For scientists wondering how bdelloid rotifers escape malicious parasites, the answer is blowing in the wind.
The tiny freshwater invertebrates, which live in ponds, streams and moss the world over, are notorious among biologists for two unique traits. They can survive for up to nine years as dry, desiccated grains, and they have never been caught having sex. The species has only females, as far as scientists can tell, and one generation creates the next by cloning.
New work appearing in the Jan. 29 Science shows how these traits might be related. Drying up and blowing away helps rotifers avoid parasites. And that may explain how the species can survive without sex, a costly but necessary evolutionary strategy for most organisms.
“We showed that this very unusual, perhaps unique lifestyle trick … may be allowing them to play hide-and-seek with their coevolving parasites,” says study coauthor Christopher Wilson of Cornell University.
Evolutionary biologists consider sex a puzzling nuisance in some ways. Combining genomes with a relative stranger to reproduce is inefficient, possibly dangerous and dilutes the original genome, Wilson says.
“It would make much more sense evolutionarily not to bother with males or sex,” Wilson says. “And yet 99 percent of things do.”
One hypothesis for why most species put up with sex is that the constant genetic reshuffling it provides protects from diseases and parasites. If every individual in a population is genetically identical, then one parasite can wipe them all out.
If true, organisms that don’t reproduce sexually ought not to last very long, and mostly they haven’t. But somehow bdelloid rotifers have persisted asexually for millions of years.
“We were really interested in how these rotifers … have been able to capitalize on all these advantages of asex, without paying the expected cost in terms of diseases,” Wilson says.
To find out, Wilson and his colleague Paul Sherman, also of Cornell, exposed six groups of Habrotrocha elusa rotifers to their nemesis, Rotiferophthora angustispora, a parasitic fungus whose Latin name means “rotifer destroyer.” The researchers kept one group wet as a control, and let the other five groups dry out for different lengths of time before rewetting them.
The fungus killed off the entire hydrated population within two weeks. The groups that spent one to two weeks drying out didn’t fare much better after they were returned to a moist environment. But after three weeks of drying, 60 percent of one group’s population was fungus-free after rewetting. The effect was even greater for the rotifers that spent even longer in the dried-out state: In the four-week group, 85 percent of rotifers survived while 90.5 percent survived in the five-week group.
“The parasite was unable to survive for quite so long as the rotifers,” Wilson says.
To determine whether rotifers could run away from the parasite, Wilson and Sherman put an infected group of rotifers in a wind chamber set to simulate a light breeze. The rotifers dried out, blew away and landed on petri dishes set 30 to 40 centimeters distant.
After one week in the breezy chamber and then rewetting, 58.5 percent of the rotifers had shed the fungus in flight and survived.
“I was really quite surprised,” Wilson says. “One would think blowing wind at [the fungi] would blow spores everywhere, like a dandelion. But they really need water to be able to spread.”
Rotifers in the wild would probably endure even longer dry spells and be blown farther, Wilson notes.
“This could be the foundation of really quite a satisfying solution” to the problem of how the asexual creatures avoid destruction by parasites, Wilson says.
But, Wilson adds, it’s going to take more than one piece of evidence to decide between this and other possible explanations.
The experimental setup used is “nice, simple and clear,” comments Matthew Meselson of Harvard University. “They’re like the old kind of biology that everybody can understand.”
But the rotifer’s escape artistry doesn’t close the book on sex, Meselson says. It’s still possible that rotifers reproduce sexually on very rare occasions, and scientists just haven’t seen them, he points out.
“It may have added to their ability to do without sex, or maybe they do have sex and it added to their ability to do without it for long periods of time,” he says. “We just don’t know if they’re asexual, and we just don’t know why sex really exists.”
