This Parasitic Fungus Turns Flies Into Zombie Insects

Imagine a fly going about its day, only to suddenly start behaving strangely. Its movements slow down, its abdomen swells, and white fuzz appears on its body.

As evening approaches, the infected fly climbs to a high point, like the top of a plant or a stick, and releases a sticky substance that attaches it firmly to the perch. It then lifts its wings and dies.

From the dead fly’s body, white spores are released, infecting other unsuspecting flies below. And the cycle of infection begins anew.

This white fuzz that consumes the flies is actually a fungus known as Entomophthora muscae, which translates to “destroyer of insects.” This fungus infects flies, turning them into “zombies” that carry out its bidding.

Discovered over 160 years ago, the actions of this fungus have puzzled scientists for years. How does it control the fly’s brain? How does it time its manipulation to occur at sunset? What genes in its genome enable it to become a master manipulator?

Today, researchers are conducting experiments to uncover the science behind this eerie mind control.

The Horrifying Behavior

Henrik H. De Fine Licht, an evolutionary biologist at the University of Copenhagen, is fascinated by the behavior of “zombie” houseflies infected with Entomophthora muscae. He was initially drawn to study this fungus to understand obligate pathogens but found himself captivated by the behavioral manipulation aspect.

After infecting the fly, the fungus doesn’t immediately attack vital organs. Instead, it feeds on fats and other nutrients, gradually starving the fly while keeping it alive. Only when the non-vital organs are consumed does the fungus begin to control the fly’s behavior, ensuring its spores are spread widely by forcing the fly to climb to a high point.

De Fine Licht was particularly intrigued by reports of how the fungus manipulates flies by making female fly carcasses attractive to healthy males. By analyzing chemicals extracted from infected and uninfected fly carcasses, his team discovered that the fungus releases volatile chemicals that lure males in.

One interesting observation by Annette Jensen, an organismal biologist at the University of Copenhagen, was that earwigs, predators of insects, were drawn to the infected fly cadavers. They preferred feeding on these cadavers over others, suggesting that the volatiles from Entomophthora muscae are highly nutritious.

Expanding the Victims List

While most research on zombie flies has focused on houseflies, molecular biologist Carolyn Elya from Harvard University turned her attention to fruit flies after discovering zombified ones in her backyard. Infecting Drosophila melanogaster, a common lab model, with Entomophthora muscae, she aims to leverage the genetic toolkit of fruit flies to study how the fungus manipulates the fly’s brain.

In a recent study, Elya and her team found that the fungus might secrete substances into the fly’s hemolymph, manipulating its neurons. Injecting hemolymph from infected flies into healthy ones resulted in the latter displaying zombie-like behavior.

Furthermore, Elya discovered that specific circadian neurons in the fly’s brain might be involved in the height-seeking behavior observed in infected flies around sunset. Silencing these neurons inhibited the climbing activity in infected flies.

Elya and De Fine Licht recently sequenced the genome of Entomophthora muscae infecting fruit flies. They found genes similar to one called “white-collar 1,” which is involved in circadian rhythms in another mold. This gene might play a role in driving the precise timing of infected flies’ behavior before death.

Additionally, the researchers identified genes that help the fungus utilize the fly’s tissues and nutrients effectively. These genes code for enzymes that digest sugars, proteins that break down chitin in the fly’s exoskeleton, and lipases that break down fats.

The quest for more insights continues, with researchers analyzing the RNA of genes to understand how the fungus manipulates the fly’s behavior. By studying the active genes inside the fly’s head at different time points post-infection, they hope to uncover the mechanisms behind the manipulation.

One intriguing finding was the activity of a fungal gene similar to one found in zombifying viruses. These viruses force infected caterpillars to climb to high locations before melting and releasing viral particles. The researchers now aim to determine if this gene in Entomophthora muscae is crucial for inducing climbing behavior in infected flies.

Moreover, recent studies suggest that Entomophthora muscae may be infected by a virus while parasitizing flies. The role of this virus in controlling the fly remains unclear.

Researchers are still investigating how the fungus initiates its manipulation of the fly. One theory proposes that the fungus releases a chemical that activates the neurons involved in climbing behavior, while another suggests that the fly’s physiological changes trigger its own neurons to kickstart the process.

De Fine Licht hopes to introduce the zombie fly-fungus system into educational curricula to engage young science enthusiasts. He recently published instructions on observing the fungus in a lab setting, which could be a fun experiment for high school students.

Whether it’s collecting fly cadavers from the field or infecting healthy flies in a laboratory, studying this bizarre phenomenon is not only fascinating but also educational.

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