166,700 Neurons: A Male Fly's Brain and Nerve Cord Map Is Published

A team co-led by the University of Cambridge and HHMI's Janelia Research Campus has published a complete wiring map of a male fruit fly's central nervous system on Sept. 3 in Cell, covering the brain and the nerve cord in a single reconstruction.

Cambridge says the map traces "the 166,700 neurons and the millions of connections between them" in the male Drosophila brain and nerve cord. Janelia's announcement of the reconstruction puts the total at more than 166,000 neurons.

Every adult fly connectome before this one was female, per Janelia: the partial "hemibrain" it released with collaborators in 2020, and the full adult female brain published last year by researchers at Princeton and Cambridge. Cambridge calls the new dataset the first full-scale wiring map of an adult animal's brain and nerve cord. "It is the first time we can compare both sexes of an animal with complex social behavior," said Gerry Rubin, the Janelia group leader who helped lead the project.

According to Cambridge, the male-female comparison showed that neurons can route the same sensory signal into sex-specific circuits: male flies that detect the smell of another male respond with aggression, while females respond to the same smell with courtship.

Janelia imaged slices of the fly's nervous system with high-resolution electron microscopy, Google Research segmented the images, and staff at Janelia and Cambridge proofread the neurons and their connections. Cambridge says those human checks "required the equivalent of 44 years of human labour."

The data are free. Janelia says the dataset and the tools needed to use it are available at no cost to researchers worldwide. The team has also imaged the nervous system of a female fly and is in the early stages of reconstructing it.