The microscopic aquatic creatures known as bdelloid rotifers are used to enduring dry spells—in more senses than one. In their common habitats of moss, soil, and seasonal pools, these minuscule, transparent animals routinely survive periods of complete dessication that can last from days to years. They also hold the record for celibacy among animals: All 460 known species of bdelloids consist exclusively of egg-laying females that have essentially been cloning themselves for 100 million years. Their endurance has long posed a kind of scientific mystery, as the majority of asexually reproducing species tend to fade away over time. But a genetic study published in May in Science [subscription required] hints that bdelloids emerging from a drought might have a kind of bizarre sex after all.
For most life-forms, going for long periods without water spells certain doom. But dehydrated bdelloids somehow reconstitute themselves when moisture returns, even though their metabolic activity stops, their cell membranes rupture, and their DNA probably gets fragmented too. “You add water, they fix themselves up, and they swim away,” says lead investigator Matthew Meselson of Harvard University.
Meselson’s study suggests that upon patching up their own DNA, the bdelloids simultaneously incorporate random scraps of DNA from other organisms. This so-called horizontal gene transfer is extremely rare among animals, and in the bdelloids’ case can include DNA from almost anything that was in their soupy habitat at the time things dried up, including whatever they just ate. In only 1 percent of the bdelloid genome, Meselson found dozens of foreign genes from bacteria, plants, and fungi inserted among the native nucleotides. It’s likely, he says, that during recovery from dessication, bdelloids pick up genes from members of their own species, too—dead members, that is, whose genes spill out of ruptured cell membranes. That process would provide the kind of genetic reshuffling that other animals achieve through sexual reproduction.
“It may be their form of sex,” Meselson says. “But their partner is essentially dead. So you’d have to call it necrophilia. Actually, since they’re all females, lesbian necrophilia.”