This video shows all the types of neuron cells in the central nervous system (brain and ventral nerve cord) of the male Drosophila fruit fly. Data acquired and analyzed by the FlyEM Project Team at HHMI-Janelia, the Cambridge Connectomics Group, and Google Research. Video by Philip Hubbard.

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. 

The new wiring diagram, a joint effort by Janelia, the MRC Laboratory of Molecular Biology, and the University of Cambridge, builds on the team’s previous nerve cord and brain datasets to reveal the complete circuitry linking sensory perception to behavior.

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. A team of researchers also recently unveiled an initial female fruit fly central nervous system connectome, which is in the process of being mapped.

The new male connectome is an important tool in understanding how the brain works, particularly its role in complex social behaviors—like mating and aggression—that can vary by sex.

“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. “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.”

Some neurons in the central nervous system of the Drosophila fruit fly are “dimorphic”, existing in both males and females but connecting to different neighboring neurons. The neighbors may be “isomorphic”, the same in both male and female, or sex specific, or dimorphic themselves. This video shows one example, the type AOTU012, which is present in left and right instances. Data acquired and analyzed by the FlyEM Project Team at HHMI-Janelia, the Cambridge Connectomics Group, and Google Research. Female data acquired by the FlyWire project. Video by Philip Hubbard and Isabella Beckett.

Moving fly connectomics forward

Buoyed by advances in imaging and computing, connectomics—the large-scale mapping and studying of neurons and their connections—has taken major steps forward in recent years, resulting in the latest fly central nervous system connectomes.

The latest dataset, which reveals the circuitry of the fly nervous system, will enable scientists to see all the neurons and circuits that might be involved in carrying out behavior. This is a huge advancement for neuroscientists who have been studying fly behavior for decades by painstakingly identifying and testing circuits one by one, allowing them to only chip away at understanding how the brain enables complex behavior.

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. Uncovering the nervous system of the tiny but behaviorally interesting fruit fly is a first step at cracking the neural code underlying complex functions.

“It basically lets us get from eyes to legs in one go,” Greg Jefferis, a neuroscientist at the LMB and Cambridge who also helped to lead the project.

The central nervous system of the Drosophila fruit fly has visual-motor pathways, which connect the visual neurons that help the fly detect objects, and the motor neurons that help the fly move in response to those objects. This video shows an example of such a pathway, from R1-R6 visual neurons to the DNg13 motor neuron. Data acquired and analyzed by the FlyEM Project Team at HHMI-Janelia, the Cambridge Connectomics Group, and Google Research. Video by Philip Hubbard and Alexandra Fragniere.

A collaborative effort

The latest connectome, like the others generated at Janelia, would not have been possible without the research campus’s FlyEM Project Team, which has developed and honed a process for generating high-quality connectomes for more than a decade.

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. 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.

While neuroscientists start to dig into the new CNS connectome, the team is already working to further improve the process for generating connectomes using new imaging tools and artificial intelligence. They have also imaged the central nervous system of a female fruit fly and are in early stages of reconstructing the connectome. Other groups at Janelia are also working to produce wiring diagrams for other key model organisms, like the larval zebrafish.

These brain maps are reshaping the way scientists think about the nervous system, which plays a critical role in everything the body does, from the most basic functions like breathing to the most complex actions like learning and memory. Uncovering how brains process information and turn it into action can help scientists better understand what causes different diseases.

“Connectomes are really one of the most powerful ways we have to understand how brains give rise to different behaviors,” Jefferis says.

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Citation:

Stuart Berg, Isabella R Beckett, Marta Costa, Philipp Schlegel, Michał Januszewski, Elizabeth C Marin, Aljoscha Nern, Stephan Preibisch, Wei Qiu, Shin-ya Takemura et al. "Sexual dimorphism in the complete connectome of the Drosophila male central nervous system." DOI: 10.1101/2025.10.09.680999