February 4, 2012 | David F. Coppedge

Rethinking Parasitism

Parasitism is bad.  Parasitism is evil.  Parasites wage war against innocent hosts.  This is our mindset.  What if parasites can do good?  This change of heart seems to be happening in one case, the case of transposable genetic elements.  If they are only doing harm to the host, why did some biologists find that “positive selection” seems to be maintaining them?  That makes it sound like the cells need them.

An article in Science Daily began with the warfare metaphor: “Many living organisms suffer from parasites, which use the hosts’ resources for their own purposes. The problem of parasitism occurs at all levels right down to the DNA scale.”  The article went on to describe the “intracellular battle that is constantly being played out between the host and invading DNA.”

While no one would deny the suffering parasites cause, a team of scientists at the University of Veterinary Medicine in Vienna was surprised to find a possible beneficial side of genetic parasites known as transposable elements.  These are pieces of DNA that “are capable of moving around within and between genomes, generally represent a drain on the host’s resources and in certain cases may lead directly to disease, e.g. when they insert themselves within an essential host gene.”

The scientists logged all the transposable elements (TE’s) they could find in a population of fruit flies.  “The findings were dramatic,” the article said.  The insertion points of TE’s varied widely between individuals, meaning that many of them were apparently not “fixed” or permanent in the population.  At first, this led Christian Schlötterer to maintain the warfare metaphor: “the genome is like a record of past wars between hosts and the parasitic DNA. There have been waves of attacks and the majority of them have been repelled, with only few transposable elements managing to survive and spread throughout the population.”

But then another surprise cast doubt on that metaphor, and supported a more cooperative picture:

Even more surprisingly, the scientists found about a dozen sites of insertion that were more frequent in the population than would be expected from their age (assessed via a different method). It seems, then, that there is positive selection for transposable elements at these sites, suggesting that insertion has a beneficial effect on the host. Such an effect had previously been shown for two insertions that give increased resistance against insecticides and these cases were refound by Schlötterer’s analysis.

The researchers do not know what benefit these TE’s confer such that they are being maintained by selection.  Schlötterer is considering a new, happier metaphor: “perhaps we shouldn’t really think of transposable elements as parasites at all. They represent a way for organisms to increase their genetic repertoire, which may be advantageous in helping them meet future challenges.

The suggestion is too premature to apply to other parasites, let alone all of them, but provides an occasion for thinking about metaphors in science.  The human mind naturally tends to classify phenomena in nature into moral categories: good kitty, bad dog; good dog, bad kitty.  We do that with parasites because we know how often some of them harm or kill us.  No one is going to suggest from this story that there is anything good about a guinea worm or malaria.  The suggestion of a benefit in TE’s may be entirely isolated to a few cases in fruit flies.  What’s instructive about the story is that the team started with the assumption that TE’s are bad.  They are at war with the host, Schlötterer  said, using the host’s resources for their own good.  The evidence they gathered suggested the opposite; they might be beneficial.

Biologists classify parasitism on one end of the scale of interactions among many things: sapping the resources of the host for the parasite’s own benefit.  Examples are mistletoe and tapeworms or parasitic roundworms.  At the other end are mutualistic interactions, where each organism benefits from the other: examples are the yucca moth and cleaner fish (1/13/2003).  Some of these interactions are so finely tuned, it is difficult to imagine how they could have evolved.  Perhaps our categories are too simplistic.  We often put viruses in the “bad” category, since they invade a host cell and use its machinery to make copies of themselves.  Why, then, do only a small fraction of viruses cause disease?  What are the others doing?  Many of them kill bacteria; that’s “good,” isn’t it?  Maybe we should take a look afresh.  In biology there are many pushes and pulls.  Pushes and pulls are not good or bad; they are opposite forces that maintain homeostasis.  For more along that line, see the 3/14/2003 commentary.

A possibility open to those of a Biblical perspective is that these interactions began as all beneficial but became destructive, either as direct judgment on sin, or as an indirect judgment through the relaxation of controls that would have kept them beneficial.  Like machines run amok, mutations might have planted bugs in the software that led to disease and suffering. The Genesis account of the curse suggests this: it was easy and pleasant in the garden, with the Tree of Life, but once the judgment came, life became toilsome and difficult, ending in death.

Such thoughts stray beyond what science can reveal.  The findings in this article, though, do lend themselves to new ways of thinking about parasites as designed mechanisms originally intended for health, a few of which have subsequently gone bad.  For readers who cannot accept that, the story is a lesson about not trusting metaphors as reliable guides to understanding the natural world.  See “Metaphors Bewitch You,” 7/04/2003.

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