Scientists have mapped the entire central nervous system of an adult male fruit fly, capturing more than 166,000 neurons. Completed through a multi-institutional effort by the Howard Hughes Medical Institute’s Janelia Research Campus, Google Research, and collaborators from the scientific community, the detailed wiring diagram enables direct anatomical comparisons with a female fly brain map released in 2024 to decode sex-specific behaviors like mating and aggression.
Charting 166,000 Neurons in a Male Fruit Fly
A fruit fly’s brain is roughly the size of a poppy seed — and yet that tiny package contains over 100,000 neurons. Researchers have constructed a complete wiring diagram — also called a “connectome” — mapping every single neuron in an adult male fruit fly’s central nervous system. The project charts over 166,000 neurons, encompassing the brain, both optic lobes, and the ventral nerve cord. This multi-year initiative was driven by the Howard Hughes Medical Institute’s Janelia Research Campus, Google Research, and collaborators from the scientific community in a project where AI is making it possible for scientists and researchers to exponentially scale projects in the field of connectomics, which precisely reconstruct the connections between brain cells.
Artificial intelligence enabled researchers to scale the rigorous reconstruction of brain cells in this years-long project. This male connectome builds upon the 2024 release of a complete female fruit fly brain map, which covers about 140,000 neurons. Together, these two datasets establish a foundational resource for neuroscience for years to come, promising to advance neuroscience experiments on this key model organism.
Comparing Brains Across Sexes
Having complete connectomes for both sexes of an organism allows scientists to investigate the neurological roots of behavioral differences, marking the first time we can compare both sexes of an animal with complex social behavior.
“Male and female flies have a lot of differences in their behavior, and neuroscientists want to understand how the brain controls those behaviors. This now allows us to easily home in on the neurons that are causing those differences.”
Gerry Rubin, head of biology and a senior group leader of the Howard Hughes Medical Institute’s Janelia Research Campus
Rubin noted that male and female flies exhibit distinct behavioral repertoires, including behaviors reflected during mating or in aggressive actions, including sex-specific fighting moves. The new map allows researchers to easily study these differences using data acquired and analyzed by the FlyEM Project Team at HHMI’s Janelia Research Campus, the Cambridge Connectomics Group, and Google Research.
Decoding Taste, Vision, and Locomotion
Initially released as a preprint, the new fly brain map was published in the journal Cell and Current Biology Thursday (Sept. 3). The male fly connectome debuted alongside three other papers, each of which uses the new data to explore a specific aspect of fruit fly neurobiology. One of these investigations, led by scientists at the Champalimaud Foundation in Lisbon, Portugal, focused on the neural circuits dedicated to the insect’s sense of taste.
Fruit flies have taste receptors in many body parts, including their legs, wings, mouthparts and inside of the throat. Researchers identified these taste receptors and traced their connections back to the fly brain. From there, they examined how those circuits interacted with those that govern behaviors such as swallowing or walking. When it comes to eating behaviors, this circuitry helps the fly determine whether a given morsel is safe or harmful and ultimately choose whether to eat it, the researchers concluded.
“Suppose you are interested in how taste controls locomotion. Now you can go to the map and ask: which sensory neurons are connected to the neurons controlling locomotion? Which intermediate neurons should I manipulate? It gives you a place to start.”
Inês de Haan Vicente, a research technician in Ribeiro’s lab
The companion papers published alongside the map respectively explore the circuits behind vision processing in the fly brain and sex-specific differences in male and female fly brains.
A Technical Roadmap for Future Connectomics
Beyond insect neurobiology, researchers point out that the fly nervous system performs remarkably sophisticated computations with relatively few neurons and little energy, and its architecture could suggest principles for designing more efficient artificial systems.
“This work also provides a technical roadmap for more ambitious connectomics projects in the future, such as those for mice and humans.”
Carlos Ribeiro, a principal investigator at the Champalimaud Foundation in Lisbon, Portugal
In the near term, the scientists aim to map the brains of larval zebrafish (Danio rerio) and adult danionin fish (Danionella). In the long run, the goal of this research is to understand how vertebrates’ brains enable complex behaviors, and then use that knowledge to help unravel the basis of neurological and psychiatric disorders in humans.
