The Resilient Water Bear Reveals its Genetic Secrets

water bear

The water bear can survive extreme conditions–and its foreign DNA may explain why. (Photo credit: Eye of Science/Science Source)

Though cute, at less than a millimeter in length, water bears aren’t exactly what you might call cuddly. Water bears are known for their ability to survive extreme conditions ranging from the depths of the oceans to the soaring heights of the Himalayas. Recent research indicates these tiny creatures have another unusual trait – nearly 20 percent of their DNA comes from other species.

Water bears, which belong to the phylum Tardigrada, get their name from both their four-legged bear-like shape and their slow, bear-like gait when they move. These resilient creatures can survive a wide spectrum of conditions including temperatures ranging from -272 degrees Celsius to 149 degrees Celsius, the vacuum of space, ionizing radiation at doses 100 times greater than would be lethal for humans, and elevations ranging from 6000 meters below to 4000 meters above sea level.

One of their most impressive traits is the ability to survive without food and water for up to a decade. Water bears can do this by going into a dormant state. When conditions improve and water returns, the water bears are able to rehydrate and go about their normal activities such as foraging and reproducing.

Recently, researchers at the University of North Carolina–Chapel Hill, led by Thomas Boothby, a postdoctoral fellow in the lab of Bob Goldstein, discovered bacterial genes in the water bear species Hypsibius dujardini. At first, Boothby thought that the sample had been contaminated. But further genome sequencing indicated that in addition to bacterial DNA, H. dujardini also contained genes from archaea, fungi, plants, and viruses. In total, 17.5 of the water bear’s DNA came from other species.

The process of transferring DNA directly from one organism to another is called horizontal gene transfer. This DNA-swapping commonly occurs in bacteria, but is thought to be atypical in animals.

Researchers hypothesize that the water bear picks up these foreign DNA during the desiccation process. When an organism dries out, its membranes become leaky and this allows DNA and other molecules to seep out into the environment. While this process kills most organisms, water bears have the ability to remain dormant until conditions improve. When water returns and the invertebrates are able to rehydrate, their leaky membranes are stitched back together and during this process they may incorporate foreign DNA from nearby dead organisms into their own genes.

“Animals that can survive extreme stresses may be particularly prone to acquiring foreign genes—and bacterial genes might be better able to withstand stresses than animal ones,” Boothby said.

Interestingly, Boothby’s research indicates that many of the foreign genes provide additional support for a water bear’s survival when water stressed. For example, a water bear’s catalase enzymes—which help prevent cell damage due to oxidation—are all from foreign sources.

These findings put a new spin on how some organisms obtain new traits.

“We think of the tree of life, with genetic material passing vertically from mom and dad,” Boothby said. “But with horizontal gene transfer becoming more widely accepted and more well known, at least in certain organisms, it is beginning to change the way we think about evolution and inheritance of genetic material and the stability of genomes. So instead of thinking of the tree of life, we can think about the web of life and genetic material crossing from branch to branch. So it’s exciting. We are beginning to adjust our understanding of how evolution works.”

In an interesting twist, not long after Boothby and his colleagues published the results of their tardigrade study in the November 23 edition of the Proceedings of the National Academy of Sciences (PNAS), a countering article was posted on bioRxiv, a free online archive sponsored by the renowned Cold Spring Harbor Laboratory that allows scientists to publish raw manuscripts that have not yet gone through the peer-review process. In the article, Georgios Koutsovoulos, lead author, and his colleagues at the University of Edinburgh and the University of Oxford dismiss the idea that H. dujardini contains any foreign DNA at all. In sequencing the same strain of H. dujardini, the researches found “15,000 fewer protein-coding genes, and a hugely reduced impact of predicted [horizontal gene transfer] on gene content in H. dujardini.”

In a comment on the bioRXiv article, Bob Goldstein, an author on the original PNAS publication wrote: “This paper reports an independent genome for the tardigrade Hypsibius dujardini and raises some reasonable concerns about contamination in our recent paper. We thought seriously about the possibility of contamination—it was of course the most likely initial explanation for the large amount of foreign DNA found in our assembly—and much of the analysis in our paper was designed specifically to address this issue. We view the independent data and analysis of Koutsovoulos and coauthors, including their analysis of our data, as valuable toward resolving questions of broad interest. We will work now to try to further resolve the issues that were raised. We plan to refrain from commenting more until we’ve done additional analyses that can shed more light on this issue, and we’ll be happy to share what we learn between groups. We appreciate that the bioRxiv preprint server is a valuable way to move science like this forward without delay, and we’re grateful to Koutsovoulos and coauthors for making use of it.”

More to Explore
Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade [abstract]
The genome of the tardigrade Hypsibius dujardini [PDF]
A huge chunk of a tardigrade’s genome comes from foreign DNA
What the World’s Toughest Animal Is Really Made Of
Water Bears are Genetic Mash-Ups
Tough Tardigrades

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