Scientists have uncovered intriguing new details about a bizarre “zombie fungus” that manipulates the behavior of the carpenter ants it infects—with gruesome results.
When the fungus—known as Ophiocordyceps unilateralis sensu lato—enters the body of these ants, it overrides their movements, causing the insects to climb up to elevated locations and bite onto vegetation with their mandibles, where they remain permanently fixed, according to a study published in the Journal of Experimental Biology.
Once in this position, the fungus then devours the ant from the inside before scattering its infectious pores.
“Roughly 2-3 weeks after an ant becomes infected with this fungus, it begins to exhibit changes in behavior,” Colleen Mangold, lead author of the study from Pennsylvania State University, told Newsweek. “For example, it starts to have muscle spasms and has difficulty walking and moving around. Ultimately, it will climb to a higher elevation and bite down onto a leaf vein or twig. Shortly after the ant has bitten down onto vegetation, it dies, and the fungus is able to grow, and later emerge from inside the ant.”
Mangold’s team say that the fungus “extensively colonizes” the muscles that control the mandibles—one of the mouth parts of the ants—but not the brain.
For the study, Mangold and colleagues sought to find out how exactly the fungus is able to influence these muscles despite causing widespread muscle damage.
“We knew from previous research that the fungus builds interconnected networks around host mandibular muscle cells,” Mangold said. “We wanted to get a better understanding of what the host muscle looked like and what the fungus looked like at the time of biting. This would hopefully give us some clues as to what may be happening to cause the infected ant to bite down onto vegetation.”
However, studying the fungal infection was tricky because it can only affect free-roaming ants, not those that live in nests. Furthermore, it only thrives in very humid environments, so the team had to recreate these conditions in the lab in order to take samples of the infectious pores.
The scientists took infected ants and froze them before studying their body structures with an electron microscope.
These investigations revealed that the fungus had entered the muscles but had not affected the local nervous system in the process. Instead, the scientists found evidence of what they call “hypercontraction.”
Essentially, the fungus penetrates the muscle by breaking through the membrane covering the muscle fibers, causing it to contract forcefully. This is what forces the ant to clamp its mandibles to a piece of vegetation. They also identified the presence of tiny bead-like toxins known as vesicles that may play a role in this contraction by causing the muscles to spasm.
“We were able to see that the host mandibular muscles appear to be in a state of forceful contraction at the time of biting,” Mangold said. “Additionally, we were able to see that the fungus physically penetrates host muscle cells. We also saw the presence of extracellular vesicle-like particles associated with fungal cells.”
“We can hypothesize that these vesicles may serve as a means of communication between host and fungal tissue, or between fungal cells, that may contribute to muscle contraction at the time of biting,” Mangold said. “However, further research needs to be done to test that hypothesis.”
In fact, the researchers say there is much more to learn about the mechanisms the fungus uses to manipulate its host.
“The next steps we want to take include isolating those vesicles and determining whether they are coming from the fungus or the host,” Mangold said in a statement.
The zombie ant fungus is one of many types of unusual parasitic fungi that have developed highly specialized relationships with plants or insects. In general, these fungi tend to penetrate the outer defenses of the organisms that they infect before feeding off living material in their bodies, often causing disease and death.
One bizarre genus known as Septobasidium infects scale insects which feed on trees, according to the Encyclopaedia Brittanica. The fungus forces these insects to permanently fix their proboscis into the bark of these trees and suck sap for the remainder of their lives.
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