BIO 203

Form and Function

Prominent features of Ophiocordyceps unilateralis include the formation of a stalk from the base of an ant host’s head, as well as the spread of mycelium throughout the host body’s muscle and brain tissue. Yeast-like growth of this fungus throughout the body of its host does not invade host cells but rather works around them and spreads the tissue apart. According to recent research, dissection of ants in post death-grip positions revealed signs of muscle atrophy of the mandibular muscles along with hyphal body growth indicative of ants infected by Ophiocordyceps unilateralis (Hughes 2011).
Courtesy of David Hughes
Complex adaptations to a particular host are a hallmark of specialized parasites. The manipulation of behavior by the parasitic fungus Ophiocordyceps unilateralis upon the Camponotus leonardi ant is suggestive of such an adaptation (Hughes 2011). Worker ants on the forest floor pick up spores dispersed by a mature fungus, allowing the spore to secrete enzymes and enter the host’s body (Evans, et al. 2011). Once inside, the fungus begins to spread throughout the ant’s body and head, occupying muscle tissue and cavities (figure). When the fungus makes an established foothold in the ant’s body, it is able to manipulate its host’s body to increase its own fitness. In more specific terms this relationship is an example of an extended phenotype, which is defined by Dawkins as the capacity of an individual or group of parasites to manipulate host behavior to improve fitness (Dawkins 1982).

It is suggested that the invading fungus, through the use of extended phenotypes and mechanisms, is able to cue for a specific form of behavior as has been observed by researchers who timed the death-grip bite of infected ants around the noon hour (Hughes 2011). These adaptations to manipulate behavior are essential in directing the host to an environment that promotes optimal growth. Though these mechanisms are not well known, it is believed that the parasitic fungus produces compounds that allow it to direct the host to perform an action (Evans, et al. 2011). Some of these adaptations, such as the use of clock genes to synchronize behavior are commonly found in other fungi (Hughes 2011). It is interesting to consider what might happen when scientists eventually discover what chemical or pathways the fungus utilizes to control behavior in its host to be both horrifying and beneficial.



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