Throughout the tropical forests of the world, there's a parasitic fungus that turns unwitting ants into "zombies." Just how the fungus is able to control the brains of its insect slaves is unknown, but Charissa de Bekker, a post-doctoral researcher at Penn State University, is determined to find out. We caught up with de Bekker to learn more about her fascinating work.
Image: Charissa de Bekker inspecting dead ants that have been infected by a parasitic fungus (marked by orange tape).
The Birth of a Scientist
If you asked Charissa de Bekker if she wanted to be a research scientist 20 years ago, she'd tell you "no." But that's not to say she had an aversion towards science as a child — in fact, it was quite the opposite. "I've always been interested in the natural sciences," she told io9. "But I never saw myself as becoming a researcher."
De Bekker, a child of two non-scientists, grew up in the Netherlands. When it came time to start thinking about her future and what colleges she wanted to attend, she decided to stay in the Netherlands and become a veterinarian. However, things didn't go exactly according to plan.
"There's only one university in the Netherlands that you can study this at," de Bekker explained. And unfortunately for her, there were many other people who had the same life plan as she did. "You have to kind of be in a lottery to get in [to the program], and I didn't make the cut."
The setback forced de Bekker to think hard on what she really wanted to do with her life. Did she want to spend a year working until she could apply to the University of Utrecht's faculty of Veterinary Medicine again? Or would it be better to just study something else in college? If she went to work, she'd run the risk of never returning to school; if she studied something else, it may also take her away from her original goal. In the end, she figured the latter option was better, so she attended the University of Utrecht to study biology — the subject she thought was the most interesting.
As she began her new college career, research science was still the farthest thing from her mind. At the time, she simply loved learning about the things that scientists had discovered in the past. But this didn't last forever.
Falling in Love with Fungus
While doing her undergraduate work, de Bekker got intrigued by genetics and microbiology. It was a reversal for her — before taking microbiology, she thought fungi and other microbes were "lame," she admitted. "People always said they were lower forms of life and don't do anything interesting," she said. Upon finishing her undergraduate studies, she went on to do her Master's and PhD work at the University of Utrecht on the fungal genetics of Aspergillus niger, a species that is "fairly boring to most people."
Micrograph of Aspergillus niger. Credit: Wikimedia Commons.
A. niger is the most common species of the Aspergillus genus, and is behind the vegetative disease black mold, which can ruin certain fruits and vegetables. The fungus is sometimes cultured in the industry to prepare different substances, such as citric acid and gluconic acid. It is also commonly used in the production of high-fructose corn syrup.
De Bekker wasn't interested in figuring out new applications for A. niger, however — she wanted to know what made the fungus tick on the individual, single-celled level. "When you think about a fungal colony, you think about it not being able to move, and the way the fungi grow from the inside out in a radius," she said. The outside cells are, in a sense, exploring and finding new carbon sources to use; the inside cells have a different pH and different set of nutrients than the outer cells. "It's kind of logical that there are different things happening in those different parts of the colony." But, she wondered, are there different things happening in the same region of the colony?
In most studies, researchers look at the composite activity of a bunch of fungal cells in the same area, which doesn't tell you what each cell is doing on its own. So for her PhD thesis, de Bekker developed techniques to investigate the gene expression of individual A. niger cells. When she looked at the activity of five cells at the edge of an A. niger colony, which all lived in the same environment, she saw that the cells were each doing something different. "They kind of had a division of labor," she explained. "This tells us that if you really want to study the mechanisms involved in these fairly simple microbes, you might want to go into the smallest amount of material as you can."
While knee-deep in her research on A. niger, de Bekker was unaware of the strange ant-infecting fungi that would soon become her new passion. Then, one day, she watched BBC's "Planet Earth" and saw a segment on the Cordyceps fungus, which induces "zombie-like behavior" in ants. "Before I saw this BBC movie, I never thought about microbes manipulating behavior," she said. "That really blew my mind."
Though de Bekker was fascinated by the ant-fungus pair, she didn't have the opportunity to work with the system at the time. But a serendipitous encounter changed that — and her whole life direction. Towards the end of her PhD project, she attended a conference in Scotland, where she bumped into a researcher who had done a lot of work on the system: David Hughes. Hughes, who runs a Penn State University lab that investigates the mechanisms behind parasite behavioral manipulation, offered de Bekker a post-doc position. She accepted.
