Theo – Year 12 Student
Editor’s Note: Year 12 student Theo has submitted this excellent essay to the Peterhouse College, Cambridge, annual Kelvin Science Prize essay competition. Theo’s chosen topic is the Cordyceps fungus, a genus so truly remarkable in its modus operandi that it really does have to be seen to be believed; therefore, I have also shared here a clip from the BBC’s Planet Earth series in which Sir David Attenborough describes what I believe to be one of the most chilling wildlife sequences ever recorded. Don’t read or watch this alone after dark… CPD
The zombie ant fugus, Cordyceps, manipulates ants by taking control of their brains. Can the ants evolve a response to this or are they locked into a zombie apocalypse forever?
Cordiceps[1] is a marvellously unique genus that is notoriously difficult to combat; therefore, it is unlikely that ants will be able to evolve a way to defeat it.
The way in which ‘Cordiceps’ is referred to in the question surfaces problematic misconceptions around the fungus. Cordiceps is the name of a genus that does not include the parasitic fungal species that creates the so-called ‘mind control’ on the ants. The fungus, called Ophiocordyceps unilateralis, is entirely separate to the Cordiceps genus and is instead part of the Ophiocordyceps genus. Furthermore, the belief that the fungus infects the brain and uses it to control the body sounds logical enough. However, the fungus never actually grows into the ants’ brain until after it has finished controlling the ants’ actions[1]. Instead the fungus grows around and sometimes into the ants muscles and it is therefore suggested that the fungus releases chemicals to cause these muscles to contract and relax in order to make the ant walk. This function is needed to transport the fungus to the desired area[2] in which it can finish growing and spread its spores.
There is strong argument that ants will be able to evolve some sort of response to this fungus, given enough time, due to the nature of evolution. Ants that have mutated genes to combat the fungus are more likely to survive, reproduce, and pass on these beneficial genes. As such, if an ant evolves a mechanism that disarms Cordiceps when it infects it, the ant will pass on these genes to many generations. These genes will become more and more prevalent in the colony until eventually it is too difficult for the fungus to find a host to infect and it may become extinct, thus ending the ants’ zombie apocalypse.
The significant counter argument to this is that ants have not evolved a survival mechanism in millennia. Fossil records of bite-scarred leaves indicate that ants have faced this hellish opponent to their survival for at least 48 million years.[2] There are some plausible explanations as to why the ants haven’t already evolved this mechanism. The ants could have been unlucky and not developed the right mutation; or those that have managed to form the mutations have died before they could pass on these genes.
Another explanation is that the fungus cannot be beaten by any evolution in the ants’ DNA. The Cordiceps fungus could be impossible to challenge by any adaptation of the ants because of its’ unique infection cycle. Starting with a spore that lands onto the exoskeleton of an ant, we can see how difficult it is for an ant to combat this fungus. The spore builds up a pressure equal to that of the pressure inside the wheel of a 747 plane in order to burst open the ant’s exoskeleton and allow access to the ant’s innards. It would therefore be very difficult to evolve an exoskeleton thick enough or strong enough to protect it from such pressure whilst remaining the size typical of an ant. [3]
Another key feature in the infection cycle is the very few visible symptoms until the fungus has control of the ant. This is fundamental in the protection of the colony since ants have developed a system known as ‘social immunity’ where they exile members showing symptoms of disease in order to protect the rest of the colony – very much like today’s social isolation. However, due to the aforementioned trait of no visible symptoms, Cordiceps has made this ineffective. This again demonstrates how difficult it is for ants to combat the fungus as previous methods that have worked at reducing the impact of parasitic fungi on the colony are now obsolete.
Although the odds seem very much stacked against the ants, there are some glimmers of hope. One notable source lies with Japanese Cicadas that are susceptible to infection from a different species of Ophiordiceps genus, which was considered parasitic until it was discovered some Cicadas had formed a symbiotic relationship with them.[4] In this relationship, the fungus replaces a bacteria known as Hodgkinia in the gut of the Cicada and can synthesise amino acids, vitamins, and other metabolites for the Cicada. [5] This shows how it is possible for insects to evolve beneficial responses to the Cordiceps genus, but it potentially offers little to directly help ants as they are attacked by a different strain of Cordiceps.
Further encouraging research conducted by Hughes and De Bekker identified a chemical called sphingosine, which De Bekker suggests is responsible for affecting the ant’s nervous system. It is a component of neural signalling molecules and was only found in ants that were susceptible to behaviour manipulation by the fungus. This is one example of the 70% of fungal proteins and other metabolites that were found only in the ants that were susceptible to the behavioural manipulation.This is a significant indicator that ants could evolve to resist the fungus because losing any one of these proteins could mean an end to behavioural manipulation, and so the ant could mutate a way to supress, destroy or isolate it, thus saving it from the infection.
Any slight interference with the so-called mind control might be enough to rid the ants of the fungus, since it could cause symptoms to show before the fungus has full control. The social immunity behaviour would then kick in and render the fungus harmless to the rest of the colony. Finding it difficult to spread, the fungus may even eventually become extinct.
