Jacinta Bowler
Infection starts with a run. That’s according to researchers who analysed hundreds of ‘zombie’ fruit flies hour by hour to see how their behaviours change once infected with a pathogenic fungus.
“It’s actually this burst of locomotor activity that starts about two and a half hours before the flies die,” says Harvard ‘zombiologist’ Dr Carolyn Elya.
Entomophthora muscae is one of many fungi that can cause ‘zombie’ behaviours in insects. Its prey is flies.
Although ‘zombie’ seems like not much of a scientific term, the paper – published in eLife, calls the flies ‘zombies’ when the fungus takes over and controls the behaviour of the flies.
Researchers know that once infected, around sunset the fruit flies will make their way to an elevated position – known as summiting. Most studies about summiting had only been observations of dead house flies. No one had ever observed how fruit flies behave in their last hours of life.
This project started over half a decade ago when Elya found zombie fruit flies on rotting fruit she’d set up to study microbes carried by the flies.
As Eyla and her team set out to painstakingly track what happens in these zombies’ final hours.
The researchers put 128 flies into a tray so they could monitor them simultaneously, and analysed what happened once they were infected.
First was the summitting or climb, which occurs around sunset.
“The climbing is very important as it positions the fly in an advantageous location for the fungus to spread to the most possible hosts,” says Elya.
“The fungus jumps to the new host by forming very specialised and temporary structures that burst through the fly’s skin and shoots spores into the environment that are only good for a handful of hours. It’s a fleeting process, so an advantageous position is everything to survival.”
Then, a droplet from the mouthpart or proboscis then sticks the infected fly to surface. The wings raise themselves up away from the body and lastly the fly dies.
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The research also analysed the flies’ genes and neurons required for flies to start the summiting process.
“Overall, we found the flies hormonal axes was mediating summiting behaviour. When we silenced these neurons the flies were really bad at summiting,” Elya says.
“We think the fungus is actually driving the activity of these neurons in order to drive the release of this hormone, which is causing the flies to have this burst of locomotor activity.”
This fungus is not going to be spreading to humans any time soon unlike the scenario portrayed in the 2023 TV series ‘The Last of Us’ in which a pandemic is caused by a type of fungi.
Zombie flies have been known to science since 1855, and there’s no evidence that they’ve ever been able to transmit to humans. But even with the researchers’ thorough analysis, there’s still much that we don’t understand about this fungus.
“There are still a lot of open questions here,” Elya says.
“What the fungus is doing is still a mystery.”