Friday, September 9th, 2011

Awesome parasite tricks

Stories about parasites just creep me the heck out, but I can’t resist them. The idea that parasites can “rewire” the brains and/or behavior of their victims isn’t new. A great example is that of Toxoplasma gondii, which can cause mice and rats to change their behavior, essentially causing them to seek out cats that will eat them (and thus pass along the Toxoplasma gondii). There are even scientists who believe that Toxoplasma infection causes mental illness in humans.

While waiting for a photographer to set up this morning, I read a nifty PLoS ONE paper on parasitic wasps from some Czech researchers that might add a few good parasite examples to your cocktail party conversation bank. The chief example, of course, is the larva of the wasps themselves, who force their spider hosts to build the sort of snuggly web-den that they would normally do as they are preparing for winter. The researchers gather that the larva get the same advantage from the winter webs as the spiders do, namely protection from weather and predators. Then, presumably, the larva eat their hosts from within. Eh, don’t feel too bad. Unless you are a big fan of spiders, Neottiura bimaculata and Theridion varians are not the sort I’d hesitate to squish. But maybe that’s just me.

Interestingly, both spiders make different kinds of winterized webs, where N. bimaculata creates a dense wad of webbing while T. varians builds a cupola-like structure. So, despite the fact that the hosts are two distinct species who build two distinctly different types of webs, the wasp larva effects them in more or less the same way, presumably by tinkering with the same winterizing mechanism (yay evolution!).

The paper’s intro also provides a few good examples, which I’ll just paste here for reference:

Many parasites and parasitoids have evolved remarkable strategies to manipulate the behavior of their hosts in order to promote their own survival and reproduction [1], [2]. The behavioral manipulations described include altered phototaxis, changes in locomotion, and the alteration of foraging and defensive behaviors [2]–[19]. The most fascinating manipulations are those that lead to unnatural host behaviors. The parasitic trematode, Dicrocoelium dendriticum Rudolphi, forces its intermediate ant-host to move up onto blades of grass during the night and early morning. This action increases the ingestion of infected ants by grazing sheep, the final host [3]. Mermithid nematodes induce their terrestrial arthropod hosts to commit suicide by jumping into water, after which the hairworms desert the host to spend their adult stage in their natural habitat [8].

Behavioral manipulations often result in the induction of innate behaviors. Acanthocephalan, Polymorphus paradoxus (Connell & Corner), evokes evasive behavior in the amphipod intermediate host, Gammarus lacustris Sars, which is then eaten by ducks [4]. The braconid parasitoid, Glyptapanteles spp., makes their caterpillar host behave as a bodyguard of the parasitoid pupae [15]. The caterpillar stands bent over the parasitoid pupae and violently lashes out at approaching predators, resulting in reduced predation of parasitoid pupae.

Evidence for benefits of the host manipulations for the parasitoid has been gained from several host-parasitoid systems [9]–[12]. But there might be also costs involved. This has been rarely studied. Maure et al. [13] investigated bodyguarding of the braconid pupae, Dinocampus coccinellae (Schrank), by ladybird Coleomegilla maculate Timberlake. Laboratory experiments revealed that duration of bodyguarding suppressed predation by lacewings but also decreased the parasitoid fecundity.

You can find the entire article here, for free, because PLoS ONE is awesome like that.