"Everyone told me I was crazy to work on something that was not a model organism," de Bekker recalls. "And a lot of people called me crazy for wanting to move to Pennsylvania to study it." But she did it anyway, and is now a postdoctoral Marie Curie Fellow at Penn State University. Her goal: To unravel how the fungal parasite Ophiocordyceps unilateralis (formerly known as Cordyceps unilateralis) controls the behavior of ants.
The Zombie Ants and Their Fungal Overlords
So far, most of what scientists know about the fungus and how it infects ants comes from a natural history perspective, de Bekker said.
The ants — which live in a variety of habitats, including colonies underground, in the forest canopy or in rotting wood — are safe from mind-controlling parasites in their nests. But when they go out to forage, they can come across fungal spores littered on the ground. When an ant becomes infected, the fungus quickly spreads throughout the ant's body.
"At one point there are enough fungal cells to be able to control the brain and make sure the ant leaves the nest," de Bekker said. The fungus hijacks the ant's brain and forces it to climb up vegetation and clamp down on a leaf or twig with its mandibles before dying. In this spot, the parasite has the optimal temperature and humidity to grow.
In 2009, Hughes and his colleagues discovered that the fungus converts the ant's internal tissues into sugar (a source of food); however, it doesn't degrade the muscle tissues controlling the mandibles, allowing the insect to remain attached to the vegetation even after death. Interestingly, O. unilateralis grows into any cracks in the ant's outer shell, a behavior that both reinforces weak spots and prevents other microbes from entering the ant's husk. Eventually, the fungus sprouts from the ant's neck, and after a couple of weeks, spores rain down on the forest floor to infect more insects.
The Hughes Lab at Penn State is using a variety of approaches to better understand how the parasite manipulates the ants, such as by looking more closely at the ant's behavior, studying how the fungus infects the ants and spreads, and investigating what the fungus does on the molecular level while inside its host. For their work, the team is focusing on O. unilateralis and its host in South Carolina, the carpenter ant Camponotus castaneus.
Charissa de Bekker digging up colonies of carpenter ants in South Carolina. Credit: Lauren Quevillon.
Uncovering the Parasite's Brain-Controlling Powers
With her background in fungal microbiology and genetics, de Bekker spearheads the team's molecular work. "One of the things I am doing is trying to overcome the complexity of the system and trying to make things simpler," de Bekker said. To do this, she designed a technique in which she keeps ant tissues alive outside of the body and grows O. unilateralis next to the ant cells. Then, she can look at what kind of molecules, or metabolites, the fungus secretes to manipulate the ant. "We are not taking the whole ant at once — we are making the fungal cells react to different tissues in different cultures."
In a related study, published last year in the journal PLOS ONE, de Bekker, Hughes and their colleagues tested the method on another insect-fungus system that has been well studied. The work revealed several new toxin metabolites that scientists hadn't described before. What's more, the researchers discovered that the fungus secreted specific metabolites when in the presence of brain tissues, which it didn't secrete next to muscle and other tissues.
De Bekker is also working on another experiment, which she just got funding through the science research crowdfunding platform, Microryza. Rather than looking at metabolites, the research involves determining the different genes the fungus expresses at different points in the infection. To do this, she will be falling back on her PhD work, and using a technique called Laser Capture Microscopy, which allows her to zoom in on the gene activity of single cells.
De Bekker recently tested if the equipment is powerful enough to cut through and isolate the ant's brain and muscle tissues — it is, as you can see in the video below. She plans to compare the genes expressed while the fungus is manipulating the ant host with the genes expressed while the fungus is growing and after the ant dies. This work will yield important clues as to how, exactly, the fungus can do what it does.
Better understanding the fungus will no doubt have medicinal applications, de Bekker said, adding that the Cordyceps fungi are already well known for some of the compounds they secrete. For example, the species Cordyceps sinensis has played a large role in Chinese medicine for a long time — a 2006 study showed that the fungus may help protect patients against certain radiation-induced injuries. Additionally, the species Cordyceps subsessilis produces the compound cyclosporine, which is used as an immunosuppressing drug for transplants. Researchers may find similar useful compounds from O. unilateralis. There are also potential applications for the fungus in neuromedicine and pest control, de Bekker said.
The work with fungus O. unilateralis and the ant C. castaneus will likely keep de Bekker busy for years to come. She doesn't know what's in store for her after this post-doc position, but thinks that she will stick to researching fungi, and possibly other fungal species that are involved in behavior manipulation. There are a number of interesting systems out there, she said, and researchers are trying hard to figure out the mechanisms behind their brain-controlling powers.
"We could really learn a lot from parasites by studying them more," de Bekker said.