Bdelloids Surviving on Borrowed DNA

15 November 2012 5:40 pm

Chiara Boschetti

She-creature. Water-going microorganisms called bdelloid rotifers are all female.

In Mother Nature's edition of the TV reality show Survivor, the bdelloid rotifers would probably be the last animals standing. These tiny aquatic creatures can survive high blasts of radiation and years of desiccation—and they've persisted for tens of millions of years without sex.

Now, a study published online today in PLOS Genetics hints at how the bdelloids do it. A new genetic analysis shows that roughly 10% of the bdelloids' active genes were pilfered from other species, such as fungi, bacteria, and plants. These foreign genes have endowed bdelloids with talents that no other animal can boast, which could help explain their ability to shrug off extreme conditions of aridity. Ultimately, the bdelloids' appropriation of foreign genes may hold the key to their success despite celibacy, which usually results in a species's extinction.

The bdelloid is a "genetic mosaic. It takes pieces of DNA from all over the place," says study author Alan Tunnacliffe, a molecular biologist at the University of Cambridge in the United Kingdom. "Its biochemistry is a mosaic in the same way. It's a real mishmash of activities."

For creatures of such superherolike ability, microscopic bdelloids—which are distantly related to flatworms—are happy in humble surroundings. The roughly 400 species of bdelloids live in fresh and brackish water, including puddles, sewage-treatment tanks, and drops of moisture adhering to soil. They have a handy ability to survive the sudden disappearance of their aquatic homes; the desiccation-survival record is 9 years.

What's even stranger, from an evolutionary biologist's point of view, is the bdelloid's long-term asexuality. For perhaps 80 million years, all bdelloids have been shes, contentedly reproducing without males -- and defying biologists' ideas about the centrality of sex. Sexual reproduction, the thinking goes, introduces genetic variation and so allows a species to adapt to a changing environment and to genetic degradation. It's commonly thought that animals that forgo sex eventually go extinct, but the bdelloid provides a glaring exception to the rule. Legendary biologist John Maynard Smith was so flummoxed by the bdelloids that he called them an "evolutionary scandal."

In 2008, a separate group of researchers found that bdelloids contain some foreign DNA in a small region of their genomes. Tunnacliffe and his colleagues decided to find out the extent of that foreign genetic material. So they turned to the bdelloid Adineta ricciae, which was discovered in a small Australian billabong, or lake. When the scientists sequenced the bdelloid DNA that provides the blueprints for active genes, they found that roughly 10% of that DNA had been borrowed from some other creature. All told, the bdelloid had adopted DNA from more than 500 different species.

By comparing the foreign sequences to genetic databases, the researchers learned that many of the sequences are responsible for directing the production of enzymes found in simple organisms but unknown in complex animals. Two genes, for example, give rise to bacterial enzymes that help break down the toxic chemical benzyl cyanide. Two more genes, these from parasitic protozoa, direct the manufacture of a compound that can ward off cellular damage. Nearly 40% of the animal's enzymatic activity includes a foreign component, Tunnacliffe says.

The proportion of the bdelloid's active genes that it got from other sources "is a really surprising big deal," says evolutionary biologist David Mark Welch of the Marine Biological Laboratory in Woods Hole, Massachusetts, who has studied bdelloids but was not involved in the present study. "If this is all true, they are bringing in DNA all the time."

The analysis of foreign gene products is especially interesting because it provides a clue to the bdelloid's endurance in the face of bad conditions, says biologist Bernard Angers of the University of Montreal in Canada who has studied atypical reproduction and genetic diversity.

Tunnacliffe says it's not clear how the bdelloids come by the foreign DNA, but it may supply them with enough fresh genetic material to overcome the disadvantages of asexuality. And the bdelloids' genetic hodgepodge makes an appealingly strange group of animals even stranger.

"We have a joke in the lab that every time you investigate these animals … they come out with something weird," Tunnacliffe says. "It's like they're here to keep us entertained and surprised."

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