By Wesley Killinen
What is the Zombie-ant Fungus?

Ophiocordyceps Unilateralis, the zombie-ant fungus, is an entomopathogen, a pathogen that infects arthropods such as insects, mites, and ticks (1).  The fungus is characterized by a stalk, stroma, that grows out of the back of an ant’s head.  The stroma is where spores can be released from the perithecial plate.  The species is highly complex and is able to form not only sexual stages (teleomorphs), but also asexual stages (anamorphs) (2).  The diversity of forms acts as a built-in survival mechanism to ensure the success of the species if the primary form (teleomorphs) fail (2).  Ophiocordyceps are a diverse species that can infect different species of ants.  Of the 29 orders of ants it is believed that Ophiocordyceps  are able to colonize 19 of these orders (3).  This wide diversity of potential hosts increases the fungus’s infectious strategies to ensure successful transmission and fill a specific ecological niche.  The fungus modifies the host behavior so that it increases the likelihood of transmission to other ants.  Transmission is aided when an ant ascends a small plant and bites onto the vegetation and dies (3).  The spores from the fungus can then descend to potential ants below to infect a new host.  The reason that the fungus must attack ants outside of their nest is because of the natural behavioral immunity inside ant colonies that can limit the growth and transmission of fungi (3, 4).  Ants are very social insects and they have hygienic behavior, where they clean each other, that reduces transmission of fungi (4).  The phenomenon of altering host behavior is known as an extended phenotype; a term coined by Dawkins (1982) to describe the relationship between a host and parasite, where the parasite will express the trait in any aspect of host shape or behavior (3).  This allows the fungi to be very diverse and fill different ecological niches wherever they are found.  As far as we know, parasitism is one of the most common modes of feeding traits: herbivore, carnivore, and parasitism; to have evolved with as many as half of Earth’s species being parasites (4).

Where can this parasite be found?

The Zombie-ant fungus is typically found in tropical forest ecosystems where its food source is most plentiful (2).  Ants make up less than 2% of all insect species, but they account for 50% of animal biomass in tropical forests (3).  Although ants can occupy very diverse habitats the fungus prefers a warmer and moister environment, such as a tropical forest.  Because ants are dominant members of their terrestrial biomes, they are the most encountered host species for Ophiocordyceps world-wide (3). 

How is the fungus transmitted between ants?

Transmission between ants is caused when a fungal spore from the air or soil encounters an ant (1).  The hypha drill, on the spore, bores through the ant exoskeleton using the enzymes lipase, chitinase, and protease (1).  Over the next few days, the fungus grows in the body cavity as a free-living yeast cell (2).  The fungus then secretes neural toxins to alter the ant’s behavior.  Not much is known about the process of host manipulation; however, there are speculations, but are not concrete enough to be included in this analysis.  The ant will move away from its colony and climb some vegetation where it will begin its “death grip”.  This is where the ant will die, and the fungus stroma begins to bud from the back of the ant’s head (1).  This process allows the fungus to consume the ant and use its body as protection while it waits for the ideal time to release its spores when ant activity has peaked and potential hosts are in the area (2).  The spores are then released to rain down onto more potential victims to continue the cycle. 

What is special about the Zombie-ant Fungus?

Ophiocordyceps Unilateralis produces secondary metabolites called naphthoquinones which have antibacterial, anticancer/antitumor, antiviral, and antimalarial activities (5, 6).  Research is currently underway into how these fungi repel other secondary infections as it protects its host’s body from scavengers.  Antiviral testing has showed some promise with some of the sub-compounds showing significant antiviral activity for Influenza A and Herpes Simplex Virus (HSV-1) (6).  The hope is to use these compounds to develop an antiviral drug to help fight infections.  Also, one of the compounds showed significant anticancer activity against Lymphoid Leukemia (6).  More research into fungal resistance mechanisms may reveal more possibilities for potential drugs.  The race is on to discover more antibiotics that will affect the developing “super bugs” that are becoming more antibiotic resistant.   

References

1.        Mckenna A. Ophiocordyceps unilateralis: The Zombie Ant Fungus – BIOL421 @UNBC – Insects, Fungi and Society.

2.        Evans HC, Elliot SL, Hughes DP. 2011. Ophiocordyceps unilateralis:  A keystone species for unraveling ecosystem functioning and biodiversity of fungi in tropical forests? . Commun Integr Biol 4:598–602.

3.        Araújo JPM, Evans HC, Kepler R, Hughes DP. 2018. Zombie-ant fungi across continents: 15 new species and new combinations within Ophiocordyceps. I. Myrmecophilous hirsutelloid species. Stud Mycol 90:119–160.

4.        Hughes DP, Libersat F. 2019. Parasite manipulation of host behavior. Curr Biol 29:R45–R47.

5.        Amnuaykanjanasin A, Panchanawaporn S, Chutrakul C, Tanicharoen M. 2011. Genes differentially expressed under naphthoquinone- 2 producing conditions in the entomopathogenic fungus 3 Ophiocordyceps unilateralis. Mol Ecol.

6.        Tandon VK, Singh R V., Yadav DB. 2004. Synthesis and evaluation of novel 1,4-naphthoquinone derivatives as antiviral, antifungal and anticancer agents. Bioorganic Med Chem Lett 14:2901–2904.