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Sunday, June 15, 2008

Bizarre parasitisms a challenge to evolution

A caterpillar of the geometrid moth Thyrinteina leucocerae with pupae of the Braconid parasitoid wasp Glyptapanteles sp.

Full-grown larvae of the parasitoid egress from the caterpillar and spin cocoons close by their host. The host remains alive, stops feeding and moving, spins silk over the pupae, and responds to disturbance with violent head-swings. The caterpillar dies soon after the adult parasitoids emerge from the pupae. Photograph by Prof. José Lino-Neto. Picture courtesy of Universiteit van Amsterdam


Previously my arguments about co-evolution were restricted to the evolution of obligate mutualism (i.e., total co-dependence of two different kinds of organisms) because I thought that only co-evolution of obligate mutualism could require that a mutation in one kind of organism be immediately answered by a corresponding mutation in another kind of organism at the same geographical location in order to produce a benefit or even just for survival. I didn't see parasitism per se as a problem for evolution because I assumed that mutations involving parasitism do not require an immediate corresponding mutation in the other organism. However, I have just discovered literature about bizarre parasitisms that may require changes in the traits of the host and so also may be a problem for evolution.

The abstract of a scientific article titled "Parasitoid Increases Survival of Its Pupae by Inducing Hosts to Fight Predators" says,

Many true parasites and parasitoids modify the behaviour of their host, and these changes are thought to be to the benefit of the parasites. However, field tests of this hypothesis are scarce, and it is often unclear whether the host or the parasite profits from the behavioural changes, or even if parasitism is a cause or consequence of the behaviour. We show that braconid parasitoids (Glyptapanteles sp.) induce their caterpillar host (Thyrinteina leucocerae) to behave as a bodyguard of the parasitoid pupae. After parasitoid larvae exit from the host to pupate, the host stops feeding, remains close to the pupae, knocks off predators with violent head-swings, and dies before reaching adulthood. Unparasitized caterpillars do not show these behaviours. In the field, the presence of bodyguard hosts resulted in a two-fold reduction in mortality of parasitoid pupae. Hence, the behaviour appears to be parasitoid-induced and confers benefits exclusively to the parasitoid.

The Merriam-Webster online dictionary defines "parasitoid" as "an insect and especially a wasp that completes its larval development within the body of another insect eventually killing it and is free-living as an adult."

Here are some excerpts from the article (the numbers are reference numbers) --
Diseases, parasites and parasitoids can induce spectacular changes in the behaviour of their host [1]–[11]. Some of these changes, such as behavioural fevering [12] and exposure to cold temperatures [13], are thought to benefit the host, but others have been suggested to result in increased transmission of parasites [1], [3], [4], [14]–[17] or increased survival of parasitoids [18]–[22]. One of the most famous examples is the parasitic trematode Dicrocoelium dendriticum, which induces its intermediate host, ants, to move up onto blades of grass during the night and early morning, and firmly attach themselves to the substrate with their mandibles [3]. This is believed to enhance parasite transmission due to increased ingestion of infected ants by grazing sheep, the final host [23]. In contrast, uninfected ants return to their nests during the night and the cooler parts of the day. Other examples of such spectacular behavioural changes include parasitoid larvae (Hymenoepimecis sp.) that induce their spider host (Plesiometa argyra) to construct a special cocoon web in which the larvae pupate [7], rodents infected by Toxoplasma that lose their innate aversion to odours of cats, the parasite's final host [9], and hairworms that 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. [6], [8]

Regarding the caterpillar and wasp,

Parasitoid larvae are known to interfere with host endocrine functions, causing the host to stop feeding before parasitoid larvae egress [10], [28], [31]–[35]. Levels of juvenile hormone, ecdysteroids and neurotransmitters (e.g. octopamine) have been found to increase shortly before parasitoid egression [33]–[35]. However, it is not clear whether parasitoid larvae produce these substances in sufficient quantity to change host behaviour [10], [34]. Moreover, the most important behavioural changes in the present study occur only after the parasitoids have egressed. The egression usually takes about 1 hour, and the caterpillars do not respond strongly to disturbance during egression, but only 1–2 hours after the event. This casts doubt on the role of the parasitoid larvae in the behavioural changes. However, when we dissected caterpillars from which parasitoids had egressed 3–4 days before, we found 1–2 active parasitoid larvae that had remained behind in the host, as has been found in another system [36]. We hypothesise that these parasitoid larvae are responsible for the changes in host behaviour. A similar mechanism has been described for the trematode D. dendriticum [37] and the liver fluke Brachylecithum mosquensis [23], which both use ants as an intermediate host. One or two of the parasites migrate to the ant's brain, where they encyst and are believed to affect the ant's behaviour. These so-called brainworms are not transmitted, and appear to be sacrificed to enable transmission of their kin [38]. If the parasitoid larvae of the system described here also stay behind to manipulate the host and do not pupate later, this would represent a cost of host manipulation: some offspring are sacrificed for higher survival of their kin [39]. This hypothesis needs further investigation.


