Tardigrades Didn't Steal A Sixth Of Their DNA From Other Organisms After All

Hey there, little fella! Katexic Publications/Flickr; CC BY 2.0

Tardigrades are near-invincible, microscopic lifeforms found from the heights of the Himalayas to the depths of the world’s oceans. Researchers also suspect that they might be prolific kleptomaniacs, stealing DNA from other organisms; a study released last year said that their DNA was one-sixth foreign.

While it is not actually unusual for DNA in a creature to have been partly imported from a foreign source – at least 8 percent of human DNA is actually comprised of ancient viruses, after all – one-sixth is a heck of a lot. A new study published in the journal Proceedings of the National Academy of Sciences has cast some doubt on this, claiming that nearly all of this “foreign DNA” is actually just bacterial contamination, with just 1 percent being stolen.

Sometimes known as water bears or moss piglets, these wriggly fellows are as adorable as they are remarkable. They can be sent into the cold vacuum of space, squashed under 600 times the normal atmospheric pressure, or left frozen in a glacier for decades, and they’ll be absolutely fine as long as they keep hydrated. Even if you actually dry them out, they become glass in order to enter a state of “suspended animation.”

Although their evolutionary history is uncertain, scientists estimate that they’ve been part of a larger group of critters around for at least 520 million years. This means that these microscopic beasts have possibly survived at least five mass extinctions events, including the one that killed off at least 90 percent of all life on Earth 252 million years ago. What exactly gives them the ability to be so resilient in the face of extreme environmental and ecological change?

It looks like they aren’t as much of a thief as scientists previously thought: Their genetic sequence, top left, shows very little foreign DNA. Aziz Aboobaker & Mark Blaxter

Some have looked to a study released last December, which claimed that these little creatures are able to steal the genes of other organisms. One tardigrade species, Hypsibius dujardini, was revealed to have foreign DNA incorporated into its genome – roughly one-sixth. These 6,000 extra genes were acquired by a process known as “horizontal gene transfer,” where DNA is given to, or stolen by, other organisms without involving reproduction.

In the case of these tardigrades, when they're dried out, their cell membranes are often broken and leaky. This allows DNA from the environment to sneak inside, allowing them to make it part of their DNA when they rehydrate and repair themselves. It’s possible that it’s this successful DNA acquisition that’s allowed them to become so adaptable to so many environments.

This new study, led by the University of Edinburgh, looked at the genome of the very same species of tardigrade, and has come to a radically different conclusion: It’s almost all bacterial contamination, and the previous study failed to eliminate these contaminants from their genome sequence data. Their independent analysis suggests just one percent of this tardigrade’s DNA is foreign.

“We hope this paper will finally correct the scientific record,” Professor Mark Blaxter, an evolutionary biologist at the University of Edinburgh and coordinator of the study, said in a statement. “Tardigrades are amazing organisms, but these suggestions about their DNA were a step too far, even for their eight legs.”

What is still true, though, is that these animals are some of the most adaptable, hardy creatures known to science. If it’s not foreign DNA that’s giving them their near-invincibility, then it must be their own.

Main image credit: Katexic Publications/Flickr; CC BY 2.0

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