Cordyceps and Zombie Ants
Cordyceps is a fungi genus that belongs to a group of fungi known as ascomycetes or such fungi. As such, they are characterized by a sac-like appendage that contains spores. Genus Cordyceps is made up of over 400 species of fungi, most of which exist as endoparasites. While many other endoparasitoids exist, it is the manner in which Cordyceps infect and affect the hosts that make them truly scary.
Formerly known as Cordyceps unilateralis, Ophiocordyceps unilateralis is one of the most popular species of genus Cordyceps. In order to complete its life cycle, the fungus infects its hosts and ultimately takes control of its action. The following is a life cycle of the fungus that ultimately causes the host to be zombified.
Phase 1: Dead ants on the underside of tropical rainforest leaves
For the sake of the cycle, we will assume that is this the first stage of the life cycle. Typically, Ophiocordyceps unilateralis infect a specific species of ants known as Carpenter ants (Camponotus leonardi). In this particular phase, infected ants are found under the leaves of tropical rainforest plants with thread-like fungal tissue growing from the back of their heads. Spores are released from the perithecia pad into the environment. These spores, however, cannot grow on the ground given that they require certain special conditions for growth. This takes us to the second phase of the life cycle of the fungus.
Phase 2: Infection
Infection of the ant begins when they come in contact in the fungal spores. Once the infective spores land on the ant, they go through a series of changes that result in the production of adhesive substances and chemicals that support the infection. Whereas the adhesive substances allow the spores to adhere and attach to the ant's cuticle, cuticle-degrading enzymes degrade the cuticle thus allowing the hyphal threads to grow inside the insect. Within the host, the fungus continues to proliferate with increased production of hyphal bodies. As these structures spread within the insect, they exploit nutritional resources which increasingly harm the host. Here, the fungus can be compared to other types of parasites such as tapeworms that infect the host (human beings and other animals) and use nutritional sources available in the intestines of the host. However, for Ophiocordyceps unilateralis, hyphal threads and enzymes produced also break down the internal structures of the ant thus killing it from the inside. This is in addition to the production of chemicals that take control of the nervous system of the ant.
Cordyceps fungus on a moth
Phase 3: Taking control
Fungal growth within the host (ant) replaces some of its cells. For instance, according to a recent study, it was shown that by the time the fungus takes control of the host's activities, half of the cells in the head of the host would have been replaced by fungal cells. This increase in fungal cells allows the fungus to take control of the host through the manipulation of such neurotransmitters as serotonin and dopamine. For instance, according to the aforementioned research study, the fungus was shown to increase the activity of genes that regulate the neurotransmitters with a notable depletion of serotonin.
*Serotonin promotes active persistence and thus foraging
While fungal activities initially cause the host to actively crawl up the plant leaf, further impact on the neurotransmitters slow down the ant as the fungus takes full control. In addition to affecting motion, the fungus also has an impact on a set of genes that control jaw muscles. This causes the muscles to deteriorate thus resulting in lockjaw effect as the ant bites down on the leaf permanently.
As the fungus continues proliferating, it continues impacting genes responsible for immunity resulting in the suppression of the host's immune system. This allows the fungus to continue growing and spreading into all the host's tissues. At the same time, it strengthens the exoskeleton of the host, which helps provide protection for the fungus as it continues growing. The fungus also starts growing outwards with fungal threads known as mycelium breaking through the exoskeleton and attaching to the leaves thus providing additional support. This is followed by the development of reproductive stalks and sporocarps in which the spores start to develop. These spores are ultimately released into the environment which allows the cycle to continue when they fall and attach onto other insects (ants)
Other species of the genus (Cordyceps) can successfully infect other types of insects such as spiders and hoppers. However, the infection process (mechanism) is similar with the ability to control the behavior of these hosts. This allows the fungus to continue thriving in their environment given that they not only take advantage of host for nutrition but also for transportation and spreading. The hosts, in this case, are therefore referred to as zombies because the fungus gradually kills them while taking control of their bodies.
Brian Lovett and Raymond J. St. Leger. (2016). The Insect Pathogens. Microbiology Spectrum: American Society for Microbiology Press.
Nick Redfern and Brad Steiger. (2014). The Zombie Book: The Encyclopedia of the Living Dead.
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