A dead spiny ant with fungal spores erupting out of its head. (Image: David Hughes/Penn State University)
Carpenter ants of the Brazilian rain forest have it rough. When one of these insects gets infected by a certain fungus, it turns into a so-called “zombie ant” and is no longer in control of its actions. Manipulated by the parasite, an infected ant will leave the cozy confines of its arboreal home and head to the forest floor—an area more suitable for fungal growth. After parking itself on the underside of a leaf, the zombified ant anchors itself into place by chomping down onto the foliage. This marks the victim’s final act. From here, the fungus continues to grow and fester inside the ant’s body, eventually piercing through the ant’s head and releasing its fungal spores. This entire process, from start to finish, can take upwards of ten agonizing days.
We’ve known about zombie ants for quite some time, but scientists have struggled to understand how the parasitic fungus, O. unilateralis (pronounced yu-ni-lat-er-al-iss), performs its puppeteering duties. This fungus is often referred to as a “brain parasite,” but new research published this week in Proceedings of the National Academy of Sciences shows that the brains of these zombie ants are left intact by the parasite, and that O. unilateralis is able to control the actions of its host by infiltrating and surrounding muscle fibers throughout the ant’s body. In effect, it’s converting an infected ant into an externalized version of itself. Zombie ants thus become part insect, part fungus. Awful, right?
To make this discovery, the scientist who first uncovered the zombie ant fungus, David Hughes from Penn State, launched a multidisciplinary effort that involved an international team of entomologists, geneticists, computer scientists, and microbiologists. The point of the study was to look at the cellular interactions between O. unilateralis and the carpenter ant host Camponotus castaneus during a critical stage of the parasite’s life cycle—that phase when the ant anchors itself onto the bottom of leaf with its powerful mandibles.
Ants infected with late stage O. unilateralis infection. (Image: David Hughes/PLOS ONE)
“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,” said lead author Maridel Fredericksen, a doctoral candidate at the University of Basel Zoological Institute, Switzerland, in a statement. “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.”
By referring to the parasite’s “extended phenotype,” Fredericksen is referring to the way that O. unilateralis is able to hijack an external entity, in this case the carpenter ant, and make it a literal extension of its physical self.
For the study, the researchers infected carpenter ants with either O. unilateralis or a less threatening, non-zombifying fungal pathogen known as Beauveria bassiana, which served as the control. By comparing the two different fungi, the researchers were able to discern the specific physiological effects of O. unilateralis on the ants.
Using electron microscopes, the researchers created 3D visualizations to determine location, abundance, and activity of the fungi inside the bodies of the ants. Slices of tissue were taken at a resolution of 50 nanometers, which were captured using a machine that could repeat the slicing and imaging process at a rate of 2,000 times over a 24-hour period. To parse this hideous amount of data, the researchers turned to artificial intelligence, whereby a machine-learning algorithm was taught to differentiate between fungal and ant cells. This allowed the researchers to determine how much of the insect was still ant, and how much of it was converted into the externalized fungus…..more here
