Friday, November 9, 2012

When Parasites' Worlds Collide

In the world of parasites, sometimes the hunter becomes the hunted (or, perhaps more appropriately, the parasite becomes the parasitized).

As its name suggests, the life of a worker ant is all work and no play. Their time is solely devoted to tending to the queen, securing food for the colony, caring for their young, and defending the nest. As the worker ant scurries along well-traveled paths in the course of its daily routine, however, it may not discern the threat overhead: when the worker ant unknowingly picks up spores released from an overhanging 'zombie-ant fungus', the ant becomes infected with the 'zombifying' fungus. 

After infection the normally reliable and consistent worker ants begin to exhibit a number of unusual behaviors. They mindlessly depart from their usual routes, convulsing, stumbling, and wandering around aimlessly. Every fungal infection, however, ends with the same eerie ritual: right on cue, at high noon the infected ant sinks its jaws into the main vein of a leaf, orienting itself to the northwest and assuming a position where the fungus will have easy access to the soil below. At this stage of infection the ant already has a head swimming with fungal cells and nerve toxins which are presumably to blame for their strange, jerky movement, loss of balance, and convulsions. The muscles around the ant's mandibles have also begun atrophying, and so after the ant bites into the leaf it develops a case of 'lockjaw'. The ant's 'death grip' anchors its body in place, and even after the ant dies several hours later it remains unmoved. The carcass of the ant remains in place for several days as a fungal stalk grows inside of the head of the deceased ant, eventually emerging and spreading its own fungal spores to other unfortunate passing ants.

Image credit: Wikimedia Commons/David Hughes/Maj-Britt Pontoppidan/PLoS ONE

The "zombie-ant" fungus (formally known as Ophiocordyceps) has evolved to capitalize quite efficiently on all of its available resources. Ophiocordyceps propagates itself by targeting ants, the most abundant insects on earth, and uses them as vehicles for its reproduction and to facilitate its movement to new hosts.  In a 2011 BMC Ecology paper the authors cryptically explain that "while the manipulated individual may look like an ant, it represents a fungal genome expressing fungal behavior through the body of an ant"; in essence, the fungus takes control of the ant's body and exploits it for its own purposes. In its takeover of the ant, the fungus ensures that all conditions are optimal for its reproductive success. The 'zombie-ant fungus' appears to be a widespread phenomenon, as there is evidence that different fungal species are found across the globe, each specialized to target local ant species.

Had the infected ant remained within the confines of the colony, its body would have been quickly extricated and heaped upon other deceased ants in the colony's 'ant graveyard' before the fungus had a chance to grow and mature. Because the fungus appears quite unexpectedly, however, and its presence is unanticipated, unsuspecting worker ants are more prone to infection. As the fungal takeover of the ant proceeds, the unwitting host begins to stray outside of the colony, having been 'programmed' to do so by the invading fungus. The ant then wanders along the forest floor, far enough away from the colony that its carcass won't alarm the other workers, but close enough that other worker ants may pass by it in their travels. 

Image credit: The American Naturalist/ University of Chicago Press

All told, the 'zombie-ant' fungus (in all of its many specialized forms and species) has been quite successful. After it has taken over and effectively killed off its host, however, another fungal species threatens to parasitize the original fungal parasite, and in this way the host-parasite relationship comes full circle.

The second fungal parasite appears to grow specifically on the ant and 'zombifying fungus' stalk, while the surrounding environment remains largely unaffected. As the new fungus carpets the ant-fungus remnants, the original fungal parasite can no longer release its spores. It has been suggested that this ultimately works to the benefit of the first fungus- had its success gone unchecked, it could easily decimate an entire ant colony. Because the first fungus is slightly crippled by the second, however, a balance between the parasites and host colony is maintained. Parasites must be able to effectively and efficiently exploit their hosts to their own purposes, however, if the process is too efficient- if they kill off their hosts too soon, in some cases, or whittle down the population of potential hosts to a critically low level- then their efficiency can be a detriment to their own livelihood.

Mother nature works in mysterious ways, and so the host-parasite relationship is often supported by an intricate framework of checks and balances. And, as indicated by the previous blog post, parasites can come in all shapes and sizes. 

The takeaway? In a fungus-eat-(er, sterilize)-fungus world, there is never a clear biological 'winner'. 

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