Host manipulation induced by parasites is broadly distributed in nature and it has been a topic of several studies over the past decades (Thomas et al. 2010). Ophiocordyceps unilateralis (Clavicipitaceae: Hypocreales) is a fungal pathogen-specific to ants of the tribe Camponotini (Formicinae: Formicidae) with global distribution. Among these myrmecophilous species, this so-called zombie or brain-manipulating fungus alters the behavior of the ant host, causing it to die in an exposed position, typically clinging onto and biting into the adaxial surface of midribs of shrub leaves by its mandibles often followed by the growth of hyphae. Species within this group exhibit Hymenostilbe asexual morphs with the ascospores actively shooting out spores.
Ophiocordyceps (Ophiocordycipitaceae, Hypocreales, Ascomycota) is known as the largest genus of entomopathogenic fungi with around 240 defined species (Robert et al. 2005, 2013); Ophiocordyceps members do not form resting spores (i.e., chlamydospores) and cannot survive outside of their hosts.
Infections of ants by entomopathogenic fungi have been reported in several regions of the world especially in tropical forests, with relatively few reports from temperate ecosystems, such as Asia, Australasia, Africa, and the Americas (Pontoppidan et al. 2009), and diversity concentrated in the tropics and decreasing with increasing latitude. These interactions frequently happen as epizootics, contaminating a large number of ant specimens in small forest regions (Andersen et al. 2009, Pontoppidan et al. 2009). The probability of affecting directly and indirectly on the abundance of its hosts. Among these associations, it is far regarded that Ophiocordyceps species (Hypocreales: Ophiocordycipitaceae) can parasitize numerous ant species from basal groups (e.g. Ponerines) to modern genera (e.g. Camponotus) (Evans & Samson 1982, Sanjuan et al. 2011, Evans et al. 2011). In addition, the occurrence of parasitized ants is fixed upon specific host plants, plant structures, and heights above the soil. In the southeast, Evans et al. (2011) interpreted four new species with conspicuous different microscopic characteristics, but with morphology just like that of O. unilateralis. After this current discovery, minute research was made on the ecology and distribution of these fungi species.
The Fungal Infection Cycle and Host Specificity
The parasitic fungi such as the host-specific pathogen Ophiocordyceps unilateralis sensu lato can dramatically monitor insect brains and manipulate their behavior to arrive death sites that are optimal for spore dispersal; for example, crickets or grasshoppers commit suicide by drowning when infected by the parasitic hairworm Spinochordodes tellinii the so-called fungal extended phenotype (The behavior, in which genes of the parasite are expressed in the host’s phenotype (Dawkins 1982)). On the other hand, on top of physiological immune surveillance, insects (especially social insects such as ants and termites) can smell and eliminate fungal pathogens, groom each other to clear pathogenic spores, raise a fever response, or die well away from their nestmates, that is termed behavioral or social immunity. Thus colony transmission and fungal development are limited by strong social immunity demonstrated by social insect societies.
The Fungal Infection Cycle and Host Specificity Entomopathogenic fungi recognize and infect insects through the spore adhesion and formation of appressoria by attaching, germinating, and penetrating the cuticles of foraging ant workers. After reaching the hemocoel (body cavity) of an insect, fungal filaments will convert into yeast-like cells as single cells in the host hemolymph that go through budding for quick propagation and check the immune response of the hosts.
The infected, moribund ants basically behave like zombies; they walk alone and erratically climb to a definite height in the vegetation (approximately 25 cm above the soil surface), before being killed for the post mortem fungal growth. They bite onto substrate in rainforests and twigs in temperate woods and transition from wandering to biting takes place synchronously around noontime (within 11:00–14:00 h) possibly in association with a solar cue. Before mummifying its host, the fungus drives a larva close to the surface of the soil (approximately 1–3 cm), and then the fruiting body then erupts from its head to form a branching network cadaver of cells. Infections by obligate Entomophthorales pathogens could also lead to similar summit” diseases, i.e., the infected insects climbing to an elevated position before death. Such manipulation functions benefit the fungi by maximizing spore transmission efficiency to begin the next infection cycle.
The fresh stage represents the preliminary phase of fungal development in the ant, which denotes that the host had been recently killed (about 5 days before). The immature stage refers to specimens lacking ascomata but presenting stromata. The rise of compact hyphae from the host’s dorsal pronotum constructing evident stromata (stalks) is characteristic of this group of fungi. Ascomata (fertile parts) initiate on these stromata and disclose the sexual spores that are ejected and spread out after they arrive at their maturity(Kepler et al. 2011; Evans et al. 2011b). Thus the fungus ends the parasitic life cycle by lysing the host tissue. During the ~ 1–2 weeks in which hyphal development occurs in the ant, the fungus is under selective pressure for host resource exploitation and defense against invaders.
Control of Insect Behavior by Parasitic Fungi
Generally, the manipulated behaviors include convulsion, erratic walking. The ants adhered to the plants using their mandibles, normally on the abaxial surface and central veins of leaves, a behavior which is not apparent in healthy ants of this species. According to Andersen et al. (2009), this behavior is congenital to the fungus, ensuring a stable microclimatic niche for its development after the host’s death and facilitating aerial dispersion of the spores. Species belonging to O. unilateralis complex initiate a series of synchronized events within the host to induce it to leave the colony towards the understory, where it perishes in an elevated and strategic position, characteristically attaching itself to abaxial vein or edges of leaves or twigs in a “death-grip” and move and die away from the nest. Once there, the spore-producing structure (stroma) emerges from the anterior intersegmental region to the pronotum, from which the ascospores actively liberate their spores after a few weeks of maturation (Andersen et al. 2009, Araújo et al. 2015). After releasing, the spores are deposited on the forest floor, where they will infect other ants that eventually go on through the site. Invasion of the host body appears through the cuticle and the corpses are predominantly observed in areas where the density of ants previously manipulated and killed by the fungus is already quite intense, called graveyards (Pontoppidan et al. 2009). In Brazil, natural epizootics resulting due to the fungi O. unilateralis were demonstrated in adult individuals of ants Camponotus sp. in the Amazon Forest (Andrade 1980, Evans & Samson 1982, 1984).
Hyperparasitic fungi can play a vital role in the control of Ophiocordyceps–host interactions, when successfully found in the population (Morozov et al. 2007) were hyperparasitized (Andersen et al. 2012), the much lower proportions of hyperparasitism observed suggest a relatively weak impact on the entomopathogenic.
Good to know
Hyperparasitism: the parasitic habit of one species upon another parasitic species.
Ophiocordyceps species infecting ants – the so-called zombie-ant fungi – incorporate one of the most intriguing and fascinating relationships between microbes and animals. Despite their massive importance for ecosystem functioning, these fungal pathogens are still faintly documented, especially regarding their diversity, ecology, and evolutionary relationships. The natural occurrence in Camponotini ants suggests that this fungus may play a significant role in ant control, and should exhort more widespread studies on the possibility of manipulating this fungus to be used in biological control programs.
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- Harry C. Evans, Simon L. Elliot, David P. Hughes. (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): e17024.
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- Kai-Wen Tu, Ming-Chung Chiu, Wei-Jiun Lin, Yen-Ping Hsueh, Chung-Chi Lin, Jui-Yu Chou. ‘Bacillus Thuringiensis Predominated in the Zombie Fungus Ophiocordyceps Unilateralis S. L.-Infected Ant Cadavers and Investigation of its Biological Properties.’
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