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A bdelloid rotifer (Habrotrocha elusa) that has been infected by a lethal fungal parasite (Rotiferophthora angustispora). Spore-bearing hyphae can be seen emerging from the digested corpse.
A bdelloid rotifer is infected by a fungal parasite.

Image courtesy Kent Loeffler, Kathie T. Hodge and C.G. Wilson

Ker Than

For National Geographic News

Published February 1, 2010

Scientists have finally solved the mystery of how one tiny creature has flourished for up to 50 million years without sex: it dries up.

Bdelloid rotifers, microscopic asexual freshwater invertebrates (animals without spines) are the ultimate escape artists, able to outpace their enemies by drying up and blowing away.

For most animals, sex is not only a way of producing the next generation, but a means of keeping enemies at bay.

"If an organism stops having sex and crystallizes its genome, all of [its enemies] catch up with it evolutionarily and can quickly overwhelm it," explained Paul Sherman, a neurobiologist at Cornell University in New York.

This idea, known as the Red Queen Hypothesis, helps explain why most animals go to great lengths to find mates and have sex.

But rotifers, the only confirmed "ancient asexuals" in the animal kingdom, have survived by abstaining from sex for the last 30 to 50 million years. In that time the rotifer has proliferated into more than 450 species found around the globe.

In contrast, other creatures that reproduce without sex--such as the nematode worm--are expected to die out after several hundred thousand years.

When faced with the threat of parasitic fungi, the rotifers dry up and allow themselves to be blown away by the wind. They come back to life when exposed to freshwater. Scientists estimate that nearly 100 rotifers can fit into a single drop of water.

So while most animals are locked in evolutionary arms races with their foes, bdelloid rotifers escape them altogether simply by being carried on the wind.

Evolutionary Hide-and-Seek

To figure out the bdelloids' trick, Sherman and his colleague Chris Wilson, also at Cornell, infected populations of rotifers in freshwater with deadly fungi and found they all died within a few weeks.

The team then dried out other infected populations for various lengths of time before re-hydrating them. They found that the rotifers could live longer without water than their fungi enemies. The longer the infected populations remained dried out, the more likely they were to survive.

In a second experiment, the scientists placed the desiccated, fungus-exposed rotifers in a wind chamber. They observed that the rotifers were able to blow away and leave the fungi behind.

The scientists think that by drying out and drifting--sometimes for thousands of miles--the rotifers can continually establish new, uninfected populations.

"The bdelloids are playing this never-ending game of hide-and-seek," said Sherman, whose research appeared recently in the journal Science.

More Mysteries

Tim Barraclough, an evolutionary biologist at Imperial College London in the U.K. who was not involved in the study, said the research was "really exciting."

"Bdelloids have been under the spotlight since [biologist] John Maynard Smith dubbed them an evolutionary scandal--how have they managed to survive so long, and to be so successful, in the absence of sex?"

While the new study suggests the bdelloids' ability to take to the wind in the face of danger is one key aspect of their survival, it's likely not the only one, Barraclough said.

"Other factors--such as the ability to take up and use DNA from their environment--may also play a role," he said.

But other mysteries remain. "Although many bdelloids live in desiccation-prone habitats, such as ephemeral ponds, there are substantial numbers found in permanently aquatic habitats that are unable to desiccate. How can they avoid accumulating parasites?"

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