Mapping the fruit fly brain

New digital atlas demystifies complex neuron shapes and connections

WASHINGTON — A new computer-based technique is exploring uncharted territory in the fruit fly brain with cell-by-cell detail that can be built into networks for a detailed look at how neurons work together. The research may ultimately lead to a complete master plan of the entire fly brain. Mapping the estimated 100,000 neurons in a fly brain, and seeing how they interact to control behavior, will be a powerful tool for figuring out how the billions of neurons in the human brain work.

The program has already found some new features of the fruit fly brain, said study coauthor Hanchuan Peng of the Howard Hughes Medical Institute’s Janelia Farm Research Campus in Ashburn, Va. “We can see very beautiful and very complicated patterns,” said Peng, who presented the results April 9 at the 51st Annual Drosophila Research Conference. “If you look at neurons at a better resolution, or look at regions you’ve never looked at before, you’ll find something new.”

A new analysis technique creates a digital neuron atlas of the fruit fly brain, highlighting how individual brain cells link up. Hanchuan Peng

Peng and his colleagues developed a method, also described in the April Nature Biotechnology, which incorporates many different images of fruit fly brains. The brains come from flies that were genetically programmed so that select neurons glow when struck with a particular type of laser light. By combining thousands of these digital images from different flies, the researchers can create maps of how these different neuronal populations fit together. The full map of the fly brain isn’t yet complete, but it will grow as more images are added.

These kinds of large-scale studies that focus on how neurons are connected are “very important for the future,” commented geneticist Wei Xie of Southeast University in Nanjing, China. Understanding how all of the neurons work together is much more meaningful than studying how a single brain cell connects to another cell, Xie said. “Just a neuron is not enough.”

“What we want to do in the next few years is to add more and more neuron reconstructions into this map,” Peng said. He likened the process to a Google Earth resource. “If you think about the fruit fly brain as the Earth, the little neurons will be the streets. We want to map a lot of neuron streets onto the Earth,” he said.

Peng and his colleagues have started combing their preliminary brain map for interesting features and comparing different flies’ brains to one another. For the most part, patterns of neuron-connecting pathways don’t vary much from brain to brain, the researchers found.

On the other hand, the shapes of cells in the same brain structure can differ dramatically. For example, the variety of shapes found in the neurons of a wheel-shaped brain structure called the ellipsoid body “are just amazing,” Peng says. In the same fly, some of the cells spread inside the ring, while others point outward in a complex lock-and-key arrangement.

The results are preliminary, but finding such unexpected variation could mean that these neurons — which were thought to be nearly carbon copies of each other — have important functional differences.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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