In a way, fruit flies are just like us. They have eyes, legs, a nervous system, and they love fruit. Unlike us, however, they only have a few thousand neurons in their brains, meaning scientists can map not only all cells but also all the connections between them, producing a complete digital “connectome” for the first time. of a living creature that is, when you think about it, basically a human.
Perhaps I’m exaggerating our similarities to fruit flies, commonly called by their scientific name, Drosophila (melanogaster, though that part isn’t usually needed), but there’s a reason we use them in so many biological experiments. You might not think you look much like one of these creatures, but you definitely look more like a fruit fly than a bacterium or dinoflagellate. Understanding even a relatively simple animal like the fruit fly teaches us a lot about animals and life in general.
Despite being, along with yeast, perhaps the best known organisms, a single drosophila is still too complex to simulate all its aspects. Hell, we’re having trouble simulating a single cell correctly. However, if you consider a creature not as a gestalt but as a collection of interrelated systems, you can start to bite the elephant.
The most recent bite, from a team led by biologists at the University of Cambridge, is a “synapse-by-synapse map” of a larval drosophila brain. With 3,016 neurons and 548,000 synapses, it’s 10 times the complexity of the last organism whose brain was mapped, a member of Congress. (Actually, it was one of the worst types of worms, an annelid. Humans have about 86 billion neurons and nearly countless synapses.)
The fruit fly larva is, of course, not a fly, but it is already a sophisticated creature, with adaptive behaviors, structures analogous to adult fly brains, short- and long-term memory, and other expected brain functions. Also, they are easier to catch. More importantly, it has “a compact brain with several thousand neurons that can be visualized at the nanoscale with electron microscopy (EM) and their circuitry reconstructed in a reasonable time frame.” as the article published today in Science says. In other words, it’s the right size and not too weird.
The brain was sliced into incredibly thin layers and imaged via EM, and the resulting slices were carefully examined to determine how neurons, axons, and other cellular structures continued between them. “We developed an algorithm to track signal propagation throughout the brain through polysynaptic pathways and analyzed feedback (sensory-to-output) and feedback pathways, multisensory integration, and interhemisphere interactions,” they write.
Serial section volume electron microscopy revealing the structure of the Drosophila brain. Image Credits: michael winding
The result is the model you see, which looks like a slug in a clown wig (I need not add that this is not what it looks like). live).
Of course, there are many interesting observations about the way the brain is organized, from nested recurring loops, multisensory integration, interactions between hemispheres, and all that good stuff. But having a complete connectome of a complex living creature is fundamentally exciting for anyone in that space – there’s a lot you can do when you have a decent simulation of a brain. While previous studies have replicated individual subsystems or smaller brains, this is the largest and most comprehensive characterization yet, and as a 3D digital resource, it will almost certainly be used and cited throughout the discipline.
Some of these things are even found in artificial neural networks; studying how such complex behavior occurs in such a sparsely populated brain could “perhaps inspire new machine learning architectures.”
Interestingly, we already have a detailed mechanical model of the adult fly’s body and movements, and while the question is obvious, the answer is no: we can’t put this brain in that body and say we’ve simulated the whole thing. But maybe next year.
