|Dead ants infected with Ophiocordyceps unilateralis|
(Tul.) Petch (1931)
Torrubia unilateralis Tul. (1865)
Ophiocordyceps unilateralis is an entomopathogenic fungus predominantly found in tropical forest ecosystems. Also known as "zombie fungi," it was first discovered by British naturalist Alfred Russel Wallace in 1849. In order to increase its own fitness, O. unilateralis manipulates the behavioral patterns of an infected ant, specifically Camponotus leonardi of the tribe of campotini. The infected ants leave their canopy nest and foraging trails, heading for the forest floor in search of an area with a temperature and humidity level that is suitable for fungal growth. The infected ant will then use its mandible to affix itself to a major vein on the underside of a leaf and eventually die.
The species can be identified at the end of its lifecycle by its reproductive structure, consisting of a wiry yet pliant darkly pigmented stroma stalk extending from the back of the deceased ant's head. The stalk has perithecia just below its tip. The fungus infects ants, most known as the carpenter ants, in which the fungus creates a single stalk arising from the dorsal neck region on which the sexual structures are borne horizontally, which creates the spores. Once infected with the fungus the ant will climb down from its normal habitat and bite down on the underside of a leaf. This is known as "the death grip" occurring in very precise locations.
Like other fungi pathogenic to insects in the Ophiocordyceps genus, O. unilateralis targets a specific host species, the Camponotus leonardi ant. However the fungus may parasitize other closely related species of ants with lesser degrees of host manipulation and reproductive success.
The fungus's spores attach to the ant, eventually breaking through the exoskeleton using mechanical pressure and enzymes. Yeast stages of the fungus spread in the ant's body and presumably produce compounds that affect the ant's hemocoel and utilizes the evolutionary trait of an extended phenotype to manipulate the behavioral patterns exhibited by the ant. These infected ants begin to have irregularly spaced full body convolutions which sends them to the forest floor. The insect climbs up the stem of a plant and uses its mandibles to secure itself to a leaf vein, with abnormal force, leaving dumbbell-shaped marks on it. A search through plant fossil databases revealed similar marks on a fossil leaf from the Messel Pit which is 48 million years old. Once the mandibles of the ant are secured to the leaf vein, atrophy quickly sets in destroying the sarcomere connections in the muscle fibers, and reduces the mitochondria and sarcoplasmic reticula. The ant is no longer able to control the muscles of the mandible and will remain affixed here. This lockjaw trait is popularly known as the death grip, and is essential in the fungus's fitness.
The fungus then kills the ant, and continues to grow as its hyphae invade more soft tissues and structurally fortify the ant's exoskeleton. More mycelia then sprout out of the ant, and securely anchor it to the plant substrate while secreting antimicrobials to ward off competition. When the fungus is ready to reproduce, its fruiting bodies grow from the ant's head and rupture, releasing the spores. This process takes 4 to 10 days.
The changes in the behavior of the infected ants are very specific, giving rise to the popular term "zombie ants," and tuned for the benefit of the fungus. The ants suffer from convulsions causing them to fall to the ground and preventing them from finding their way back to their canopy. The ants generally clamp to a leaf's vein at a mean height of 25.20 ± 2.46 cm above the forest floor, on the northern side of the plant, in an environment with 94–95% humidity and temperatures between 20 and 30 °C (68 and 86 °F). Infections may lead to 20 to 30 dead ants per square meter. "Each time, they are on leaves that are a particular height off the ground and they have bitten into the main vein [of a leaf] before dying." When the dead ants are moved to other places and positions, further vegetative growth and sporulation either fails to occur or results in undersized and abnormal reproductive structures.
O. unilateralis has been known to destroy entire ant colonies. In response, ants have evolved the ability to sense that a member of the colony is infected; healthy ants will carry the dying one far away from the colony in order to avoid fungal spore exposure.
Throughout the life cycle, there are unique challenges that must be met by equally unique metabolic activities. The fungal pathogen must attach securely to the arthropod exoskeleton and penetrate it—avoiding or suppressing host defenses—then, control the behavior of the host before killing it; and finally, it must protect the carcass from microbial and scavenger attack.
The Ophiocordyceps fungus contains various known and untapped bioactive metabolites, and is being investigated as a new source of natural drugs with immunomodulatory, antitumor, hypoglycemic and hypocholesterolemic functions.
Six bioactive naphthoquinone derivatives have been isolated from O. unilateralis, namely
- 4-O-methyl erythrostominone
- deoxyerythrostominol, and
- 3,5,8-trihydroxy-6-methoxy-2-(5-oxohexa-1,3-dienyl)-1,4-naphthoquinone showed anti-malarial activity in vitro.
