Scientists in the UK have built the first-ever complete map of a fruit fly larva’s brain. The map showing the neural connections in the insect’s brain comprises 3,016 neurons and 548,000 synapses, they said. The scientists used thousands of slices of the larva’s brain imaged with a high-resolution electron microscope, to reconstruct a map of the fly’s brain.
“If we want to understand who we are and how we think, part of that is understanding the mechanism of thought,” says Joshua T. Vogelstein, a biomedical engineer at Johns Hopkins University. “And the key to that is knowing how neurons connect with each other.”
To create this magnificent multi-functional map called a connectome, researchers scanned thousands of slices of the baby fruit fly’s brain with a high-resolution electron microscope. They then put the pictures together and added them to data they had already gathered, meticulously marking each and every connection between neurons.
That includes both the cells that talk to each other within each half of the brain as well as those that communicate between the two hemispheres, making it possible to study the interactions across the brain in depth. The brain’s hemispheres have unique and important functions, but how they integrate and use information from each side for complex behavior and cognition is not as well understood.
“The way the brain circuit is structured influences the computations the brain can do,” explains neuroscientist Marta Zlatic from the University of Cambridge. “But, up until this point, we’ve not seen the structure of any brain except the roundworm Caenorhabditis elegans, the tadpole of a low chordate, and the larva of a marine annelid, all of which have several hundred neurons.”
Recently, scientists have made significant progress in charting the human brain and tracking neural activity in mice, but the focus has been on specific regions and current technology is still not advanced enough to complete a connectome for larger animals such as humans.
However, Zlatic explains, “All brains are similar – they are all networks of interconnected neurons – and all brains of all species have to perform many complex behaviors: they all need to process sensory information, learn, select actions, navigate their environments, choose food, recognize their conspecifics, escape from predators, etc.” Fruit flies (Drosophila melanogaster) are a popular scientific research model due to their easy-to-study features, their complex but compact brains, and because they share many biological similarities with us humans.
Notably, the connecting structures the researchers observed were determined to be most repetitive among the ingoing and outgoing neurons in the part of the brain that allows us to learn and remember what we’ve learned. They also found that some of the identified features worked in a similar way to some computer networks for machine learning.
“What we learned about code for fruit flies will have implications for the code for humans,” states Vogelstein. “That’s what we want to understand – how to write a program that leads to a human brain network.”
The team suggests the next step will be to learn more about the neural structure involved in certain behavioral functions, such as learning and decision-making, and to look at the activity of the whole connectome while the insect is engaged in the activity.
The first effort to map a brain was a 14-year study of C. elegans that started in the 1970s. It yielded an incomplete map of the roundworm’s brain and eventually earned the scientists a Nobel Prize.
“It’s been 50 years and this is the first brain connectome. It’s a flag in the sand that we can do this,” Vogelstein says.
“Everything has been working up to this.”
The research has been published in the journal Science.