Why Are Tardigrades the World’s Hardiest Creature? DNA Offers Clues

They may be tiny, but don’t let their stubby stature fool you

Ramazzottius varieornatus, a species of tardigrade, photographed with scanning electron microscope Kazuharu Arakawa and Hiroki Higashiyama

Despite their tiny stature and their adorable nicknames—moss piglets, water bears—the tenacious tardigrade has some tremendous capabilities. Well-known for being one of the hardiest-known forms of life, tardigrates can survive desiccationdeadly radiation, and even the vacuum of space. Now researchers may finally be starting to tease out the genetic basis of tardigrade superpowers.

In 2015, a study published in the Proceedings of the National Academy of Sciences, suggested that some of their superpowers could come from an another oddball accomplishment of the microscopic creature: DNA theft. The researchers sequenced a tardigrade species' genome and found that roughly one-sixth of its DNA (around 6,600 genes) appeared to come from other organisms, mainly bacteria. These sections of DNA were thought to be picked up through the process of so-called horizontal gene transfers, which is a common in bacteria and other microbes (scientists have only recently discovered some animals can also do this).

“If they can acquire DNA from organisms already living in stressful environments, they may be able to pick up some of the same tricks,” researcher Thomas Boothby, a Life Sciences postdoctoral fellow at the University of North Carolina, Chapel Hill, told Smithsonian.com in 2015.

But just a week after it was published, the study faced steep opposition. Another group of tardigrade researchers claimed that much of the supposedly "stolen" DNA likely came from contamination of the samples from bacteria that lived alongside the tardigrades. "There is no way, biologically, these can be part of the same genome," geneticist Mark Blaxter told Ed Yong of the Atlantic in 2015.

Now Blaxter and his team are back with a new analysis of the tardigrade genome, published in the journal PLOS Biology. "I have been fascinated by these tiny, endearing animals for two decades," Blaxter says in a statement. "It is wonderful to finally have their true genomes, and to begin to understand them."​

This latest study compares the genomes of two tardigrade species:Hypsibius dujardini and Ramazzottius varieornatus. Though the research hints at some of the reasons behind tardigrade superpowers, it also sheds light on how little we know about this adaptable critter. 

The main superpower the researchers focused on was how the creatures can dry out at years at a time. For most life, desiccation means death. So the team examined genes that are activated under dry conditions, discovering a set of proteins that appear to fill in for water lost in tardigrade cells. By taking the place of the missing water molecules, the proteins prevent the cells structures from collapsing and allows the tiny tardigrade to revive itself when water returns.

The latest study is also providing clues into how tardigrades came to be. Scientists previously suspected that tardigrades may be closely related to the phylum of arthropods, which includes insects and spiders. But this latest study strongly suggests that tardigrades are actually more closely related to nematodes, also known as roundworms. The researchers examined a set genes that determine the layout of an embryo called "HOX genes." They found that, similar to nematodes, both species of tardigrade lack five commons genes from this set.

As for the controversy over how much gene transfer really takes place? It appears to be mostly settled now, reports Tina Hesman Saey from Science News"The authors' analysis methods, and their methods for getting clean DNA, are certainly an improvement over our own earlier methods," Bob Goldstein, who supervised Boothby's 2015 research, tells Saey.

But the debate about tardigrades amazing superpowers and where they belong on the tree of life is far from settled. Are tardigrades more closely related to arthropods or nematodes? "It’s still an open question," phylogeneticist Max Telford tells Saey.

Even so, Blaxter hopes that his team's tardigrade genomes will continue to help tease out tardigrade's tangled relationships as well as assist in the development of useful applications for the creatures superpowers. "This is just the start," Blaxter says in a statement. "With the DNA blueprint we can now find out how tardigrades resist extremes, and perhaps use their special proteins in biotechnology and medical applications."

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