Ophiocordyceps and its Effects on Ant Communities

By Iain Oothoudt | Graphic Design Major

There have been many zombie movies/TV shows created as thrillers, horror or action films which depict what life would be like in an apocalypse. These forms of entertainment illustrate a fictional reality for the audience to enjoy, but how would people react to a real-life scenario? What would it be like if a zombie infection attacked the world as we know it? People may raid stores to pack up on food medicine and supplies, others might storm weapons shops to get anything as a means of protection. Overall the world could fall into anarchy and chaos. Most people do not know this, but there is a real version of this zombie apocalypse except it is on a significantly smaller scale and with a very different species. This zombifying infection is called Ophiocordyceps unilateralis and is one of many different fungi that targets ants. By manipulation of physical, genetic and behavioral traits this deadly fungus promotes its own growth and reproduction.

The full scientific name of this fungus is Ophiocordyceps unilateralis and it is an organism that attacks/targets insects. The fungus is predominantly found in tropical forest ecosystems. The organism was discovered in 1895 by the British naturalist Alfred Russel Wallace (Penn State News, 2015), However, it has only been in the last couple of years that scientists have uncovered details about the mechanisms of this fungus. O. unilateralis is considered both a parasite and a fungus. It is a parasite because it attacks a host and has a relationship with the host to where it only benefits. The organism is also recognized as a fungus because it has “cell walls made of chitin, it has hyphae that digests the organisms food before ingesting and it releases spores as a mode of reproduction” (Biology Wise, 2018). When Ophiocordyceps unilateralis infects an ant the mortality rate is one hundred percent and the chance of its own reproduction increase significantly.

The genetic impact of O. unilateralis on ants is highly complex with many genomic factors weighing in, scientists have recently just started to uncover specific details about the fungus genetic history and evolution. Wichadakul et al. (2015) designed a study to discover more about this organisms adaptations over time, in the study he concluded that the organism “ophiocordyceps possesses protein-coding genes similar to those found on other insect fungi (Beauveria bassiana, and Metarhizium robertsii) which had more available genomic resources.” These two fungi were “comparatively analyzed with ophiocordyceps pathogenicity and virulence in order to gain insights into its biology and the emergence of host specificity” (Wichadakul, 2015). After completion of the analysis the researchers found that in comparison with the other fungi, O. unilateralis had a reduced number of gene families found related to pathogenicity. Furthermore, some of the common gene families in other fungi were missing in Ophiocordyceps. On the other hand O. unilateralis had an increased number of gene families related to the production of bacterial-like toxins and retrotransposable elements. (Wichadakul, 2015). Through the specific abundance and deficiency of certain gene types it shows the adaption of this fungus over time and reveals the many attributes that this fungus carries. “The loss of various genes involved throughout the pathogenesis for O. unilateralis would result in a reduced capacity to exploit larger ranges of hosts and contain a different level of host specificity, while the expansions of other gene families suggest an adaptation to particular environments with unexpected strategies” (Wichadakul, 2015). The evolution of this fungus may have limited O. unilateralis to only target the ant species, however, there is still a large variety of ant types infected, over one hundred different ant types are infected by Ophiocordyceps.

Before the parasitic fungi O. unilateralis is completely in control of its target it needs to express and manipulate certain genomic factors to do so. DeBeckker et al. (2015) worked to discover what genes O. unilateralis manipulates to control their host. In this experiment the he “experimentally infected Carpenter ants to collect tissue from both parasite and host during the abnormal behavioral period. Afterwards, samples were collected and subjected to mixed RNA-Seq analysis.” After completion of this experiment the DeBeckker found many gene related abnormalities that reveal how the fungi can control their host. This study uncovers that “the fungal parasite might be regulating immune and neuronal stress responses in the host during abnormal behavioral period, as well as impairing its chemosensory communication and causing apoptosis. Moreover, it found genes up-regulated during manipulation that putatively encode for proteins with reported effects on behavioral outputs, proteins involved in various neuropathologies and proteins involved in the biosynthesis of secondary metabolites such as alkaloids” (De Bekker, 2015). These manipulations show the interesting genomic factors that the fungus affects: apoptosis (programmed cell death), chemosensory communication (use of chemicals to communicate certain messages), and up-regulated genes (increase of responses to a stimulus). Additionally, it displays how these genes are used to influence the ants behavior and physical state.

The most notable behavioral abnormality is called the death grip; the death grip is a behavioral action the ant is forced to take. When the ant is infected, the fungus forces it to find a location optimal for the fungus reproduction. When the area is identified, the ant bites into a vein of a leaf and dies. Preceding this death grip phenotype there are two smaller behavioral abnormalities that take place. The first is random walking. Before the ant is infected and influenced by the fungus its movements are purposeful and it usually stays away from the forest floor: “Ants that dwell in the canopy rarely descend to the forest floor and when they do they always travels on well defined trails. Trail individuals do not forage on the forest floor and trails normally ascend into the canopy within 3–5 m from where they descended” (Hughes, 2011). Once the ants are infected by the fungus they start to descend to the forest floor where there is shorter vegetation. During their journey to the forest floor their movements are noticeably irregular and unnatural. The second small behavioral abnormality that takes place is convulsive activity. While walking down to the shorter vegetation the ants sometimes have short convulsive episodes where they fall to the forest floor and lose their ability to navigate back up to the canopy. The small behavioral abnormalities induced by fungal infections are a complex example of behavioral manipulation requiring coordinated changes of host behavior and morphology (Hughes, 2011). This complex behavioral manipulation shows how much control the fungus has over the and and reveals the intricate way that this fungus influences behavior.

