Ophiocordyceps unilateralis

After years of research, the taxonomy of Ophiocordyceps unilateralis is becoming increasingly clear.

In tropical forests, the ant species Camponotus leonardi lives in the high canopy and has an extensive network of aerial trails. Sometimes the canopy gaps are too difficult to cross, so the ants' trails descend to the forest floor where they are exposed to O. unilateralis spores. The spores attach to their exoskeletons and eventually break through using mechanical pressure and enzymes.[8] Like other fungi pathogenic to insects in the genus Ophiocordyceps, the fungus targets a specific host species, Camponotus leonardi; despite this, the fungus may parasitize other closely related species of ants with lesser degrees of host manipulation and reproductive success.[9]

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.[10] An infected ant exhibits irregularly timed full-body convulsions that dislodge it from its canopy nest to the forest floor.[11]

The changes in the behavior of the infected ants are very specific, giving rise to the popular term "zombie ants." Behaviors are tuned for the benefit of the fungus in terms of its growth and its transmission, thereby increasing its fitness. 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 26 cm above the forest floor,[10] 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.[12] 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.[10] In temperate forests, the typical behavior of zombie ants is to attach themselves to the lower side of twigs, not leaves.[13]

A search of plant-fossil databases revealed similar marks on a fossil leaf from the Messel Pit, which is 48 million years old.[14][15] 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 reticular. The ant is no longer able to control the muscles of the mandible and remains fixed in place, hanging upside-down on the leaf. This lockjaw trait is popularly known as the death grip and is essential in the fungus's lifecycle.[11] A study led in Thailand revealed that there is a synchronization of this manipulated biting behavior at solar noon.[13]

The fungus then kills the ant and continues to grow as its hyphae invade more soft tissues and structurally fortify the ant's exoskeleton.[9] More mycelia then sprout out of the ant, securely anchoring it to the plant substrate while secreting antimicrobials to ward off competition.[9] 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.[9] Dead ants are found in areas termed "graveyards" which contain high densities of dead ants previously infected by the same fungus.[16]

The term "zombie ants" has been used in popular media as well as scientific articles, but has also been described as "catchy, yet misleading."[17][18]

Schematic representation of the ant behavioral manipulation caused by natural products secreted by O. unilateralis

O. unilateralis' life cycle includes and depends on the infection and the manipulation of a carpenter ant, principally C. leonardi.[3] The behavioral manipulation of the ant, which gives rise to the name "zombie-ant", is an extended phenotype of the fungus. It first affects the ant's behavior through convulsions that make it fall from its high canopy nest onto the forest floor.[11] This is followed by the fungus controlling the climbing of the ant and the locking of its jaw (and subsequent death) onto a leaf around 25 centimetres above the ground, which is thought to be the optimal height for fungal spore growth and dispersion.[11]

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.[8]

The behavioral manipulation of the ant would not be possible without the presence of huge fungal cell populations beside the host's brain[13] and within muscles[19] because these lead to the secretion of various metabolites known to have important behavioral consequences.[20] During the infection the parasite comes across an array of environments such as different host tissues or the immune response.[20] Studies have shown that O. unilateralis reacts heterogeneously by secreting different metabolites according to the host tissue it encounters and whether they are live or dead.[19] The identification of these natural products is important in order to understand which aspects of the ants are under control and consequently how O. unilateralis manipulates the ant.

More in-depth research is needed for the identification of other fungal compounds which act to atrophy the mandibular muscles, and for the understanding of their exact effects on the ant.

Many studies describe Ophiocordyceps unilateralis distribution as pantropical since it occurs mainly in tropical forest ecosystems.[6] However, there are some reports of the zombie-ant fungus in warm-temperate ecosystems.[13]

Its distribution includes tropical rainforests located in Brazil, Australia and Thailand, and temperate forests found in South Carolina, Florida and Japan.[5]

A 48-million-year-old fossil of a leaf stem exhibiting dumbbell-shaped marks characteristic of those made by an ant in the death-grip of Ophiocordyceps unilateralis was discovered in the Messel pit (Germany).[22]

