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….

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