Or so you would expect, but the fungus has another trick up its metaphorical sleeve. Once the fungus has full control of the ant, it forces it to leave the colony anyway. This puppeteering ensures the ant moves to a position above feeding routes of the colony, so that spores released by the fungus drop and infect working ants. This method ensures the fungus does not infect the whole colony, and therefore never runs out of hosts. However, this is contradicted by a documentary voiced by David Attenborough where he states ‘entire colonies could be wiped out’. This causes some problems as both sources are both widely believed to be reliable but a possible explanation could be in the ambiguity of David Attenborough’s words and he could be referring to the average Cordiceps fungus whereas De Bekker is directly referencing Ophiocordyceps unilateralis. Whether it wipes out the colonies or not, it still means that the ants’ method of exiling the infected makes little difference.
There is a potential weakness within the infection cycle of the Cordiceps fungus that the ants might be able to exploit. The tubes that form to connect the growing spores within the ants’ body allow communication and the sharing of nutrients and are known as conidial anastomosis tubes (CATs). The growth of these tubes is guided[8] [9] so that they grow towards each other and they are septated. Therefore, there are two recognised ways in which the ant could evolve to combat the Cordiceps fungus by countering these tubes.
The first is evolving a way to mask the CATs from each other. The CATs are believed to use chemotaxis [10] to grow in the direction of other CATs, therefore the ants could evolve their own chemical to bind with the CATs chemotaxis chemicals and thus obscure the presence of other CATs, rendering them unable to connect and so cannot communicate, or share nutrients.
The other counter to the CATs is to exploit the septated nature of their cells, that is their inability to repair themselves once damaged. The ants could evolve the ability to synthesise an enzyme that could break down the tubes. Alternatively they could form a symbiotic relationship with a bacteria that could consume the CAT cells.
Both of these would have the similar effects of disrupting the distribution of nutrients and communication within the fungus which may lead to the fungus either dying in parts of the ant, or the fungus being unable to properly control the ant, meaning the ant will not move under the fungus’ influence.
The latter effect, although appearing superficially insignificant, would have a large impact on the fungus’ ability to grow and spread as Cordiceps requires a very specific environment to grow to spread its spores. Experiments done by David Hughes have shown that the fungus won’t grow at any height higher or lower than around 25cm [11] and that all 16 studied ants would bite into the vein of the leaf at times between 11:00 and 13:45[12]. This demonstrates the importance of the ability to control ants in the fungus’ growth, and if it is disrupted it could spell death for the fungus.
The problem with these potential solutions by breaking down CATs is that should an ant evolve an ability to kill the fungus and stop it spreading, that ability will most likely not be passed down to the next generation as the fungus would still be able to grow to the stage where irreparable damage has been caused to the ant. So the Cordiceps lives on.
In conclusion, many of the potential ways the ants could evolve to combat the Cordiceps fungus are unlikely to have any major effect. I do think there is a way that the ants could escape the ‘zombie apocalypse’, but not by their own evolution as proposed by the question. David Hughes’ research on the environmental factors on Cordiceps growth discovered the fungus was unable to grow at different heights above the forest floor and that the ant bit into the leaf at a specific time. This suggests that the fungus requires a very specific temperature in order to finish growing and eventually spread its spores which would mean an environmental change in temperature could lead to the end of the Cordiceps and the continuation of the ant colonies.
Theo 949019
Bibliography
Journals
Dance, Amber. 2014, “Microbes make change”. Proceedings of the National Academy of Sciences of the United States of America 111, no.6 (February): 2051
Frederickson, Maridel. Zhang, Yizhe. Hazen, Missy. Loreto, Raquel. Mangold, Colleen. Chen, Danny. Hughes, David. 2017, “Three-dimensional visualisation and a deep learning model reveal complex fungal parasite networks in behaviourally manipulated ants.” Proceedings of the National Academy of Sciences of the United States of America 114, no.47 (November): 12590-12595
Milius, Susan. 2011, “Lunchtime of the living dead.” Society for Science & the public 179, no.13 (June): 9
Pennisi, Elizabeth. 2014, “Parasitic Puppeteers Begin To Yield Their Secrets.” American Association for the Advancement of Science 343, no.6168 (January): 239
Websites
BBC Studios. “Cordyceps: attack of the killer fungi – Planet Earth Attenborough BBC wildlife.” Accessed April 2, 2020. https://www.youtube.com/watch?v=XuKjBIBBAL8
Lu, Jennifer. “How a parasitic fungus turns ants into ‘zombies’.” Accessed March 14, 2020. https://www.nationalgeographic.com/animals/2019/04/cordyceps-zombie-fungus-takes-over-ants/
Matsuura, Yu. Moriyama, Minoru. Łukasik, Piotr. Vanderpool, Dan. Tanahashi, Masahiko. Meng, Xian-Ying. McCutcheon, John. Fukatsu, Takema. “Recurrent symbiont recruitment from fungal parasites in cicadas.” Accessed March 20, 2020. https://www.ncbi.nlm.nih.gov/pubmed/29891654
Roca, Gabriela. Arlt, Jochen. Jeffree, Chris. Read, Nick. “Cell Biology of Conidial Anastomosis Tubes in Nerospora Crassa.” Accessed March 25, 2020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1140100/
Simon, Matt. “Absurd Creature of the Week: The Zombie Ant and the Fungus That Controls Its Mind.” Accessed March 19, 2020. https://www.wired.com/2013/09/absurd-creature-of-the-week-zombie-ant-fungus/
[1] It is often interchangeably spelt ‘Cordyceps’.
[2] This place being around 25cm above the ground where the ant can bite into the vein of a leaf and usually close to one of the ant colonies foraging routes.
The GSAL Journal 