Zombie flies that climb higher and higher until they die may sound like something straight out of a horror movie, but it's actually happening in real life. How is this possible? The culprit is a fungal parasite called Entomophthora muscae, which manages to take control of the fly's brain and manipulate its behavior to suit its needs.
Researchers from Harvard University have been studying the internal clock and hormone-producing systems of infected fruit flies to find out more about this strange behavior. They discovered that the fly's brain undergoes a burst of locomotor activity about two and a half hours before it dies, which causes it to climb to an elevated spot known as summiting. But why do zombie flies climb to their death? It all comes down to the fungal parasite's need to spread its spores effectively. By forcing the fly to climb high, it ensures that its spores are released from a favorable location where they can easily reach other potential hosts.
The researchers used machine learning to assist in identifying summiting flies in real-time, which helped them measure the behavior and identify genes and neurons involved. They found that the fungus targets specific areas of the fly's brain responsible for managing circadian and neurosecretory systems, leading to the release of a hormone that triggers the burst of locomotor activity. "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," says lead author Carolyn Elya. What's more, the researchers found that the fungus increases the permeability of the blood-brain barrier, allowing substances in the fly's bloodstream to affect its brain. Transfusing the blood of summiting flies into control flies caused a burst of movement, indicating that substances in the blood contribute to summiting behavior.
While this phenomenon may sound terrifying, it's amazing to see how nature works in mysterious ways. However, there are still many unanswered questions about how the fungus manages to control its host. More research is needed to determine factors like if E. muscae can keep time, which chemicals are involved, and if fungal cells must physically reach the brain to cause a full summiting response. One thing is for sure, the zombie fly phenomenon provides yet another example of how strange and fascinating the natural world can be. As Elya notes, "what the fungus is doing is still a mystery."