There has also been research into the use of red naphthoquinone pigments made by O. unilateralis as a dye for food, cosmetic and pharmaceutical manufacturing processes.
O. unilaterlis produces naphthoquinone, which under acidic conditions show red and under basic conditions show a purple color. These pigments are stable against acid/ alkaline conditions, light and non-cytotoxic, which makes them applicable food coloring as well as dyeing of other materials. These attributes also make it a prime candidate for anti-tuberculoid testing in secondary TB patients, by improving symptoms and enhancing immunity when combined with chemotherapeutic drugs.
O. unilateralis suffers from an unidentified fungal hyperparasite, reported in the lay press as the "anti-zombie-fungus fungus," that results in only 6–7% of sporangia being viable, limiting the damage O. unilateralis inflicts on ant colonies.The hyper-parasite moves in to attack Ophiocordyceps unilateralis as the fungal stalk emerges from the ant's cadaver. This can stop the stalk from releasing its spores.
The density of infected ants and graveyards of dead ants are numerous and spread throughout the surrounding area of the colony. Even though O. unilateralis is very virulent, only about 6.5% of all fruiting bodies are viable spore-producers. This is caused by the castration of the fungus by the hyperparasite which may cause the limiting of the viable infectious spores. Ants also groom each other to combat microscopic organisms that could potentially harm the colony. This shows that the colony is not in as much danger as previously thought. Additional fungi have also shown beneficial assistance to the colony as well.
In popular culture
A fictitious variant of the fungus is featured in the game The Last of Us. The infection spreads to humanity and creates a zombie-like appearance/behavior, which causes the infected to become hostile and violent towards the non-infected.
In the television series The Simpsons episode "The Man Who Grew Too Much" (season 25, episode 13), Sideshow Bob reveals that he has spliced his DNA with O. unilateralis, and that as a consequence he can be tipped into a murderous rage by the slightest provocation.
- "Ophiocordyceps unilateralis". MycoBank. International Mycological Association. Retrieved 2011-07-19.
- Pacchioli,David. "Getting to the bottom of the zombie ant phenomenon." new.psu.edu. May 21, 2013.
- Mongkolsamrit, S.; Kobmoo, N.; Tasanathai, K.; Khonsanit, A.; Noisripoom, W.; Srikitikulchai, P.; Somnuk, R.; Luangsa-Ard, J. J. (2012). "Life cycle, host range and temporal variation of Ophiocordyceps unilateralis/Hirsutella formicarum on Formicine ants". Journal of Invertebrate Pathology 111 (3): 217. doi:10.1016/j.jip.2012.08.007.
- Evans, H. C.; Elliot, S. L.; Hughes, D. P. (March 2011). "Hidden Diversity Behind the Zombie-Ant Fungus Ophiocordyceps unilateralis: Four New Species Described from Carpenter Ants in Minas Gerais, Brazil". Plos ONE 6 (3): 1–9. doi:10.1371/journal.pone.0017024. PMC 3047535. PMID 21399679.
- Pontoppidan, M. B.; Himaman, W.; Hywel-Jones, N. L.; Boomsma, J. J.; Hughes, D. P. (12 March 2009). "Graveyards on the move: The spatio-temporal distribution of dead Ophiocordyceps-infected ants". PLOS ONE 4 (3): e4835. doi:10.1371/journal.pone.0004835. PMC 2652714. PMID 19279680.
- Sung, G. H.; Hywel-Jones, N. L.; Sung, J. M.; Luangsa-Ard, J. J.; Shrestha, B.; Spatafora, J. W. (2007). "Phylogenetic classification of Cordyceps and the clavicipitaceous fungi". Studies in Mycology 57 (1): 5–59. doi:10.3114/sim.2007.57.01. PMC 2104736. PMID 18490993.
- Evans, H.; Elliot, S.; H., David (September–October 2011). "Ophiocordyceps unilateralis – A keystone species for unraveling ecosystem functioning and biodiversity of fungi in tropical forests?". Communicative Integrative Biology 4 (5): 598–602. doi:10.4161/cib.4.5.16721 (inactive 2014-01-13). PMC 3204140. PMID 22046474.
- Sample, Ian (18 August 2010). "'Zombie ants' controlled by parasitic fungus for 48m years". News » Science » Microbiology. The Guardian. Retrieved 2010-08-22.
