Ophiocordyceps unilateralis

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Ophiocordyceps unilateralis
Ophiocordyceps unilateralis.png
Dead ants infected with Ophiocordyceps unilateralis
Scientific classification
O. unilateralis
Binomial name
Ophiocordyceps unilateralis
(Tul.) Petch (1931)

Torrubia unilateralis Tul. (1865)
Cordyceps unilateralis (Tul.) Sacc. (1883)

Ophiocordyceps unilateralis is an insect-pathogenic fungus, discovered by the British naturalist Alfred Russel Wallace in 1859, and currently found predominantly in tropical forest ecosystems. O. unilateralis infects ants of the Camponotini tribe, with the full pathogenesis being characterized by alteration of the behavioral patterns of the infected ant. Infected hosts leave their canopy nests and foraging trails for the forest floor, an area with a temperature and humidity suitable for fungal growth; they then use their mandibles to affix themselves to a major vein on the underside of a leaf, where the host remains until its eventual death.[2] The process leading to mortality takes 4–10 days, and includes a reproductive stage where fruiting bodies grow from the ant's head, rupturing to release the fungus's spores. O. unilateralis is, in turn, also susceptible to fungal infection itself, an occurrence that can limit its impact on ant populations, which has otherwise been known to devastate ant colonies.

O. unilateralis and related species are known to engage in an active secondary metabolism, among other reasons, for the production of substances active as antibacterial agents that protect the fungus-host ecosystem against further pathogenesis during fungal reproduction. Because of this secondary metabolism, an interest in the species has been taken by natural products chemists, with corresponding discovery of small molecule agents (e.g. of the polyketide family) of potential interest for use as human immunomodulatory, anti-infective, and anticancer agents.


The scientific name is sometimes written Ophiocordyceps unilateralis sensu lato, which means "in the broad sense", because the species is actually a complex of many species within O. unilateralis.[3]


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 dead ant's head. The stalk has perithecia just below its tip.[4][5] The fungus infects ants, most known is the carpenter ants, in which the fungus creates a single stalk arising from the dorsal neck region on which the spore-bearing sexual structures are borne horizontally. Once infected with the fungus, the ant climbs down from its normal habitat and bites down on the underside of a leaf. This is known as "the death [5] grip", and occurs in very specific locations.[6]

Life cycle[edit]

Like other fungi pathogenic to insects in the genus Ophiocordyceps, O. unilateralis targets a specific host species, Camponotus leonardi. However, the fungus may parasitize other closely related species of ants with lesser degrees of host manipulation and reproductive success.[7]

The changes in the behavior of the infected ants are very specific, giving rise to the popular term "zombie ants", and are tuned for the benefit of the fungus. The spores of the fungus attach to and eventually break through the ant's exoskeleton using mechanical pressure and enzymes.[6] Yeast stages of the fungus spread in the ant's body and presumably produce compounds that affect the ant's hemocoel, using the evolutionary trait of an extended phenotype to manipulate the behavioral patterns exhibited by the ant.[8] An infected ant exhibits irregularly timed full-body convulsions that dislodge it to the forest floor.[9] The ant climbs up the stem of a plant and uses its mandibles with abnormal force to secure itself to a leaf vein, leaving dumbbell-shaped marks on it. The ants generally clamp to a leaf's vein at a height of 25 cm above the forest floor,[8] 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.[10] "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."[7] 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.[8]

A search of plant-fossil databases revealed similar marks on a fossil leaf from the Messel Pit, which is 48 million years old.[11][12] 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 reducing the mitochondria and sarcoplasmic reticula. The ant is no longer able to control the muscles of the mandible and remains fixed in place. This lockjaw trait is popularly known as the death grip, and is essential in the fungus's lifecycle.[9]

The fungus then kills the ant, and continues to grow as its hyphae invade more soft tissues and structurally fortify the ant's exoskeleton.[7] More mycelia then sprout out of the ant, and securely anchor it to the plant substrate while secreting antimicrobials to ward off competition.[7] When the fungus is ready to reproduce, its fruiting bodies grow from the ant's head and rupture, releasing the spores. This process takes 4-10 days.[7]

Geographic distribution[edit]

The fungus occurs in the tropical forests of Thailand[13] and Brazilian rainforests.[12]

Host impact[edit]

O. unilateralis has been known to destroy entire ant colonies. In response, ants have the ability to sense that a member of the colony is infected; healthy ants carry the dying one far away from the colony to avoid fungal spore exposure.[4][5]

Throughout the lifecycle, unique challenges 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.[6]

Medicinal potential[edit]

The Ophiocordyceps fungus contains various known metabolites, as well as a number of structurally uncharacterized substances; these natural products are reportedly being investigated as potential leads in discovery efforts toward immunomodulatory, antitumor, hypoglycemic, and hypocholesterolemic targets.[14]

Six known naphthoquinone derivatives have been isolated from O. unilateralis, namely erythrostominone, deoxyerythrostominone, 4-O-methyl erythrostominone, epierythrostominol, deoxyerythrostominol, and 3,5,8-trihydroxy-6-methoxy-2-(5-oxohexa-1,3-dienyl)-1,4-naphthoquinone, which have shown activity in in vitro assays related to antimalarial drug discovery.[15][16]

The use of red naphthoquinone pigments made by O. unilateralis has been studied as a dye for food, cosmetic, and pharmaceutical manufacturing processes.[17]

O. unilaterlis produces naphthoquinone, which under acidic conditions shows a red color, and under basic conditions shows a purple color. These pigments are stable against acid/alkaline conditions and light, and are not cytotoxic, which makes them applicable for food coloring and as dye for other materials. These attributes also make it a prime candidate for antituberculoid testing in secondary TB patients, by improving symptoms and enhancing immunity when combined with chemotherapeutic drugs.[18][19]

Fungal hyperparasite[edit]

O. unilateralis suffers from an unidentified fungal hyperparasite, reported in the lay press as the "antizombie-fungus fungus", that results in only 6–7% of sporangia being viable, limiting the damage O. unilateralis inflicts on ant colonies. The hyperparasite moves in to attack O. unilateralis as the fungal stalk emerges from the ant's body, which can stop the stalk from releasing its spores.[20][21]

The graveyards of dead ants are numerous and spread throughout the surrounding area of the colony. Though O. unilateralis is very virulent, only about 6.5% of all fruiting bodies are viable sporeproducers. This is caused by the weakening of the fungus by the hyperparasite, which may limit the viability of infectious spores. Ants also groom each other to combat microscopic organisms that could potentially harm the colony. Additional fungi grant beneficial assistance to the colony, as well.[21]


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