Puppet Masters

Reminiscent of Robert Heinlein's classic novel, scientists have discovered a parasite that has succeeded in immortalising itself within the genome of its host.

"It didn't seem possible at first," says Werren, professor of biology at the University of Rochester and a world-leading authority on the parasite, called Wolbachia. "This parasite has implanted itself inside the cells of 70 percent of the world's invertebrates, coevolving with them. And now, we've found at least one species where the parasite's entire or nearly entire genome has been absorbed and integrated into the host's. The host's genes actually hold the coding information for a completely separate species."

Wolbachia may be the most prolific parasite in the world—a "pandemic," as Werren calls it. The bacterium invades a member of a species, most often an insect, and eventually makes its way into the host's eggs or sperm. Once there, the Wolbachia is ensured passage to the next generation of its host, and any genetic exchanges between it and the host also are much more likely to be passed on.

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It will be fascinating to learn how many hitchhikers the human genome has picked up along the way. How many riders are sleeping in the free seats, waiting for a chance to wake and run amok? How many of our diseases are merely the restless squirming of a misfit stowaway, how many of our moods are stirred by sleepwalking free riders?