Researchers have completed the first full neural connectome of the male fruit fly central nervous system, linking more than 166,000 neurons across the brain and ventral nerve cord according to DongA Science. The comprehensive wiring diagram was built by a joint research team from the Howard Hughes Medical Institute (HHMI) HHMI, Google Research, the Medical Research Council Laboratory of Molecular Biology, and the University of Cambridge.
Complete Central Nervous System Map of the Male Fruit Fly Unveiled
The results of the mapping effort were published in the international journal Cell. The newly mapped central nervous system includes both the brain and the ventral nerve cord, which serves as the neural structure linking the brain to the body by relaying sensory information and transmitting motor commands.
Two Decades of Technological Advancement
The effort to map the entire fruit fly central nervous system began in 2008 at HHMI’s Janelia Research Campus. When Janelia’s founding Executive Director Gerry Rubin initially proposed creating a complete map of all neurons in the fruit fly brain, many scientists believed the project would take too long and cost too much.
At the time the project started, technology constraints meant the workload was estimated to require 500 people working for 10 years. Instead, the team advanced microscopy imaging techniques and developed computational methods to automate image analysis. Rubin noted that bringing together multidisciplinary researchers increased the efficiency of connectome construction by more than 1,000-fold.
Milestones along the way included:
- An initial, fully-automated reconstruction of a female fruit fly brain released in 2019 by Google Research.
- The 2020 release of the “hemibrain,” a human-verified map of half a female fruit fly brain containing 25,000 neurons and 21 million connections, which was the largest and most detailed connectome produced at that time.
- The development of AI connectomics tools such as flood-filling networks and the state-of-the-art reconstruction system PATHFINDER, which improved the speed and accuracy of generating neural shapes.
Insights Into Behavior and Sex Differences
The complete wiring diagram is expected to help scientists trace neural circuits that transform sensory information into behavior, analyze how taste preferences and aversions turn into movement or hormone release, and provide insight into how the brain enables complex action from sensory perception to behavior.
Analysis of the connectome also allowed researchers to investigate the origins of behavioral differences between male and female flies. The team found that while most sensory and motor circuits are shared between the sexes, neurons that differ by sex are concentrated in higher-order brain regions. Specifically, about 100 classes of interneurons found only in males form complex networks in the central brain that regulate male-specific behaviors like courtship and aggression.
Future Directions and Vertebrate Models
The success of the fruit fly connectome has helped launch the field of connectomics. Researchers are now applying the lessons learned and techniques developed during the project to new animal models, with the ultimate goal of providing a mechanistic account of how vertebrate brains generate complex behavior to help understand brain function and diseases like Alzheimer’s and depression.
As part of this next phase, researchers are currently building a forebrain connectome of larval zebrafish—whose transparent bodies make imaging easier—while simultaneously recording neuronal activity during behavior.
