No Sex For 40 Million Years? No Problem

Science Daily — A group of organisms that has never had sex in over 40 million years of existence has nevertheless managed to evolve into distinct species, says new research published today. The study challenges the assumption that sex is necessary for organisms to diversify and provides scientists with new insight into why species evolve in the first place.

Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws. Have these asexual animals really diversified into evolutionary species? (Credit: Diego Fontaneto / Courtesy of PLoS Biology)

The research, published in PLoS Biology, focuses on the study of bdelloid rotifers, microscopic aquatic animals that live in watery or occasionally wet habitats including ponds, rivers, soils, and on mosses and lichens. These tiny asexual creatures multiply by producing eggs that are genetic clones of the mother -- there are no males. Fossil records and molecular data show that bdelloid rotifers have been around for over 40 million years without sexually reproducing, and yet this new study has shown that they have evolved into distinct species.

Using a combination of DNA sequencing and jaw measurements taken using a scanning electron microscope, the research team examined bdelloid rotifers living in different aquatic environments across the UK, Italy and other parts of the world. They found genetic and jaw-shape evidence that the rotifers had evolved into distinct species by adapting to differences in their environment.

Dr Tim Barraclough from Imperial College London's Division of Biology explained: "We found evidence that different populations of these creatures have diverged into distinct species, not just because they become isolated in different places, but because of the differing selection pressures in different environments.

"One remarkable example is of two species living in close proximity on the body of another animal, a water louse. One lives around its legs, the other on its chest, yet they have diverged in body size and jaw shape to occupy these distinct ecological niches. Our results show that, over millions of years, natural selection has caused divergence into distinct entities equivalent to the species found in sexual organisms."

Previously, many scientists had thought that sexual reproduction was necessary for speciation because of the importance of interbreeding in explaining speciation in sexual organisms. Asexual creatures like the bdelloid rotifers were known not to be all identical, but it had been argued that the differences might arise solely through the chance build-up of random mutations that occur in the 'cloning' process when a new rotifer is born. The new study proves that these differences are not random and are the result of so-called 'divergent selection', a process well known to cause the origin of species in sexual organisms.

Dr Barraclough adds: "These really are amazing creatures, whose very existence calls into question scientific understanding, because it is generally thought that asexual creatures die out quickly, but these have been around for millions of years.

"Our proof that natural selection has driven their divergence into distinct species is another example of these miniscule creatures surprising scientists -- and their ability to survive and adapt to change certainly raises interesting questions about our understanding of evolutionary processes."

Note: This story has been adapted from a news release issued by Imperial College London.

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Uni.Science Tags: Sex, Time, DNA, Biology

Sea Snakes Conquered by Salt

by Elizabeth Pennisi

PHOENIX, ARIZONA--Shipwrecked sailors shouldn't drink ocean water no matter how thirsty they get. And neither should sea snakes. Contrary to the current dogma, at least some of these serpentine mariners must have freshwater to survive. Research shows that without it, at least one group of sea snakes--and likely others--will gradually waste away, researchers reported here yesterday at the annual meeting of the Society for Integrative and Comparative Biology. The need for access to fresh water may limit where these snakes can live, explaining their patchy distribution along certain coastlines.

All organisms must work to keep dehydration in check. Kidneys concentrate urine to conserve water, and many marine animals have special adaptations for getting rid of the excess salt taken in from the surrounding environment. Sea snakes--dozens of species of which live in the open ocean, while a few others hang out inshore--have a gland under their tongues for this purpose. Researchers have long assumed that this gland worked so well that the snakes could get away with sipping salt water whenever they needed a drink.

But Harvey Lillywhite, an ecological physiologist at the University of Florida in Gainesville, began to suspect otherwise when he had trouble keeping file snakes, which live almost fulltime in the ocean, alive in his lab. He discovered the snakes did fine once he put them in fresh water and began to wonder if the same was true of other marine snakes.

With the help of Ming Tu from the National Taiwan Normal University in Taipei, Lillywhite and his colleagues collected three species of sea kraits, snakes that live in the coastal waters of islands off Taiwan but, at the very least, come ashore to lay their eggs, usually in rocky caves close to the intertidal zone. Two of the species also visit land occasionally. All have the brine-secreting gland, suggesting they are well adapted to constant immersion in salt water.

For their experiments, the researchers first took the snakes out of water long enough to allow them to dehydrate. They then put the snakes in different concentrations of seawater. None of the dehydrated snakes tried to drink anything that was 50% or more salt water (They live in full-strength seawater.) But they did gulp down water fresh water and imbibed 25% saltwater concentrations, Lillywhite reported.

In a second study, Lillywhite's team tracked the weight of the snakes for 10 days. For the experiment, they kept the snakes in the seawater without food. The researchers placed half of the snakes in fresh water every other day for an hour. All the snakes experienced dehydration and lost weight, but the ones exposed to fresh water lost significantly less, says Lillywhite.

The results help explain the demographics of these Taiwanese snakes, Lillywhite says. They tend to be most plentiful along the shore, where there are springs or other sources of fresh water nearby. Furthermore, there are more sea snake species in areas with higher mean annual rainfalls, notes Lillywhite. Under calm conditions, thin layers of rain will float on top of the salt water, apparently providing ample supplies for the snakes.

It's a "major finding," says Harold Heatwole, an ecologist at North Carolina State University in Raleigh. Physiologist Lisa Hazard from Montclair State University in Upper Montclair, New Jersey, agrees. "He shows pretty clearly that [sea snakes] have to have access to fresh water," she says.

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