Part One: Introduction to Parasitism and Host Manipulation
By Dr. Kyle Lesack
Nature provides countless examples of ingenious adaptations that allow each species to survive in their environment and, ultimately, reproduce. Some of the most striking examples facilitate parasitism, a term describing organisms that exploit a host species to obtain the resources they require to survive. Parasites have evolved a wide range of evolutionary strategies, which are reflected in the diversity of hosts and habitats that they are associated with. While the specific selection pressures may vary considerably between parasitic species, the success of all parasites depends on their ability to survive and exploit the microhabitats within their hosts, as well as their ability to infect their next host. The adaptations are often surprising in their complexity. For example, the larvae of certain nematodes, such as Ascaris lumbricoides and Strongylus vulgaris, undergo extensive tissue migrations throughout their host’s body. Other parasites have evolved “complex” life cycles that require transmission between multiple host species. Altogether, parasitism has been an incredibly successful lifestyle, having evolved independently hundreds of times (1).
Some of the most striking strategies involve profound manipulation of the host’s behaviour or morphology. Horsehair worms, as an example, infect and alter the behaviour of grasshoppers and crickets to seek water once the parasite has developed into the adult stage (2). After encountering water, the worms emerge from the host and mate, thereby completing their life cycle. The flatworm Dicrocoelium dendriticum provides another example of host behaviour manipulation[kl1] . The definitive hosts (where reproduction occurs) of these worms include ruminants, such as cattle and sheep. To complete its life cycle, the parasite must first infect a snail intermediate host, followed by an ant, which is then consumed by the definitive host. How the parasite manipulates the ant into being consumed by the grass feeding ruminant host is an incredible feat of evolution. At dawn, the infected ants are induced into seeking out and clamping onto the tops of blades of grass where they remain for the night. Most impressively, the ant returns to its nest the next morning, thereby avoiding the heat during the day. This process can be repeated daily until either the ant is consumed by a ruminant or dies from other causes. The parasite’s ability to dramatically modify its host’s behaviour has earned this relationship the nickname of “zombie ants”.
Many parasites have complex life cycles that involve transmission between multiple host species. Typically, this occurs when infected eggs excreted from one host are ingested by another or when a host is susceptible to a predation by another. Certain parasites can even manipulate their host’s appearance to facilitate predation. One noteworthy example occurs in the flatworm Leucochloridium paradoxum. Surprisingly, insectivorous birds serve as definitive hosts for this parasite species despite snails being the intermediate. This parasite species evolved the remarkable ability to form a broodsac in the snail host, which resembles a caterpillar (https://www.youtube.com/watch?v=ZO-4f41Gaf8). This in turn attracts birds that predate upon the snail and ingest the parasite. Strategies where a parasite manipulates its host into resembling prey of another are a type of mimicry, which includes a broad range of adaptations characterized by an evolved resemblance to another species or object. Mimicry is common enough among parasites, especially at the molecular level, but alterations to host appearance of this magnitude are quite rare.
In part two of this series, the discovery of the parasite Myrmeconema neotropicum will described in a fascinating example of serendipity in science.
References
1. Kaya H, K. Horsehair worms, Hairworms [Internet]. 2013. Available from: https://ipm.ucanr.edu/PMG/PESTNOTES/pn7471.html
2. DeLaCruz D. Leucochloridium paradoxum. 2003; Available from: https://animaldiversity.org/accounts/Leucochloridium_paradoxum/
3. Yanoviak SP, Kaspari M, Dudley R, Poinar G. Parasite-induced fruit mimicry in a tropical canopy ant. Am Nat. 2008;171(4):536–44.
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