Moselio Schaechter


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July 23, 2012

An Evolutionary Tale of Zombie Ants and Fungal Villains & Knights

by Gemma Reguera

Infected ant with spore stalk

Fig. 1: An ant infected by O. unilateralis bites the underside of a
leaf. The fungal stroma emerges from the back of the ant’s head and
develops a fruiting body with a capsule full of spores. Once matured,
the capsule is released and the spores disperse on the forest floor.
Courtesy of David P. Hughes. Source.

In a recent post I shared with you some amazing things I had learnt about coprophilous (‘dung-loving’) fungi that spit their spores like pros. What I did not tell you then is that my six-year-old son also fell in love with the spitting fungi (dung + spit = child’s interest!) and wanted to learn more. So we spent hours watching online videos until we stumbled upon a BBC’s Planet Earth video narrated by the great David Attenborough about ant parasitic fungi in the genus Cordyceps. The video shows a carpenter ant (genus Camponotus) that has been infected by spores of the fungus Ophiocordyceps unilateralis. The spores germinate inside the ant’s respiratory track and the mycelia grow towards the brain while feeding on soft tissues. Once the fungus reaches the brain, it induces behavioral changes such that the ant climbs up vegetation and bites the underside of the leaves. There the ant awaits its death while the fungus continues to grow within. The stroma stalk of the fungus eventually protrudes from the back of the ant’s head and a fruiting body bearing a capsule filled with spores forms near its tip (Fig. 1). Once the spores are sexually mature, the capsule is released, and then explodes, either in the air or upon hitting the ground. This delivers the spores into the path of healthy ants, there to start a new cycle of infection.

Right away, I searched the fungus name in the STC blog and found a beautiful post by Ilham Naili that describes the life cycle of this parasitic fungus in more detail. I also started my own search and learned that the fungal spores can attach to the ant’s body and secrete enzymes to penetrate through the cuticle. These are indeed some aggressive parasites capable of a generalized body invasion. In fact, some pictures of dead ants show the fruiting body protruding through their abdomen, rather than the back of the head. But spores also enter the ant though its respiratory spiracles (aka the exoskeleton openings that allow the air to enter the trachea). This appears to be the most efficient route because the spore can use the nutrients provided by non-essential soft tissues within the respiratory track to grow its mycelia in close proximity to the ant’s brain. There, bioactive compounds secreted by the mycelia enter the ant’s brain and effectively block pheromone reception. This disorients the ant and leaves it under complete fungal control. Like a zombie, the ant climbs up the trees and bites a vein on the underside of a leave with great force. Interestingly, the shape of the grip bite mark is very distinctive and is similar to marks identified in 48 million year old fossil leaves!!!!! This biting behavior effectively immobilizes the dying ant upside down under a leaf, but also lowers its head (Fig. 1). It appears as if, in doing so, the ant submissively bows to its destiny and exposes the soft tissue between the head and the trunk to facilitate the protrusion of the fungal stroma stalk.

Napthoquinones

Fig. 2: O. unilateralis growing on potato dextrose agar for 7 days secretes six type of naphthoquinones. The formulas are shown in (a) and the growth of the fungus on the agar in (b). After 12 days of incubation the naphthoquinone secretion is so copious that it forms droplets on the plate (c). Source.

Concerned as I was about prematurely destroying my son’s love for microbes, I set out to learn more about Cordyceps, avidly searching for that ‘positive something’ about them that could alleviate the bad memory of the brain-eating fungus. Cordyceps, I learned, is a fungal group (in addition to a genus) in the Ascomycota that comprises hundreds of species. All are endoparasitic; most parasitize arthropods, though a few infect other fungi. Amazingly, each fungus specializes in infecting a single type of host. While some can infect other hosts, reproductive success is often compromised. Not all affect the host’s behavior, but those that do are known to produce copious amounts of bioactive compounds. O. unilateralis, for example, secretes at least six types of secondary metabolites broadly known as naphthoquinones (Fig. 2). All six have antibacterial, anticancer, and antimalarial activity. Well, there goes something positive ...

