Second complete fruit fly brain map: every neuron, twice

What happened

Researchers announced Friday the completion of a map of every neuron in the brain of a male fruit fly, Ars Technica reported. The connectome—cataloging neurons, their locations, and the synapses between them—joins a female Drosophila connectome finished earlier this year, giving neuroscientists a pair of complete wiring diagrams for the same species. The male map involved a collaboration between biologists at HHMI’s Janelia Research Campus and computer scientists at Google, combining electron-microscopy slices with AI models that stitch tissue, outline cells, and spot synapses.

Even a fly brain is not small work. The nervous system has on the order of 150,000 neurons, and the new effort found more than 300 million synaptic connections in the brain portion studied. Building that catalog took roughly four years from an intact brain to a finished map. Comparing male and female connectomes let the team quantify sex-specific neurons, shared neurons with different shapes or connections, and how those differences relate to genes such as doublesex and fruitless that help turn chromosomal sex into neural identity.

The point of the tool is speed for everyone else. When a lab finds an interesting neuron, researchers can open a browser and see what it connects to instead of rebuilding the wiring by hand.

Why it matters

Connectomes are to neuroscience what genomes were to molecular biology: incomplete by themselves, transformative as shared infrastructure. With two fly brains mapped, theorists can ask how a known wiring diagram produces a known behavior, and experimentalists can test whether a neuron of interest sits in sensory, integrative, or motor neighborhoods. The sex-comparison results already hint at a hierarchy: sex differences often show up more in integrative and decision-making areas than in the most basic sensory detectors or motor interfaces.

Scale is the next problem. One finding noted a neuron present in the female map missing from one hemisphere of the male, likely a developmental glitch. Saying anything statistical about that kind of variability will take many more brains, which means the four-year pipeline has to get faster. Janelia’s Gerry Rubin told Ars the hope is that a future mouse connectome might still need a team of about fifty people even with a thousand times more neurons—if software keeps absorbing the scaling.

Conclusion

A second complete fruit fly brain map is live, male beside female, AI-assisted and synapse-dense, with sex-linked wiring differences now countable instead of guessed. Full-color strip. Source: https://arstechnica.com/science/2026/09/brain-of-a-male-fly-completely-mapped-joins-earlier-map-of-a-female-brain/