Ithaca, NY, USA. Some very small invertebrates named bdelloid rotifers haven't had sex in 30 million years. They should be extinct by now but a recent finding explains their evolutionary longevity.

The rotifers are microscopic escape artists. Faced with parasites and pathogens, they dry up and blow away with the wind. The bdelloids come back to life when re-exposed to fresh water.

Paul Sherman, professor of neurobiology and behavior at Cornell, co-authored the paper in Science with lead author and doctoral candidate Chris Wilson.

Bdelloid rotifers (DELL — oyd ROW-tiff-ers) are tiny, freshwater invertebrates that have long puzzled scientists because, as completely asexual animals, they should have been extinguished by parasites and pathogens long ago in evolutionary time. Instead, the bdelloids have proliferated into more than 450 species. Asexual animals like rotifers reproduce by cloning and this makes for a fixed gene pool.

The Red Queen

Habrotrocha elusa. A spore-bearing and deadly fungal parasite (Rotiferophthora angustispora) emerges from the digested corpse of a bdelloid rotifer (Habrotrocha elusa).

Click Pic for Details

In biology, The Red Queen hypothesis posits coevolving parasites and pathogens that rapidly extinguish asexuality. If true, alternative mechanisms that deal with biotic enemies must exist that grant long-lasting asexual lineage.

The discovery that bdelloids desiccate and wisp away with the wind helps explain their ancient asexuality and success. "It also helps answer one of the deepest puzzles in evolutionary biology — why sex is nearly ubiquitous," said Wilson.

"These animals play an evolutionary game of hide and seek," said Sherman. "They can drift on the wind to colonize parasite-free habitat patches where they reproduce rapidly and depart again before their enemies catch up. This effectively enables them to evade biotic enemies without sex, using mechanisms that no other known animals can duplicate."

To study the bdelloids' adaptations, Wilson infected populations of rotifers with deadly fungi and found that they all died within a few weeks.

He then tried drying out other infected populations for varying lengths of time before rehydrating them. He found that the fungi were far more sensitive to dehydration than the rotifers. The longer the infected populations remained dried out, the more successful they were at completely ridding themselves of fungi and eluding death.

In a second wave of experiments, Wilson placed dried, fungus-infected rotifers in a wind chamber. The scientists observed that the rotifers were able to disperse without the fungi and establish parasite-free populations. After just seven days of blowing around, there were as many fungus-free rotifer populations as there were after three weeks of dehydration without wind. So, by drying and drifting passively on the wind — sometimes for hundreds of miles — bdelloids can continually establish new, uninfected populations.

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