Microscopic Water Bears Incorporate Foreign DNA into Genome

From the peaks of the Himalayas and the ocean’s deepest depths to frigid Antarctica and the searing deserts, tardigrades are animals that thrive in extremes.

Dry them out, and tardigrades can survive for years, even decades. Add water, and they spring back to life, raring to reproduce, feed and live their normal lives. Radiation? Not a problem, these microscopic animals can survive doses thousands of times more intense than humans can. The vacuum of space? Yeah, they survived that too.

“These abilities to survive these extreme stresses is what really got me interested in studying tardigrades,” says Thomas Boothby, a postdoctoral fellow at the Univ. of North Carolina at Chapel Hill, in an interview with R&D Magazine.  

Boothby and Bob Goldstein, a faculty member of UNC’s College of Arts and Sciences, recently sequenced the genome of these microscopic cosmopolitans. Surprisingly, the discovered 17.5% of their genome, around 6,000 genes, are from foreign DNA.



Most animals, according to UNC, get less than 1% of their genome from foreign DNA. The previous record-holder was the rotifer, another microscopic animal. However, the amount of foreign DNA in its genome is about half that of the tardigrade’s. “Just like tardigrades, rotifers can survive all these extreme environmental conditions, including desiccation,” says Boothby.  

“What we speculate is that the ability of the tardigrades to survive drying out might be linked to their ability to take up all this foreign DNA,” Boothby says. “This catalog of genes (will) allow us to start to tease apart how it is that these animals survive these extreme conditions.”

Also known as water bears or moss piglets, tardigrades were first described in 1773 by German pastor J.A.E. Goeze. They’re diminutive, only measuring between 0.05 to 1.2 mm, and have four pairs of legs, which end in claws. Over 900 separate species have been discovered, and they can feed on plant cells, animal cells or bacteria.

Boothby and Goldstein believe the desiccation process may provide a gateway for foreign DNA into the tardigrade’s genome. “There are three things that happen to a cells when it dries out that makes us think that,” says Boothby.

When a cell dries out, its DNA fragments and breaks apart. But upon rehydration, the plasma membrane surrounding the cells becomes temporarily leaky, giving macromolecules, such as DNA, the chance to pass back and forth between the environment and the animal’s cells. “As the tardigrades are stitching their own genomes back together, they may accidently incorporate some of this foreign environmental DNA,” says Boothby.

The process is known as horizontal gene transfer, a sort of an antithesis to the usual process of inheriting DNA from parents.

“We’re doing some follow-up experiments to try to test that hypothesis,” Boothby says. Additionally, he wants to test and see if the foreign genes are responsible for the tardigrade’s ability to survive extreme conditions.