Can fungus turn ants into zombies?

May 23rd, 2023

Written by: Marissa Maroni

The recently-released HBO series “The Last of Us” features a parasitic fungus that infects humans, hijacking their behavior to promote its spread. Though “The Last of Us” is fiction, this terrifying premise is not far from what happens to some organisms living today. Ants can be infected by a fungus that controls its behaviors, referred to as a ‘zombie ant’. The zombifying fungus, called Ophiocordyceps, infects ants through spores. The spores spread and cause ants to leave their nest, go into the daylight, and sink their jaw into a leaf as they await death¹. Although bizarre, this behavior gives the fungus the perfect opportunity to infect new, unsuspecting victims.

The journey of a zombie ant

The goal of the Ophiocordyceps fungus is to spread to new hosts where it can continue its life cycle. The fungus begins by dispersing spores, a reproductive cell capable of generating more fungus. These spores are similar to the helicopter seeds that maple trees drop, resulting in a new tree. These spores are dispersed from the fruiting body, a stalk of the fungus that grows from an infected ant’s head (see Video 1). Once dispersed, the spores can land on an ant’s cuticle and begin its infection life cycle².

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Yes, the Cordyceps fungus is real—and it can destroy entire insect colonies. 😵‍💫 #NatGeoTikTok

♬ original sound – National Geographic

The first signs of infection in the ant are altered travel patterns. Uninfected ants typically travel above ground in the forest canopy and stick together. In contrast, infected ants travel alone, are hyperactive, and stumble off leaves due to uncoordinated movements that resemble someone that’s intoxicated³. Interestingly, researchers have hypothesized that this change in behavior might be a protective mechanism for the fungus as non-infected ants will attack ants showing signs of infection⁴.

 In the next phase of infection, the ant climbs onto a leaf and bites down where the ant remains locked in place. At this point, the ants continue to display unusual behaviors. To start, the leaf biting behavior, referred to as the death grip, is a new behavior altogether only seen in infected ants. The death grip is thought to be important for keeping the ant in a protected location while the fungus grows⁴. Through observing infected ant behavior, researchers found the death grip is time specific, occurring at a precisetime in the morning³. Further, the ants often choose to bite into leaves that are in direct sunlight⁵.  Because of where the infected ants position themselves and at what time of day, it’s thought that the ant dies in an optimal location for fungal growth and protection from predators⁴. The ants journey ends with its jaw still locked onto a leaf. In time, the fruiting body of the fungus, like growing a new limb, erupts from the ant’s head¹. The fruiting body excretes spores that can infect a new host, completing the fungal parasite’s life cycle and ending the ant’s.

Fascinatingly, even though the ant head is surrounded by fungus cells the brain is left untouched⁶. Could it be protected on purpose so the fungus can continue to control its host?

What changes in the zombified ant brain?

To understand what changes occur in the brain of an infected ant, researchers wanted to look at their DNA, specifically what genes are turned on or off. An organism’s DNA is the code that builds proteins important for performing cellular functions. A transcript or messenger RNA is an in-between molecule in the process of making proteins from DNA. Researchers use a method that can determine the number of transcripts to get a snapshot of which genes are being turned on, called RNA-sequencing. Researchers can use this data to answer questions like “how does infection change what molecules are being made in the brain?” and “can these newly made molecules change ant behavior?” As previously mentioned, an infected ant’s behavior promotes the spread of the fungus by giving it the time and optimal environment to grow and spread. Clearly, from the fungus’ perspective, it is beneficial to protect its host, the ant. Through RNA-sequencing, researchers found evidence of this. They found immune and stress-related genes were decreased in the ant brain during infection. Researchers hypothesize that the fungus may be turning these genes off to prevent premature death in their host (creepy, right?!)⁷.

But what could control the changes in behavior that promote fungal spread? If the fungus preserves the brain, it must be for a reason. Neurons communicating in the brain build and control the various behaviors in any given organism and communicate using signaling molecules called neurotransmitters. Researchers found that in the infected ant brain there was an increase in genes that can generate neurotransmitters, suggesting that the parasitic fungus can alter the neuronal signaling in the ant brain⁷! Although there is not conclusive proof, the changes in neuronal signaling could be tool for the fungus to control the ant’s mind. Another study supports this finding, where, instead of the increases in proteins that make neurotransmitters, researchers found changes in signaling molecules themselves⁶. There are still missing pieces needed to understand how fungus controls ant behavior. However, the science points to the fungus turning on sets of genes that regulate and control the ant’s mind!

Although a bit unsettling to face, zombified organisms do currently exist! On top of that, the ant fungal parasite is not the only parasite able to manipulate its host. In fact, a previous PNK article described various parasites that brainwash organisms upon infection! Understanding how parasites control other organisms gives insight into how behaviors are formed to begin with. For example, if researchers can pinpoint the pathway used by the fungus to control ant behavior, they could harness similar pathways in humans. Imagine being able to control unwanted behaviors like tremors in Parkinson’s disease. Further, this is a compelling example of zombie science fiction actually happening in the real world!

References

1. Jennifer Lu (2019) How a parasitic fungus turns ants into ‘zombies’. National Geographic
2. Hughes, D. P., Andersen, S. B., Hywel-Jones, N. L., Himaman, W., Billen, J., & Boomsma, J. J. (2011). Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection. BMC ecology, 11, 1-10.
3. Hughes, D. P., Andersen, S. B., Hywel-Jones, N. L., Himaman, W., Billen, J., & Boomsma, J. J. (2011). Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection. BMC ecology, 11, 1-10.
4. de Bekker, C., Beckerson, W. C., & Elya, C. (2021). Mechanisms behind the madness: how do zombie-making fungal entomopathogens affect host behavior to increase transmission?. MBio, 12(5), e01872-21.
5. Andriolli, F. S., Ishikawa, N. K., Vargas-Isla, R., Cabral, T. S., de Bekker, C., & Baccaro, F. B. (2019). Do zombie ant fungi turn their hosts into light seekers?. Behavioral Ecology, 30(3), 609-616.
6. Loreto, R. G., & Hughes, D. P. (2019). The metabolic alteration and apparent preservation of the zombie ant brain. Journal of insect physiology, 118, 103918.
7. De Bekker, C., Ohm, R. A., Loreto, R. G., Sebastian, A., Albert, I., Merrow, M., … & Hughes, D. P. (2015). Gene expression during zombie ant biting behavior reflects the complexity underlying fungal parasitic behavioral manipulation. BMC genomics, 16, 1-23.

Cover photo by Roman Grac from Pixabay