A fungal parasite that infects ants and manipulates their behavior to benefit the fungus' reproduction does this without infecting the ants' brains, according to a study led by Penn State researchers. These findings are significant as they demonstrate that the zombie ant fungus parasite does not need to physically invade the brain in order to exert behavior control and that parasite cells may coordinate to change host behavior.
Ophiocordyceps unilateralis sensu lato—a complex of species sometimes called zombie ant fungus—surrounds and invades muscle fibers throughout the ant's body, and fungal cells form a 3D network that may enable them to collectively control host behavior, the researchers explained.
Previous research showed that this parasite controls the behavior of carpenter ant workers, compelling them to climb vegetation and bite into the underside of leaves or twigs, where they die. As the fungus grows in the ant cadaver, it produces a stalk that protrudes from the ant's head and discharges infectious spores onto the ground below, where they can infect other foraging ants.
"To better understand how such microbial parasites control animal behavior, we looked at cell-level interactions between the parasite and its carpenter-ant host at a crucial moment in the parasite's lifecycle—when the manipulated host fixes itself permanently to vegetation by its mandibles," said lead author Maridel Fredericksen, former master's degree student in entomology at Penn State, now a doctoral candidate at the University of Basel Zoological Institute, Switzerland.
"The fungus is known to secrete tissue-specific metabolites and cause changes in host gene expression as well as atrophy in the mandible muscles of its ant host," she said. "The altered host behavior is an extended phenotype of the microbial parasite's genes being expressed through the body of its host. But it's unknown how the fungus coordinates these effects to manipulate the host's behavior."
To investigate this, the research team infected ants with either O. unilateralis s.l. or with a generalist fungal pathogen, Beauveria bassiana, to distinguish between effects that are common to pathogenic fungi from those that are specific to O. unilateralis s.l.
Using serial block-face scanning-electron microscopy, the team created 3D visualizations to determine the distribution, abundance, and interactions of the fungi inside the bodies of the ants. The researchers then took slices of tissue at 50 nanometers and captured images of each slice, using a machine that could repeat that process 2,000 times over a 24-hour period.
The team found that O. unilateralis s.l. cells were present throughout virtually all regions of the host ants' bodies, including in the head, thorax, abdomen and legs. In addition, a large proportion of these fungal cells were connected, suggesting that they form a network to control the host's behavior collectively.
However, although fungal cells were concentrated directly outside the brain, researchers observed no fungal cells inside the brain. "Normally in animals, behavior is controlled by the brain sending signals to the muscles, but our results suggest that the parasite is controlling host behavior peripherally," explained senior author David Hughes, associate professor of entomology and biology, Penn State.
Although the host brain isn't invaded by fungal cells, previous work has shown that the brain may be chemically altered by the parasite, Hughes noted. "We hypothesize that the fungus may be preserving the brain so the host can survive until it performs its final biting behavior —that critical moment for fungal reproduction. But we need to conduct additional research to determine the brain's role and how much control the fungus exercises over it."
Image: An ant manipulated by the "zombie ant fungus" bites onto the underside of a twig—it's last act before dying and becoming a platform for fungal reproduction. Image courtesy of Kim Fleming, Penn State.