The question that lingered in my mind was what mechanism, if any, could protect the ants from the deadly fungus. The ants build their nests in the above-ground vegetation. But the parasitic fungus controls the behavior of the infected ants and forces them to climb upwards so they die close to the colony’s nest. This maximizes the chances of infecting healthy individuals. Furthermore, dead ants are often found at high densities (approximately 26 per m2) in localized areas of the vegetation termed graveyards. This helps concentrate the released fungal spores in the ants’ path on the forest floor beneath. Healthy ants are known to actively avoid the zombie ant graveyards and only venture to the forest floor occasionally—strategies that prevent infection. This spatially limits their resources but, hey, giving up some food sources seems like a good evolutionary trade off to ensure the survival of the colony.

Stroma

Fig. 3: A stroma of Ophiocordyceps nutans infected by Polycephalo-
myces
sp. is shown on the left. As a comparison, a healthy, non-
parasitized stroma with its red fruiting body is shown on the right.
Credit Alisha Owensby. Source.

Just when I thought this was it for the defenseless ants, I stumbled upon a paper by David P. Hughes’s group published in PLoS One about a white fungus that comes to the ant’s rescue. Who is this mysterious knight? None other than a cousin of the Cordyceps! The fungal family Clavicipitaceae includes genera with sexual cycles such as the Cordyceps as well as asexual genera such as the Polycephalomyces. Cordyceps are often parasitized by Polycephalomyces spp. This makes the Polycephalomyces white knight a hyperparasite (aka a parasite of a parasite). Cordyceps are especially susceptible to infection during the stroma stage. Once the hyperparasite infects a stroma, it feeds from it and prevents the formation of its fruiting body (Fig. 3). More than half of the dead ants in the graveyards surveyed by the researchers were hyperparasitized. The brain-eating fungus has a slow maturation process, needing approximately 4 weeks to form the stroma and 4 more weeks to produce sexually mature fruiting bodies. This allows sufficient time for the white fungi to infect the Cordyceps stroma. As a result, the white fungi prevent the formation of a sexually mature fruiting body and ‘castrate’ the brain-eating fungus on the spot.

The lengthy maturation process of the brain-eating fungus also exposes the fungal structures to environmental insults for prolonged periods of time. In fact, as many as 13% of the fungal structures in the dead ants surveyed in the study were damaged. Furthermore, in 25% of the dead ants, the fungus had not completed the maturation of their fruiting bodies, possibly because of inadequate temperature and humidity. The combined action of the white fungus and environmental insults rendered most (93-94%) of the dead ants sterile and posing no risk of infection. To make things worse for the Cordyceps fungus, more than half of the remaining infective fruiting bodies suffered from some sort of ‘sexual dysfunction’ and could not shoot their spores when tested under laboratory conditions. Yet, once matured, the fruiting bodies were robust; they were immune to the white fungi and able to survive in the environment for prolonged periods of time. This capability for long-term survival compensates, at least partially, for the low dispersal potential and increases the reproductive success of the parasitic fungus.

In the end, the white knight, with a little help from the environment, ensures that only a few, but very robust, fruiting bodies are produced over time. This is enough to increase the size of the ant graveyards slowly, but steadily, so sexually mature spores can be produced at regular intervals for longer periods of time. The researchers compared this evolutionary trade-off to a phenomenon called iteroparity, a reproductive strategy in which an organism produces offspring in successive cycles, rather than in a single event, to ensure long-term survival. Every zombie ant that joins the graveyard is like a new ‘pregnancy’ for the fungal villain but also food and shelter for the white knight. This along with the ants’ preventive measures to avoid contact with infected individuals and areas of high spore concentration ensures a perfect balance between the survival of the ant colony and the parasitic and hyperparasitic fungi, so none goes extinct. And although the video of the brain-eating fungus should be rated PG-13 for the animal-loving audience, I can’t help but admire how beautifully nature intertwines the lives of animals and microbes to keep a harmonious balance. This is indeed a true tale of co-evolution, with zombies, villains, and white knights included. I just need to wait a few more years until I can help my son understand that this is, indeed, a story with a happy ending.