- Andersen, S. B.; Gerritsma, S.; Yusah, K. M.; Mayntz, D.; Hywel‐Jones, N. L.; Billen, J.; Boomsma, J. J.; Hughes, D. P. (September 2009). "The life of a dead ant: The expression of an adaptive extended phenotype". The American Naturalist 174 (3): 424–433. doi:10.1086/603640. JSTOR 10.1086/603640. PMID 19627240.
- Hughes, D. P.; Andersen, S. B.; Hywel-Jones, N. L.; Himaman, W.; Billen, J.; Boomsma, J. J. (May 2011). "Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection". BMC Ecology 11 (1): 13–22. doi:10.1186/1472-6785-11-13. PMC 3118224. PMID 21554670.
- "Fossil Reveals 48-Million-Year History of Zombie Ants". Science Daily. 18 August 2010. Retrieved 2010-09-12.
- Hughes, D. P.; Wappler, T.; Labandeira, C. C. (23 February 2011) [18 August 2010]. "Ancient death-grip leaf scars reveal ant fungal parasitism". Biology Letters (Royal Society) 7 (1): 67–70. doi:10.1098/rsbl.2010.0521. PMC 3030878. PMID 20719770.
- "Zombie Ants have fungus on the brain, new research reveals". Science Daily. 2011. Retrieved 2012-11-27.
- Attenborough, David (3 November 2008). "Cordyceps: attack of the killer fungi". Planet Earth. BBC Worldwide. https://www.youtube.com/watch?v=XuKjBIBBAL8. Retrieved 2013-04-21.
- Harmon, Katherine (31 July 2009). "Fungus makes zombie ants do all the work". Scientific American. Retrieved 2010-08-22.
- Xiao, J. H.; Zhong, J. J. (2007). "Secondary metabolites from Cordyceps species and their antitumor activity studies". Recent Patents on Biotechnology 1 (2): 123–137. doi:10.2174/187220807780809454. PMID 19075836.
- Kittakoopa, P.; Punyaa, J.; Kongsaeree, P.; Lertwerawat, Y.; Jintasirikul, A. (1999). "Bioactive naphthoquinones from Cordyceps unilateralis". Phytochemistry 52 (3): 453–457. doi:10.1016/S0031-9422(99)00272-1.
- Wongsa, P.; Tasanatai, K.; Watts, P.; Hywel-Jones, N. (August 2005). "Isolation and in vitro cultivation of the insect pathogenic fungus Cordyceps unilateralis". Mycological Research 109 (Pt 8): 936–940. doi:10.1017/S0953756205003321. PMID 16175796.
- Unagul, P.; Wongsa, P.; Kittakoop, P.; Intamas, S.; Srikitikulchai, P.; Tanticharoen, M. (April 2005). "Production of red pigments by the insect pathogenic fungus Cordyceps unilateralis BCC 1869". Journal of Industrial Microbiology and Biotechnology 32 (4): 135–140. doi:10.1007/s10295-005-0213-6. PMID 15891934.
- Isaka, Mashahiko, Prasat kittakoop, Kanyawim Kirtikara, Nigel l. Hywel-Jones, and Yodhathai Thebtaranonth. "Bioactive Substances from Insect Pathogenic Fungi." Accounts of Chemical Research 38.10 (2005): 813-23.
- Yuan, Wang, DAI Enlai, and JIA Zhong. "A Retrospective Analysis of Cordyceps Anti-Tuberculosis Capsule Combined with Chemotherapy for 614 Cases of Secondary Tuberculosis." Journal of Traditional Chinese Medicine 2013.15.
- "The Zombie-Ant Fungus Is Under Attack, Research Reveals". Pennsylvania State University. 2012-05-02. Retrieved 2013-03-04.
- Andersen, S. B.; Ferrari, M.; Evans, H. C.; Elliot, S. L.; Boomsma, J. J.; Hughes, D. P. (2 May 2012). "Disease Dynamics in a Specialized Parasite of Ant Societies". PLOS ONE 7 (5): e36352. doi:10.1371/journal.pone.0036352. PMC 3342268. PMID 22567151.
- Anderen, Sandra B, et al. "Disease Dynamics in a Specialized Parasite of Ant Societies." PLOS ONE 7.5 (2012).
- Hill, Kyle (2013-06-25). "The Fungus that Reduced Humanity to The Last of Us". Scientific American. Retrieved 2014-03-10.
- Ophiocordyceps unilateralis at UniProt.org. Accessed on 2010-08-22.
- An Electronic Monograph of Cordyceps and Related Fungi