would respond.

The connectome, and all the data associated with it, reveals patterns of healthy brain functions and could thus also be consequential in the treatment of brain diseases. Of the 8,453 types of neurons the scientists identified, 4,581 were newly discovered. The brain map also provides insight into the neurotransmitters, such as dopamine and serotonin, secreted by the various types of neurons, per a statement.

Anita Devineni, a fruit fly expert at Emory University who was not involved in the studies, tells the New York Times that she relies on the team’s publicly available data to plan new experiments: “We’re using this for everything we do.”

Scientists hope to eventually map the brain of a male fruit fly to complement this female brain, then move on to even more ambitious projects, such as creating a connectome for a mouse brain, which has around 1,000 times as many neurons as the fruit fly.

“We’re not done, but it’s a big step,” Bock, the neuroscientist who began slicing up the fly brain in 2013, tells Nature News.

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.