Gemma_Reguera_crop




Gemma is associate professor in the Department of Microbiology and Molecular Genetics, Michigan State University and an Associate Blogger at STC.

ResearchBlogging.org

Pontoppidan MB, Himaman W, Hywel-Jones NL, Boomsma JJ, & Hughes DP (2009). Graveyards on the Move: The Spatio-Temporal Distribution of Dead Ophiocordyceps-Infected Ants. PLoS One, 4 (3) PMID: 19279680

Andersen SB, Ferrari M, Evans HC, Elliot SL, Boomsma JJ, & Hughes DP (2012). Disease dynamics in a specialized parasite of ant societies. PloS one, 7 (5) PMID: 22567151

Comments

It seems there is a great possibility that the ant's behavioural change is due to some form consciousness; a realisation that they are marked for death, conscious that they are a detriment to their hive-mind psychology. That their behaviour is that of a conscious and dutiful action.. Science, prove me wrong... =]
I do wonder how Darwin would interpret and respond to these discoveries.

Thanks for raising this interesting topic.

There is a general belief that the ultimate aim of a parasite is to be adapted and cause limited damage to the host; from a well-repeated quote
“A truly successful parasite is commensal, living in amity with its host, or even giving it +ve advantages. A parasite that regularly and inevitably kills its host cannot survive long, in the evolutionary sense…….”
Star Trek
The Ophiocordyceps case, however, is clearly an exception and a very nice example to counter the above idea; that was how I were introduced into the topic from a mini-debate.
________________________________________
There are uncertainties in regard to the “mind-controlling” theory, since ant or other insects are known to climb & exploit the sun-ray in against infection: the peculiar position & gesture of the dead-ant might just be a failed attempt of the process. Nevertheless, researches are rather limited in the field at the moment, and I don’t really feel the need to exclude any possibilities; in the end, the “zombie ant” title is an excellent “catch” for new student, including myself.
________________________________________
More fascinating stories of mind-manipulation in the nature can be found in a greatly written book by Janice Moore: “Parasites and the behavior of animals” of the Oxford series.

Plsease forgive me for any mistakes/ typos; it's 3 in the morning

Thanks for the post Gemma. So much happening around us that we're not aware of. Nature works in such mysterious and fascinating ways. Seems cruel at times but at the end of the day its all about survival of the species and somehow balance prevails. We have so much to learn from nature.

I wonder if their behavior is driven purely by their instinct for survival...or is there more to it...maybe world domination!

I found out the existence of /Cordyceps/ in December 2010, when a read a story on a Spanish weekly magazine about one of the most protected species (sic) growing in Buthan, much appreciated due to their medical properties and known as "yagtsa guen bub". It was described as "a plant-animal hybrid --a plant in summer, a worm in winter-- that grows at 4000 m. above the sea level." I could not imagine such a chimaera, it was against any biological principle, and I did a search in Google general and Google Schoolar to know more about it.

I finally came across the fascinating relationship between /Cordyceps sinensis/ (its current name could be /Ophiocordyceps sinensis/, but I am not sure) and the moth /Thitarodes namnai/, which make up the famous "yarsta goenbub". By then, my daugther was spendign a long stay in Nepal, where I had known this medicine was also sold. I asked her to buy me some yarsta goenbub. However, when she told me the price of a yarsta goenbub can, I gave up.

I wrote a post on my blog about this story and, for almost one year, it was the one that got more visits. I assume people that sought information about the medicine or because I embedded an excerpt of Attenborugh's video that show the zombie ants.

I very much appreciate this post by Gemma, because it provides me with more information on that amazing relationship.

I saw a picture of these ants in one of the photography magazines I read (I forget which one).

This is a most fascinating article you've written here! I found it through a link on Google Plus, and I'm so glad I did. Wonderful information!! Thank you for posting, Ms. Reguera!

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