Ophiocordyceps unilateralis
Ophiocordyceps unilateralis, commonly known as zombie-ant fungus, is an insect-pathogenic fungus, discovered by the British naturalist Alfred Russel Wallace in 1859. Zombie ants, infected by the Ophiocordyceps unilateralis fungus, are predominantly found in tropical rainforests.
These fungi thrive in warm, humid environments, which are ideal for their growth and reproduction. However, they can also be found in warm-temperate forest systems. The fungus primarily targets ants from the tribe Camponotini, including carpenter ants.
O. unilateralis infects ants of the tribe Camponotini, 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 attach themselves to a major vein on the underside of a leaf, where the host remains after its eventual death. The process, leading up 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.
Related species, like Tolypocladium inflatum produce secondary metabolites that have been a source of valuable pharmaceuticals. Organisms like O. unilateralis have often evolved to produce specific metabolites for their antibacterial and/or host modulating activities. As a result, some of these metabolites may prove to be useful pharmaceuticals.
Systematics
After years of research, the taxonomy of Ophiocordyceps unilateralis is becoming increasingly clear.''Cordyceps'' vs ''Ophiocordyceps''
Throughout history there has been confusion about the distinction between the genera Cordyceps and Ophiocordyceps. There have been many debates about whether the zombie-ant fungus belonged to one or to the other as Ophiocordyceps was only recently brought forward.The genus Cordyceps comprises over 400 species, historically classified in the family Clavicipitaceae within the order Hypocreales. The classification was based on different morphological characteristics such as filiform ascospores and cylindrical asci.
When Cordyceps were first classified, there was no concrete evidence for the genus Ophiocordyceps. However, in 2007, important new molecular data was tested, and enabled them to reorganize the family Clavicipitaceae. It was found that Clavicipitaceae was in fact three distinct monophyletic families: the Clavicipitaceae, the Cordycipitaceae and the Ophiocordycipitaceae.
The new molecular phylogenetics studies contradicted the older classification and moved all Cordyceps species forming a sister group with Tolypocladium, into Ophiocordycipitaceae. Fungi able to parasitize ants were also included in the transfer, such as Cordyceps unilateralis which was later renamed Ophiocordyceps unilateralis. Following this study, multiple traits such as the production of darkly pigmented, hard to flexible stromata were defined as characteristics of the family Ophiocordycipitaceae.
''Ophiocordyceps unilateralis sensu lato''
The fungus's scientific name is sometimes written as Ophiocordyceps unilateralis sensu lato, which means 'in the broad sense', because the species actually represents a complex of many species within O. unilateralis.Support for this term has become increasingly important. In 2011, it was hypothesized that the zombie-ant fungus could actually be described as a complex of species which are host-specific, meaning that one O. unilateralis species can only successfully infect and manipulate one host ant species. There is a possibility that this resulted in or reinforced the reproductive isolation of the fungi, leading to its speciation. Following this, a study conducted in Brazil delimited, using morphological comparisons of the ascospores, germination processes, and asexual morphs, four different Ophiocordyceps species. Afterwards, three new species were described in the Brazilian Amazon, six in Thailand, and one in Japan.
More recently in 2018, 15 new O. unilateralis species were described based on classic taxonomic criteria, and macro-morphological data with a deeper focus on ascospore and asexual morphology.
The asexual morphologies made it possible to distinguish two different clades mainly composed of species associated with ants which they termed "O. unilateralis core clade" and "O. kniphofioides subclade."
Further analyses were conducted using a set of different traits. Morphological traits were used and included both macro-morphological characters and microscopic traits. Moreover, other traits such as the host and the location of the death grip were added to the analyses. The morphological study led to 15 new identified species, with 14 which were distributed in the core clade, and one in the subclade. Moreover, it was found that species in the O. kniphofioides subclade specialise on neotropical ants, whereas species in the core clade specialise on Camponotini species.
Species within the O. unilateralis core clade as described in 2018:
- O. albacongiuae
- O. blakebarnesii
- O. camponoti-atricipis
- O. camponoti-balzani
- O. camponoti-bispinosi
- O. camponoti-chartificis
- O. camponoti-femorati
- O. camponoti-floridani
- O. camponoti-hippocrepidis
- O. camponoti-indiani
- O. camponoti-leonardi
- O. camponoti-melanotici
- O. camponoti-nidulantis
- O. camponoti-novogranadensis
- O. camponoti-renggeri
- O. camponoti-saundersi
- O. halabalaensis
- O. kimflemingiae
- O. naomipierceae
- O. ootakii
- O. polyrhachis-furcata
- O. pulvinata
- O. rami
- O. satoi
- O. daceti
- ''O. kniphofioides''
Morphology
Typical morphology
The zombie-ant fungus is easily identifiable when its reproductive structure becomes apparent on its dead host, usually a carpenter ant. At the end of its life cycle, O. unilateralis typically generates a single, wiry yet pliant, darkly pigmented stroma which arises from the dorsal pronotum region of the ant once it is dead. Moreover, perithecia, the spore-bearing sexual structure, can be observed on the stalk, just below its tip.fruiting body.Most species within the O. unilateralis s.l. species complex have both a sexual and an asexual morph. These are different in terms of their function and characteristics. Generally, the asexual morphs identified for Ophiocordyceps are Hirsutella and Hymenostilbe, two genera of asexually reproducing fungi.
Morphological variation
O. unilateralis species exhibit morphological variations which are most certainly due to their wide geographic range, from Japan to the Americas.Moreover, it has been hypothesized that their morphological variations may also be a result of one fungus species maximizing its infection on one specific host ant species. Different subspecies of ant can occur within the same area, which means that in order to coexist they have to occupy different ecological niches. Consequently, the fungi may have evolved at the subspecies level in order to maximize its fitness.
''O. unilateralis'' core clade morphological characteristics
The O. unilateralis core clade, as described in 2018, has distinct morphological characteristics. It exhibits a single stroma with a Hirsutella asexual morph, which arises from the dorsal neck region of the dead ant and produces a dark brown perithecia attached to its stalk.These species are also recognizable through the host species they infect, which are only Camponotini species. Once the host is killed by the fungus, it is commonly found fixed through their mandibles onto the surfaces of leaves.
''O. kniphofioides'' subclade morphological characteristics
The O. kniphofioides subclade, as described in 2018, also has distinct morphological characteristics. Its species produce a stroma that grows laterally from the host's thorax which itself generates an orange ascoma. Moreover, species within this subclade share a Hirsutella asexual morph.As for the core clade, these species are also recognizable through the hosts they infect, which are usually neotropical ant species. The subclade does not present the same extended phenotype with the famous "death grip" that O. unilateralis species typically exhibit. Their hosts usually die at the base of large trees in the Amazonian rainforest, among the moss carpets.
Life cycle
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. 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.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.
An infected ant exhibits irregularly timed full-body convulsions that dislodge it from its canopy nest to the forest floor.
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 26cm above the forest floor, on the northern side of the plant, in an environment with 94–95% humidity and temperatures between. Infections may lead to 20 to 30 dead ants per square meter. 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. In temperate forests, the typical behavior of zombie ants is to attach themselves to the lower side of twigs, not leaves.
A search of 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 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. A study led in Thailand revealed that there is a synchronization of this manipulated biting behavior at solar noon.
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, securely anchoring 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 4–10 days.
Dead ants are found in areas termed "graveyards" which contain high densities of dead ants previously infected by the same fungus.
The term "zombie ants" has been used in popular media as well as scientific articles, but has also been described as "catchy, yet misleading."