Water Bear Genome Reveals Even More Weirdness

We see a lot of weird animals here, but the tardigrade continually takes the cake. The tardigrade, commonly known as a water bear or a moss piglet, has captivated researchers for two centuries with its freakish near-invincibility. Scientists recently sequenced the genome of one tardigrade species, hoping to find some answers. Instead, they found more strangeness: It turns out that tardigrades can borrow genes from bacteria, fungi, and even plants. The researchers' findings were recently published in the Proceedings of the National Academy of Sciences. 

There are more than 1000 tardigrade species, all of which are pudgy, microscopic, and unbelievably hardy. Experiments have shown that tardigrades can survive temperatures below -458°F and above 300°F. You can zap them with radiation, take away their food and water, even leave them in the vacuum of space—and they’ll be just fine. When the going gets tough, tardigrades simply go into a state of suspended animation called cryptobiosis. Their metabolisms slow to almost nothing, and they dry up into little barrel-shaped husks called tuns. They won’t come out until the coast is clear. 

Lots of animals hibernate, and plenty of others enter cryptobiosis. But no other animals can do what the tardigrades do.

For all their toughness, tardigrades are pretty hard to keep alive in the lab. The Goldstein lab at UNC Chapel Hill is one of the few places in the world that has figured it out. Some tardigrade species are more cooperative than others, so when researcher Thomas Boothby decided to sequence a tardigrade genome, he picked the easygoing species Hypsibius dujardini.

True to tardigrade form, the results were super, super weird. Boothby and his colleagues found that the moss piglet’s DNA contained scraps of genes copied and pasted from other organisms. The genes weren’t taken from other animals, either—they came from plants, and fungi, and bacteria.

This liberal use of other organisms' genes is made possible through a rare (and even disputed) phenomenon known as horizontal gene transfer (HGT). As Ed Yong notes in the Atlantic, ticks, wasps, aphids, and fruit flies have all been caught swiping genes from fungi, bacteria, and viruses.

But these species only take one or two genes, which can make up about 1 percent of their DNA. Not so with H. dujardini. Boothby and his team found that stray genes from foreign species make up 17.5 percent, or a whopping one-sixth, of the tardigrade’s genome. About 90 percent come from diverse bacteria. “The number of them is pretty staggering,” he told Yong. 

Not everybody buys into the HGT phenomenon, and some previous examples have since been debunked as contaminated specimens. (If any trace of another species gets into a sample, the DNA of both species will show up in tests.)

The UNC researchers wanted to be extra sure before they published their results, so they double-checked the genome using a system called PacBio, which analyzes single unbroken strands of DNA, as Yong describes. The PacBio analysis confirmed it: Those genes all belonged to the tardigrade. The foreign strands of DNA were so interwoven with the tardigrade’s that some of the nonanimal genes had taken on animal traits.

This is highly unusual, to put it mildly. Boothby believes the secret lies in the tardigrade’s curl-up-and-fake-death response to difficult conditions. “So we think tardigrades are drying out, and their DNA is fragmenting along with the DNA of bacteria and organisms in the environment,” Boothby said in the Atlantic. “That gets into their cells when they rehydrate. And when they stitch their own genomes together, they may accidentally put in a bacterial gene.” 

Those genes may be the key to the water bear’s invincibility. Boothby and his colleagues have already found that tardigrades switch on some of these genes in response to stress. If a tardigrade accidentally picks up genes that happen to make it tougher or more adaptable, it’s probably going to keep them.

The research team plans to peer next into the genes of other animals. What’s true for the tardigrade may be true for other species—some of it, anyway. One thing's for sure: The deeper we go into tardigrade biology, the more bizarre it gets.