Why scientists are mapping the fruit fly's surprisingly complex brain
‘There's a lot going on in that little brain,’ says neuroscientist Vivek Jayaraman
Fruit flies might be small, but their brains are mighty.
So mighty, in fact, that dozens of scientists have spent years mapping the hundreds of thousands of neurons firing between millions of synapses inside a fruit fly's poppy seed-sized brain.
"It's one of those things that people maybe don't realize when they're trying to swat a fly," neuroscientist Vivek Jayaraman told As It Happens host Carol Off.
"There's a lot going on in that little brain."
Jayaraman works for the Janelia Research Campus of the Howard Hughes Medical Institute in Virginia, which, in collaboration with researchers at Google, has been undergoing an ambitious project to map a fruit fly's brain.
Their work is still ongoing, but this week, the researchers published some of their early findings in the journal eLife — a whopping 350 pages about the complex circuitry found in just one section of the insect's brain.
What is a 'connectome' and how do you make one?
The brain map is called a connectome, and it's being built through a combination of painstaking manual labour and machine-learning algorithms.
When it's finished, it will provide a detailed diagram of the fly's 100,000 neurons and the tens of millions of connections between them.
It's a rarely used concept in science, though not a new one. In the 1960s, Nobel Prize-winning biologist Sydney Brenner and his team spent years building a connectome of a roundworm's brain.
"Brenner realized that to understand the nervous system you had to know its structure," Scott Emmons, a neuroscientist and geneticist at Albert Einstein College of Medicine, told the New York Times.
Brenner's connectome was hand-traced with coloured pens. But modern computer modelling opens new doors for building more elaborate brain maps — like that of the fruit fly.
While Jayaraman's colleagues are no longer tracing brain pathways by hand, there's still a lot of detailed human labour involved.
In order to create a digital map of a fruit fly's brain, the scientists first had to cut a real one into teeny slabs, then scan images of those slabs using a technique called focused-ion beam scanning electron microscopy.
"I know that's a mouthful, but essentially they were able to image these really tiny pieces of brain tissue and simultaneously, or right after, slice it, look at the next layer and so on down the line of this tiny poppy seed," he said
"It's actually harder than you might think to do this flawlessly for hours."
The researchers then used a computer program to stitch the images back together into a 3D map, and machine-learning algorithms to identify the neurons and trace the pathways between them — all of which are painstakingly checked over, corrected, and adjusted by human scientists.
"This is the effort of a lot of people," Jayaraman said.
'One tiny piece of that poppy seed'
In their latest paper, the researchers detail what they found in the central complex region of the fly's brain.
"We've looked at just one tiny piece of that poppy seed — just the part that, among other things, controls how the fly navigates," Jayaraman said.
The central complex allows the fly to understand its spatial relationship to the world around it, and execute decisions based on that.
That's how it can, for example, catch the whiff of a banana on your counter, figure out how to get into your home through a tiny crack in the window, eat its fill of banana, move onto the next thing, and never forget where it is or how it got there.
"Think of yourself, like, hiking in a forest or something like that. And let's say that the sun is kind of low on the horizon. It's a pretty good cue for which direction you're headed. Better than maybe even the trees that you've barely seen before. Maybe you've been on that trail just once or twice. So local cues aren't as informative, sometimes, as celestial cues," Jayaraman said.
"And so those kinds of things seem wired into the fly brain. It's going to trust certain cues more than others."
Why does it matter?
So what's the value in understanding the complex brain activity that goes into a fruit fly finding its dinner?
"All I do at dinner parties, if someone asks me about this work, I ask them to shut their eyes and then … put whatever morsel of food they have on their fork into their mouths. And they do it effortlessly. And then I ask them how they did that. And they clearly use some kind of internal model to do that. They knew where the hands would be. They knew where their mouth was. They could connect the dots," Jayaraman said.
"That's the kind of process that a fruit fly brain needs as well."
It's food for thought the next time you're about to swat a fruit fly that's buzzing around your lunch, Jayaraman said.
"At the end of the day, I mean, if an insect seems like a pest because it's competing with you for food, I understand why you may feel, you know, the need to do what you do," he said.
"But that said, I hope you at least do it with greater appreciation for the creature — the marvel that is that fly."
Written by Sheena Goodyear. Interview produced by Chris Trowbridge.