Another notable behavioral abnormality that the infection causes is the location of death. As mentioned in the paragraph above, most infected ants journey to the forest floor where the smaller vegetation is and die there. This interesting behavior further re-enforces the irregular behavior of infected ants and shows how much control the fungus has over their host. In 2009, a study was done to find out what causes the odd death location. “Our study has shown that the parasitic O. unilateralis fungus has a strong effect on the spatial structure of dead ants; that is mostly influenced by environmental conditions” (Pontoppidan, 2009). There is a wide variety of environmental conditions that could affect the spatial structure so heavily such as: time, temperature, and humidity. All of these factors are taken into account so the spores have the highest chance if infecting a new host and promoting the reproduction of the fungus.

After being infected by the fungus the ant only has a short time before their inevitable death. The entire process usually takes a couple weeks from the initial infection to the final release of spores at the end. First the ant gets infected by the fungus, over time the fungus grows and slowly starts protruding out of the back of the ants head. Even though there are different features of all the different types of O. unilateralis most types have generally the same look. All of the already discovered fungi grow out of the posterior side of the ants head and the shape of the fungus usually looks like a stem with a fruiting bud at the end of it. After the fungus matures to a certain point it begins the process of physically manipulating the host, “The fungus accurately manipulates the infected ants into dying where the parasite prefers to be, by making the ants travel a long way during the last hours of their lives” (Science daily, 2018). The ants are manipulated out of their canopy home and forced to bite onto either a small sapling on the forest floor or at the base of a tree. “After the ant dies, the fungus continues to grow inside the body, as the fungus spreads within the dead ant’s body, it converts the ant’s innards into sugars. But it leaves the muscles controlling the mandibles intact to make sure the ant keeps its death grip on the leaf. The fungus also preserves the ant’s outer shell, growing into cracks and crevices to reinforce weak spots. In doing this, the fungus fashions a protective coating that keeps microbes and other fungi out” (science daily, 2018). By feeding off of the nutrients inside the ant’s body, the fungus can produce the spores necessary for its reproduction. “After a week or two, the stroma starts raining down the spores to the forest floor below. Each spore has the potential to infect another unfortunate passerby” (Science daily, 2018). The spores that are released occasionally can be spread considerably long distances by both biotic and abiotic factors but usually the diameter of the infection site is only a couple meters.

For non-infected ants, avoidance is the best tactic to combat Ophiocordyceps. “The most important way they avoid infection seems to be staying as far away from victims as possible. That may be part of the reason why these ants make their nests in the forest canopy, high above fungal breeding zones. Ants also seem to avoid blazing their foraging trails under infected areas. This too might be an adaptive strategy to avoid infection, but more study is needed to confirm it” (Science daily, 2018). Since O. unilateralis solely targets worker ants the decrease of ants with this role in the community can have detrimental effects on the ant colony. Worker ants main purpose is to gather/hunt for the colony. Overall worker ants are expendable but, if a large number of them die it can make it harder for the colony to survive and thrive. It would be interesting to see if a different type of fungus targeted ants that have a different role in the community, such as the warriors/protectors, and learn how that would affect the community. So far, minimal, if any, research has been found showing O. unilateralis attacking other ants besides the worker class. Currently no known resistances in the ant community have been identified, but as this parasitic relationship continues there is a possibility that certain ants will develop immunity to the fungus and pass that gene onto their offspring. The chances of this happening are small, however, mutations like this could occur to further improve ant fitness and protect the ant community for future generations.

O. unilateralis is an interesting and unique disease that can have major effects on the working class of ant communities. Through the use of genetic, behavioral, and physical manipulation, this parasitic fungus uses ants as their means for growth and reproduction. With O. unilateralis adaptation over time it has developed unique tactics to influence its hosts, disperse its spores, protect itself from competing fungi, and grow into the horrifyingly deadly infectious disease it is today. Hopefully in the future scientists can learn more about O. unilateralis and provide more information about the mechanics of this extraordinary fungus.

Biology Wise. (2018). Characteristics of fungi. BiologyWise.com

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.(research article). BMC Genomics, 16, 620.

Hughes, D., Andersen, S., Hywel-Jones, N., Himaman, W., Billen, J., & Boomsma, J. (2011). Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection. BMC Ecology, 11, 13. doi:10.1186/1472–6785–11–13

Pontoppidan, M., Himaman, W., Hywel-Jones, N., Boomsma, J. J., & Hughes, D. P. (2009). Graveyards on the move: The spatio-temporal distribution of dead ophiocordyceps-infected ants. PLoS ONE, doi:10.1371/journal.pone.0004835

Wichadakul, D., Kobmoo, N., Ingsriswang, S., Tangphatsornruang, S., Chantasingh, D., Luangsa-ard, J., & Eurwilaichitr, L. (2015). Insights from the genome of ophiocordyceps polyrhachis-furcata to pathogenicity and host specificity in insect fungi.(research article)(report). BMC Genomics, 16, 881.

ABOUT THE AUTHOR:

Iain Oothoudt, a freshman graphic design student at Bethel University from Woodbury, Minnesota., would like to work as a graphic designer for a well known company. Iain likes to rock climb, play soccer, and fish at his cabin.

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