A fungal parasite that infects ants and manipulates their behavior to benefit the fungus’ reproduction accomplishes this feat without infecting the ants’ brains, according to a study led by Penn State researchers.
Instead, 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 3-D network that may enable them to collectively control host behavior, the researchers said.
Previous research showed that this species-specific 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 3-D visualizations to determine the distribution, abundance and interactions of the fungi inside the bodies of the ants. This use of the technology represented a breakthrough in the study of this parasite-host system, according to senior author David Hughes, associate professor ofentomology and biology, Penn State.
Working with co-author Missy Hazen in the Microscopy and Cytometry Facility of Penn State’s Huck Institutes of the Life Sciences, researchers 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.
“By stacking these slices, we could reconstruct them in 3-D, giving us a micron-level view of the interaction between the fungus and host, with incredibly high resolution,” Hughes said. “This is an unprecedented view of how a manipulator controls its host.”
“Our collaborators at Notre Dame were able to use deep learning to train computers to differentiate between fungal and ant cells so we could determine how much of the organism is ant and how much is fungus,” Hughes explained.