The paper is discussed in ScienceDaily.

Another bizarre example of parasitism is described on the blog of science writer Carl Zimmer. To him, it's all just a simple matter of evolution:

Scientists don't yet understand how Ampulex manages either of these feats. Part of the reason for their ignorance is the fact that scientists have much left to learn about nervous systems and metabolism. But millions of years of natural selection has allowed Ampulex to reverse engineer its host. We would do well to follow its lead, and gain the wisdom of parasites.



Blogger Larry Fafarman said...

Question for Carl Zimmer (see his comment at the bottom of the post) --

How can mindless random mutation "reverse engineer" anything, particularly something that cannot even be reverse engineered by humans?

Sunday, June 15, 2008 1:14:00 PM  
Blogger Jim Sherwood said...

This example really is amazing, alright. Fred Hoyle mentions the Dicrocelium parasite, which is discussed here also, in The Intelligent Universe (1984, p.40.)He says it's "a remarkable process difficult to explain by the haphazard modifications of evolutionary trial-and-error."

Darwin had a theological argument for his theory, which was essentially "Why would God design these damn parasites?" But theology isn't science: and that problem doesn't arise in ID anyway, since the designer or designers may not be God.

Thursday, June 19, 2008 1:50:00 PM  
Blogger Jim Sherwood said...

Christians prefer to suppose that the "designer" is God, so they would perhaps have to explain the existence of the parasites. But they are used to dealing with the problem of why evil exists, anyway. And one point that Darwinists always miss is that according to ID, all features of a critter don't have to be designed: some may be due to natural processe, such as RM+NS. So Christians, I imagine, might suppose that some natural process misled the parasites into bad habits.

Darwinists just assume that given enough time, RM+NS can do anything!
Similarly some of the ancient Epicurean materialists supposed that all of the critters had popped up suddenly when particles just chanced to come together in the right manner. Given that the universe is infinitely old, that could happen! I think that these given-enough-time arguments are pretty poor ones.

Thursday, June 19, 2008 2:12:00 PM  
Blogger Larry Fafarman said...

Jim Sherwood said...
>>>>> This example really is amazing, alright. Fred Hoyle mentions the Dicrocelium parasite, which is discussed here also, in The Intelligent Universe (1984, p.40.)He says it's "a remarkable process difficult to explain by the haphazard modifications of evolutionary trial-and-error." <<<<<

The funny thing is that science writer Carl Zimmer wrote a book -- "Parasite Rex" -- about these strange parasitic relationships, and then later wrote a book titled, "Evolution: the Triumph of an Idea." He doesn't seem to wonder about how these strange parasitic relationships evolved.

Thursday, June 19, 2008 9:48:00 PM  
Blogger Jim Sherwood said...

Zimmer is nothing but a science writer, one of those guys who thinks that he is educating the "common people" by popularizing whatever arbitrary evolutionary doctrine seems to him to be most orthodox. So he's a fool, IMHO.

Friday, June 20, 2008 1:24:00 PM  
Blogger Scythemantis said...

You don't seem to understand even the basic definition of evolution if you honestly believe the "complexity" of a natural behavior can "challenge" it.

Evolution is not a doctrine, it's an observable process that explains speciation. It does not contradict the concept of a supernatural creator; Darwin himself believed in God for a full 13 years after publishing the Origin of Species, only losing faith (and becoming agnostic, not atheist) after the torturous deaths of his loved ones, not because of science.

Saturday, November 08, 2008 3:18:00 AM  
Blogger Scythemantis said...

To Larry Fafarman -

There is positively nothing "random" about evolution, with or without an intelligent designer behind it. No scientist has ever claimed that the process happens randomly, and by using the word "random" at all, your questions become moot because you clearly do not know what the word evolution actually means.

It is a predictable, repetitive process of adaptation brought about by environmental factors.

Saturday, November 08, 2008 3:21:00 AM  

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