Google and HHMI Janelia Publish the Complete Fruit Fly Brain Connectome — Built With AI

Google Research and the Howard Hughes Medical Institute's Janelia Research Campus, together with collaborators at the MRC Laboratory of Molecular Biology and the University of Cambridge, published on September 3 the complete connectome of the male fruit fly's brain and central nervous system — a decade-long project and, by neuron count, the largest complete brain map produced to date. Partial maps of larger brains exist with more raw data — a one-cubic-millimeter slice of mouse visual cortex mapped by the MICrONS project has over a billion synapses, more than the fly's entire nervous system — but none of them cover a whole organism's brain the way this one does.

What was mapped

The paper, "Sexual dimorphism in the complete connectome of the Drosophila male central nervous system," published in Cell, documents more than 166,000 neurons and 125 million synaptic connections across the fly's brain, optic lobes, and ventral nerve cord — the insect analog of a spinal cord. Every neuron and connection in the male fly's nervous system is now catalogued, annotated, and proofread by a team of human experts at Janelia.

Where AI comes in

Connectomics starts by slicing a brain into millions of ultra-thin sections and imaging each one under a microscope. Turning that stack of flat images back into an accurate 3D wiring diagram — matching up neurons and tracing individual connections across thousands of slices — is where AI does the heavy lifting. Google's team used machine learning models to align the images and reconstruct the neurons in three dimensions, a task that would be impractical to do by hand at this scale.

Why it matters

The new map complements a previously released female fruit fly connectome, giving researchers a matched pair for the first time — useful for studying sex-based differences in behaviors like courtship and aggression, and for understanding natural variability between individual brains in the regions that don't differ by sex. The dataset is open and explorable through Neuroglancer, the visualization tool Google built for exactly this kind of dataset.

Fruit flies have driven multiple Nobel-winning discoveries in genetics, and researchers are betting the same model organism can do for neuroscience what it already did for genetics — a complete, cellular-scale map of how a real nervous system is wired, as a stepping stone toward understanding brains too large to map directly, including our own.