January 31, 2010

Worms Don’t Need Sex; They Dry Up Instead

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The Bdelloid rotifer has survived 30 to 40 million years without ever having sex. Without genetic variation from sexual reproduction to defend against predators these small invertebrates should have already gone extinct, but Cornell researchers have discovered the rotifers’ trick to staying alive: a drying-up adaptation that protects them against parasites.

The experiments addressed an important question — why the bdelloid rotifers have survived for such a long time. The study called the Red Queen Hypothesis — the idea that a species must continually evolve to stay relative to the other species and systems around it — into consideration. The name is a reference to Through The Looking Glass by Lewis Carroll, where the Red Queen tells Alice that “it takes all the running you can do to stay in the same place,” said Chris Wilson, a Cornell doctoral candidate in the lab of Prof. Paul Sherman, neurobiology and behavior.

“These animals have evolved a way to avoid parasites and pathogens by drying up and blowing away,” Sherman said.

Rotifers typically live in mossy environments that fluctuate between two extremes — moist and submerged in water until the moss desiccates to become dry, crumbly and brown. Because of this unique living habitat, rotifers are accustomed to being in both dry and wet environments and thus they have developed a “physiological trick where they lose all the water in their bodies,” according to Wilson.

This adaptation can also defend the rotifers from detrimental pathogens. In the first experiment, Sherman and Wilson infected a number of populations of rotifers with parasites and recorded the percentage of surviving rotifers after one to five weeks of dehydration. They discovered that after three weeks, the number of surviving rotifers increased significantly. The longer the rotifers were dried out, the more effectively the parasites were eradicated from the population, indicating that the adaptation is successfully used as a defense mechanism against pathogens.

Rotifers also use this defensive behavior to facilitate travel by wind.

“[It is] the idea that if some organism could move around in space and time, always moving to new populations, [then] it’s effectively no longer closely coupled to its biological enemies,” Wilson said.

In a second experiment, Sherman and Wilson took samples of rotifers in dried up brown moss and simulated wind transport of the worms and certain pathogens in a contained setting. The rotifers managed to gather in small round holes dug in the bottom of the environment, without the presence parasites.

“After a week … we managed to get rid of the parasite from 60 percent of the populations,” Wilson said. While in the air, the rotifers are in a dry environment, which forces the parasites, organisms that thrive in liquid-based environments and do not react well when shaken and blown around, to abandon their attack. The rotifers are then also spread to different environments, where they can establish new populations. “These animals are essentially playing an evolutionary game of hide and seek,” said Sherman.

In this case, the bdelloid rotifers have disproved the Red Queen Hypothesis. Their ability to rid themselves of pathogens keeps them from extinction at the hands of parasites that could potentially wipe out entire populations of asexual organisms, lacking genetic variation.

“Under that view, that’s how they have lasted so long without sex because they’re escaping parasites through space and time instead of through a genetic race,” Wilson said. “They’re just getting away from the parasites,” Wilson said.

Joanna Dong ’13, a biology major and pre-med student at the University, noted the significance of the research outcome.

“These findings provide fresh insight into what scientists and teachers long held to be true,” Dong said. “Asexual reproduction should no longer be considered wholly inferior to sexual reproduction.”

In the future, Wilson and Sherman hope to replicate such experiments in nature, “to see whether this lab-based experiment to see extends to natural habitats,” Wilson said.

Original Author: Cindy Huynh