Pity the poor bdelloid rotifers. In addition to having an unpronounceable name (it’s “dell-oid row-ti-fer”), these small, silky, worm-like animals have had a mean trick played on them: Somewhere along the line, evolution eliminated all rotifer males and, with them, all chance for rotifer sex.
Besides its obvious benefits, sex allows organisms to become more genetically varied. A lack of different sexes should have left almost all rotifers as clones, and highly susceptible to extinction. But in fact, at least four hundred species of rotifers exist—a state some have called “an evolutionary scandal.”
The reason, scientists now realize, is that rotifers have found a clever solution to the problem of not having sex—a solution that opened their eyes to a world of non-sexual delight that has nothing to do with Mendel
or meiosis.
It all traces back to another pitiable trait of rotifers—their tendency to get caught in droughts. “They can survive drying, and that underlines nearly everything that’s important about them,” says Matthew Meselson, a scientist who works on rotifers at both Harvard University and the Marine Biological Laboratory in Woods Hole, Massachusetts.
That is, rotifers can survive bouts of severe dehydration in a dormant state, during which they shrivel up and even their DNA cracks apart. When water returns to the tree or dried pond bed where bdelloid rotifers roost, their bodies have amazing abilities to recuperate and piece the cracked DNA back together.
The astounding part—the part that earned Meselson and two colleagues, Irina Arkhipova and Eugene Gladyshev, a recent paper in the journal Science when they discovered it—is that rotifers don’t just piece their own DNA back together. The DNA of everything inside them, including microbes in their gut and whatever they happened to have eaten, also cracks apart during the drying. And when piecing DNA back together, the overzealous janitor proteins inside rotifers sometimes also sweep that foreign DNA into rotifer chromosomes.
“It presents ... an organism with the opportunity to enrich its genetic blueprint with genes that are available in the environment and that maybe provide an advantage,” notes Arkhipova, who also splits her time between Harvard and the Marine Biological Laboratory. This allows rotifers to differentiate into new species and thrive in nearly every known environment.
This so-called lateral gene transfer is common in simple bacteria, but seeing it in something as complex as rotifers was startling. Yet it’s only one odd example of what scientists now realize are the many ways that animals and plants evolve without sexual reproduction or mutations. Genes, it turns out, are hardly static after birth and trade information in ways as much Lamarckian as Darwinian.
What’s more, there may be human applications. Ultraviolet rays kill cells primarily by damaging or scrambling DNA; but because of their ability to repair themselves, rotifers are more or less immune to UV light. As a result, Meselson thinks that understanding rotifers could help cells reverse the nasty effects of radiation on mammals.
For the next steps in their rotifer work, Meselson and Arkhipova will focus on a slightly different problem. They’ve shown that bdelloids can scoop up DNA from other species, but they haven’t determined if rotifers can incorporate and splice together DNA from other nearby rotifers, probably dead ones.
If that’s the case, then rotifers, far from being the “spinsters” of the animal world, may actually have the most bizarre sexual appetite of all. “It’s a form of sex,” says Meselson. “I guess you’d call it necrophilia, but it is a form of sex.”
Sam Kean is the associate editor of Science & Spirit.
