Much has been made of the parasitic fungus Ophiocordyceps unilateralis , which infects ants and takes over their bodies. He turns them into what would be known as zombies in popular culture , and forces them to hang from plants to die there. The fungus then devours the inside of the ant’s body and reinforces its exoskeleton, to firmly anchor itself to the leaf and, when the time comes, pull out the reproductive organ through the animal’s head that will release the spores of a new generation.
This fascinating behavior, however, is by no means isolated. There are many other pathogens that zombify their victims and take control of their actions. Taking advantage of the fact that February 4 is the day of Zombie Pride , we will review three of these cases, which are not as well known, but are equally curious.
A worm that induces suicide
Among freshwater aquariums it is very common to have several species of so-called killifish . They are small and easy to breed fish, among which the Florida flagfish, the panchax or the chococo stand out . Where these fish occur in the wild, they are often found with a flatworm, Euhaplorchis californiensis , which has a fairly complex life cycle.
The eggs of the parasite are released through the droppings of waterfowl, and are accidentally consumed by snails . The larvae that hatch from these eggs sterilize their host and swim out into the water. They then enter the fish through the gills, and travel along the nerves to the brain, where they encyst, lining its surface . But from the brain of the fish it must reach the digestive system of the bird, where its life cycle ends. To achieve this, it zombifies the fish: it alters the levels of hormones so that the fish becomes much more aggressive, swims with more force, shakes and jumps out of the water frequently. This erratic behavior draws the attention of birds, for whom it becomes easy prey. The cycle closes when they devour the zombiefied fish.
Who really pulls the strings
Few animals have the weaving ability of spiders, and among them, the best are those that, redundantly, receive the common name of weaver spiders. In fact, they weave two different types of webs depending on what they need at any given time: large, resistant, dense and sticky webs for hunting; and rest nets, fine, delicate, with non-sticky threads and that require few resources to rest while moulting. And it is precisely this ability to weave that the larva of the wasp Reclinervellus nielseni takes advantage of when it finds and dominates the spiders of the genus Cyclosa .
The larva actually controls the spider by injecting it with hormones, turning it into a kind of drugged puppet , and forcing it to radically change the appearance of its resting web. Reinforce the supporting fabric, remove sticky threads, if any, and decorate the netting so that it reflects UV light to prevent other insects from accidentally colliding and destroying it. Finally, it forces the puppet spider to build a dense core which it will then use as a cocoon to metamorphose into an adult wasp. The spider does not survive the process.
The triffid bacterium
It is well known by all that plants do not have a nervous system, and therefore nothing and no one can turn them into zombies . Not in the original voodoo sense—creatures whose wills have been taken away and behave like mere slaves to the will of their houngan or voodoo shaman. Nor in its most modern and cinematographic version – in which a pathogen in your head alters your behavior with the sole mission of spreading, usually through a bite. So it is difficult for something like what John Wyndham proposed in The Day of the Triffids to happen.
However, and brains aside, plants are still living beings, and as such they are still susceptible to a pathogen taking over their organism and using it for their own benefit. That is precisely what some bacteria of the genus Phytoplasma can do. When a plant is infected, the bacterium hijacks and reprograms its developmental systems, triggering abnormal growth that completely deforms the victim’s architecture . Where the infection appears, it begins to grow massively and uncontrollably, often causing the “witch’s broom” phenomenon —although it can also be caused by other pathogens—. This anomalous structure, this zombified plant, creates an optimal environment for the pathogen , and although it is usually maintained as long as the plant is alive and generally does not reduce its life expectancy, it does make it unable to reproduce.
Referencias
Evans, H.C., Elliot, S.L. and Hughes, D.P. (2011) ‘Hidden Diversity Behind the Zombie-Ant Fungus Ophiocordyceps unilateralis: Four New Species Described from Carpenter Ants in Minas Gerais, Brazil’, PLOS ONE, 6(3), p. e17024. doi:10.1371/journal.pone.0017024.
Huang, W. et al. (2021) ‘Parasitic modulation of host development by ubiquitin-independent protein degradation’, Cell, 184(20), pp. 5201-5214.e12. doi:10.1016/j.cell.2021.08.029.
Lafferty, K.D. and Morris, A.K. (1996) ‘Altered Behavior of Parasitized Killifish Increases Susceptibility to Predation by Bird Final Hosts’, Ecology, 77(5), pp. 1390–1397. doi:10.2307/2265536.
Takasuka, K. et al. (2015) ‘Host manipulation by an ichneumonid spider ectoparasitoid that takes advantage of preprogrammed web-building behaviour for its cocoon protection’, Journal of Experimental Biology, 218(15), pp. 2326–2332. doi:10.1242/jeb.122739.
