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 (genus Camponotus). 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 (and other fungi) 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. 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 (e.g. typical single stroma arising from the host's dorsal pronotum, the ascoma (perithecia) growing from the stroma) and microscopic traits (e.g. the morphology of the ascospores in terms of size, shape, septation and germination). 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:
Species within the O. kniphofioides subclade as described in 2018:
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. This complex forms the fungus' fruiting body. Most species within the O. unilateralis s.l. species complex have both a sexual (teleomorph) and an asexual morph (anamorph). 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.
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