Neuroscience crossed a quiet threshold this month: for the first time, both sexes of a complex-social animal have complete wiring diagrams of their nervous systems.[2]
The male fruit fly's brain and nerve cord are now mapped end to end — and because every earlier fly connectome was female, researchers can finally compare the sexes neuron by neuron.[2]
The complete connectome of a male Drosophila melanogaster central nervous system — a seamless map of all the neurons in the brain and nerve cord of a single male fly and the millions of connections between them — was unveiled by a team from HHMI Janelia, the MRC Laboratory of Molecular Biology, the University of Cambridge, and Google Research.[2]
Published in Cell, "Sexual dimorphism in the complete connectome of the Drosophila male central nervous system" is the result of a decade-long partnership that advances connectomics using computing and AI to build cellular-scale maps of entire brains.[1]
With over 166,000 neurons and 125 million synaptic connections, it is the largest brain map by number of neurons to date.[1]
It also includes the ventral nerve cord, analogous to the spinal cord — the map begins to expand from the brain into how the brain controls the body.[1]
Until now, all fruit fly brain connectomes have been female — the hemibrain released by Janelia in 2020, and the full adult female brain published by Princeton and Cambridge.[2]
"It is the first time we can compare both sexes of an animal with complex social behavior," says Janelia Senior Group Leader Gerry Rubin, who helped lead the project.[2]
Having both males and females mapped allows the two to be compared in places where neurons differ, and used to study the biological mechanisms for fruit fly courtship and aggression.[1]
Some neurons are "dimorphic": they exist in both sexes but connect to different neighbors.[2]
The new map makes those spots easy to find — Google highlights a neuron called AOTU008 whose male version has two additional projections; Janelia shows the type AOTU012 as another example.[1][2]
"It basically lets us get from eyes to legs in one go," says Greg Jefferis of the LMB and Cambridge.[2]
The team imaged the samples with high-resolution focused ion beam scanning electron microscopy (FIB-SEM); Google Research segmented the EM images, and teams of experts from Janelia and Cambridge proofread and labeled the neurons and their connections and determined their cell types.[2]
The male fly connectome has been annotated and verified by human experts at HHMI Janelia.[1]
All data and the tools necessary to use them are available for free to researchers worldwide, viewable and downloadable through Neuroglancer, the open-source visualization tool Google created.[1][2]
The team has imaged the central nervous system of a female fruit fly and is in the early stages of reconstructing that connectome; Janelia groups are also working on larval zebrafish wiring diagrams.[2]
In the same week, Google helped map a portion of the elephantnose fish's hindbrain in Nature — the first demonstration of a connectome combined with other information to produce a mechanistic model of learning in a vertebrate brain.[1]
The honest caveat: connectomes are structural snapshots, not behavior — they show who talks to whom, not what the conversation means.[unverified]
That is why the dimorphic-neuron comparison and the fish plasticity work, which attach function to structure, are the parts to watch.[unverified]
[1] research.google/blog/a-connectomics-milestone-mapping-the-complete-male-fruit-fly-brain — A connectomics milestone: Mapping the complete male fruit fly brain (Google Research blog, 2026-09-03)"With over 166,000 neurons and 125 million synaptic connections, this is the largest brain map by number of neurons to date, providing a fundamental resource for scientists to use fruit flies as a model organism for studying how the brain works.""is the result of a decade-long partnership that advances the field of connectomics using computing and AI to build cellular-scale maps of entire brains.""This new brain map complements the female fruit fly brain map and recently released complete female fruit fly brain and nerve cord map.""Having both male and female brains and central nervous systems mapped allows the two to be compared in places where neurons differ, and used to study the biological mechanisms for fruit fly courtship and aggression.""The male fly connectome has been annotated and verified, or proofread, by a team of human experts at HHMI Janelia.""In a study led by Columbia University and published this week in Nature, our team helped map a portion of the elephantnose fish's hindbrain that is used in signal processing."
[2] janelia.org/news/researchers-reveal-connectome-of-the-male-fruit-fly-central-nervous-system — Researchers reveal connectome of the male fruit fly central nervous system (HHMI Janelia Research Campus)"A team of researchers has unveiled the complete connectome of a male fruit fly central nervous system —a seamless map of all the neurons in the brain and nerve cord of a single male fruit fly and the millions of connections between them.""Until now, all fruit fly brain connectomes have been female, including the hemibrain released by Janelia scientists and collaborators in 2020, and the full adult female brain published by researchers at Princeton and Cambridge last year.""It is the first time we can compare both sexes of an animal with complex social behavior," says Janelia Senior Group Leader Gerry Rubin, who helped lead the project."The male CNS connectome—comprised of more than 166,000 neurons in the brain and nerve cord—will ultimately allow researchers to determine the circuits involved in key behaviors, like walking toward a mate.""The team used high-resolution focused ion beam scanning electron microscopy to image the samples, and state-of-the-art algorithms and machine learning techniques to collect, manage, analyze, and publish the data. Google Research segmented the EM images, and teams of experts from Janelia and Cambridge proofread and labeled the neurons and their connections and determined their cell types.""These data and the tools necessary to use them are now available for free to researchers worldwide."
Cross-check note: Google's blog cites the paper as Cell, DOI 10.1016/j.cell.2026.08.015; Janelia cites the same title as preprint DOI 10.1101/2025.10.09.680999. cell.com itself is behind a bot wall, so the journal page was not fetched — the Cell attribution rests on both primary sources agreeing.