When O. unilateralis-infected ants die, they are mainly located in regions containing a high density of ants which were previously manipulated and killed.[16] These areas are termed "graveyards" and can be of 20 to 30 meters in range[23] (with a local density of dead ants possibly exceeding 25 meters square).[11]

The density of dead ants within these graveyards can vary according to climatic conditions. This means that environmental conditions such as humidity and temperature can influence O.unilateralis' effects on the host population.[23] In fact, studies have described seasonal patterns in the density of previously infected dead ants, with an increase during the rainy season and a decrease during the dry season.[3] It is thought that large precipitation events at the beginning and the end of the rainy season stimulates fungal development,[3] which leads to more spores being released and ultimately more individuals being infected and killed.

Ophiocordyceps are known in the pharmaceutical world to be a medically-important group.[7] O. unilateralis fungi produce various known secondary metabolites, as well as several structurally uncharacterised substances. These natural products are reportedly being investigated as potential leads in discovery efforts toward immunomodulatory, antitumor, hypoglycemic, and hypocholesterolemic targets.[24]

In an Ophiocordyceps species within Japanese cicadas, the Ophiocordyceps replaces the symbiotic bacteria within the cicadas to help the host process sap as nutrients, unlike other related species, such as the Ophiocordyceps sinensis, which is a traditional immune booster and cancer treatment in Tibetan and Chinese culture.[25]

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.[33][34]

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 spore producers. 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 also grant beneficial assistance to the colony.[34]

In host–parasite dynamics, both the host and the parasite are under selective pressure: the parasite evolves to increase its transmission, whereas the host evolves to avoid and/or resist the infection by the parasite.

The principal hosts of O. unilateralis evolved adaptive behaviors able to limit the contact rate between uninfected susceptible hosts and infected hosts, thereby reducing the risk of transmission.

O.unilateralis' principal hosts evolved efficient behavioral forms of social immunity. The ants clean each other's exoskeletons to decrease the presence of spores attached.[11] Also, ants can sense that a member of the colony is infected; healthy ants carry the O. unilateralis-infected individual far away from the colony to avoid exposure to spores. There are also reports that most worker ants remain inside the nest boundaries; this would mean only foragers were at risk of infection.[34]

Moreover, one of the fungus' principal hosts, Camponotus leonardi, provided evidence for the avoidance of the forest floor by the host ants as a defence method.[23] In areas where O. unilateralis is present, C. leonardi builds its nests high in the canopy, and has a broad network of aerial trails. These trails occasionally move down to the ground level, where infection and graveyards occur, due to canopy gaps too difficult for the ants to cross. When the trails descend to the forest floor, their length is only of three to five meters before going back up into the canopy. This demonstrates the avoidance of the zones of infection by the ants. Additionally, more evidence participates in the favour of this defence method being adaptive as it is not observed in undisturbed forests where the zombie-ant fungus is not present.[23]

In the video game series The Last of Us, Ophiocordyceps unilateralis has evolved to infect humans, thus creating zombie-like enemies in the game. Also, in episode two of the 2023 television series The Last of Us on HBO Max,[35] Ophiocordyceps unilateralis is revealed to be the primary cause of the infected outbreak and subsequent collapse of human civilization. In the show, the fungus, having adapted to higher temperatures due to climate change, takes control of humans (as opposed to insects) as an alternative host and causes them to exhibit erratic behaviors, such as the desire to attack and infect non-infected humans. Craig Mazin, who co-wrote and produced the series, said that everything the series suggests fungi do, they have done forever in real life.[36]
In the 2014 novel The Girl With All The Gifts, its 2016 film adaptation and its prequel novel entitled The Boy on the Bridge - all written by M.R. Carey, a strain of Ophiocordyceps unilateralis is similarly able to infect the human population through exchange of bodily fluids, leading to an apocalyptic world inhabited by zombie-like "hungries" who attack non-infected. The novel “The Genius Plague” by David Walton, though not about this species specifically, expands on the idea of fungi influencing animals with a fungus that invades human brains and influences their actions toward its advantage.

• Massospora cicadina

• Ophiocordyceps unilateralis at UniProt.org. Accessed on 2010-08-22.
• An Electronic Monograph of Cordyceps and Related Fungi