Update: Another recent study looking at the genome of Hypsibius dujardini did not find evidence for a massive transfer of foreign DNA.
When humans are resilient, we say they’re made of sterner stuff.
But they've got nothing on tardigrades, tiny water-dwelling invertebrates that are considered the most robust animals on Earth.
Not only can the tough little guys—also known by the nicknames water bear and moss piglet—withstand freezing temperatures, long periods of drought, and high doses of radiation, they're the only animals known to have survived the vacuum of space.
That made us here at Saturday’s Weird Animal Question of the Week wonder—just what are these microscopic diehards made of?
Like everything about the tardigrade, the answer is startling.
Little Bit of Everything
Tardigrades have a huge amount—17.5 percent—of foreign DNA, a new study says.
A team sequenced the genome of a tardigrade species Hypsibius dujardini "to try to understand how it is that some animals can survive some amazingly extreme conditions,” co-author Bob Goldstein, a biologist at the University of North Carolina at Chapel Hill, says via email.
They found the H. dujardini genome is material from several entirely different kingdoms—mostly bacterial (16 percent) but also fungal (0.7 percent), plant (0.5 percent), archaeal (0.1 percent), and viral (0.1 percent).
“We never expected to find that an animal genome would be quite so littered with foreign genes," said Goldstein, whose study was published November 23 in the journal Proceedings of the National Academy of Sciences.
The tardigrade acquires foreign DNA via a process called horizontal gene transfer, in which genetic material is transferred directly between organisms instead of being passed down from parent to offspring.
The discovery of other DNA in the tardigrade is "really peculiar," says co-author Thomas Boothby, also a biologist at UNC.
“Many animals appear to have a small degree of horizontal gene transfer, including humans,” Boothby says. “But nowhere close to the proportion [about one-sixth of genes) that we see in the tardigrade genome.”
“What this tells us is that instead of thinking about a tree of life, we can start to think more about a web of life, where genetic material from one branch, say the bacterial branch, can cross over to the animal branch.”
Pass It On
So how exactly does the tardigrade get foreign DNA? It may have to do with one of the tardigrade's survival skills—the ability to completely dry out in drought conditions, then rehydrate. (See "Five Extreme Life-Forms That Live on the Edge.")
As a cell dries out, its DNA breaks into fragments, and the cell membrane “becomes temporarily leaky as it rehydrates," Boothby says.
This allows big molecules like proteins and fragments of foreign DNA to pass in and out of the cell, after which the fragmented DNA repairs itself.
“Ultimately we think as the tardigrade is repairing its own broken genome, it may stitch in some foreign fragments as well.”
Another factor that allows for the persistence of foreign DNA is that some tardigrades reproduce asexually, like the ones in the study.
The study animals are “all daughters of a single female tardigrade collected over 20 years ago in a pond in England,” and “essentially clone themselves every time they reproduce.” Boothby says. (See "5 Reasons Why The Tardigrade Is Nature’s Toughest Animal.")
Self-cloning stabilizes the foreign genes, since each animal has two copies of them and no contribution from a second parent that lacks the genes.
“We think that some of tardigrades’ ability to survive extremes might have come from improved foreign genes,” Boothby adds.
For instance, some of the tardigrades' acquired genes have roles in stress tolerance, which may have boosted their drought survival skills, he says. (See "5 Animals That Can Take the Extreme Heat—and Cold.")
Nothing like imports to amp up an already fine collection.
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