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Water Bears: Genome Sequence Says It Has Most Foreign DNA Of Any Animal

Nov 26, 2015 10:45 PM EST

Water bears, the strange, segmented micro-animals known to scientists as tardigrades, can not only survive some of Earth's most extreme environments, but are also the only creatures known to survive the harshness of outer space without the help of a NASA-grade spacesuit. After sequencing this nearly indestructible organism's genome, researchers from the University of North Carolina (UNC) at Chapel Hill discovered the unique species has quite a bit of foreign DNA. 

"We had no idea that an animal genome could be composed of so much foreign DNA," Bob Goldstein, co-author of the study and a faculty member in the biology department at UNC, said in a news release. "We knew many animals acquire foreign genes, but we had no idea that it happens to this degree."

To be exact, 17.5 percent of a water bear's genome is made of foreign DNA, which means that their genes come from creatures other than the animal itself. To put this in perspective, the genomes of most other animals have less than one percent foreign DNA. Therefore, water bears are considered to have more foreign DNA than any other species. The previous record-holder was a microscopic animal known as a rotifer. Ultimately, this study could change conventional views on how DNA is inherited. 

Water bears are tiny, microscopic animals that mostly live in mosses, lichens and liverworts, but some can also be found living in fresh water. They have eight legs with four claws on each, short and cylindrical bodies, and mouths with sharp, pointy teeth called stylets. When feeding, the animals use their teeth to cut into moss leaves or algae and then suck the plant's juices.

This is not the first time scientists have explored the extraterrestrial-like characteristics of water bears. In 2007, thousands of tiny tardigrades were strapped to a satellite and rocketed into space where they were supposed to fend for themselves in environments that humans can't manage without the use of specialized equipment. When the one millimeter-long creatures returned to Earth, researchers discovered many of them had survived and some of the females had even laid viable eggs in space, which later hatched healthy young. 

The little critters can also survive some of the harshest regions on Earth, too. This includes remote, ecological niches located at altitudes of 18,196 feet on a mountain in the Himalayas, as well as in Japanese hot springs and in the deep oceans of Antarctica. Additionally, water bears can withstand surprising amounts of radiation and extremely hot and cold temperatures. 

"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," Thomas Boothby, first author of the study and a postdoctoral fellow in Goldstein's lab, added

After all, bacteria have survived Earth's harsh environments and drastic climate changes for billions of years. Fossil water bears date back to the Cambrian period, over 500 million years ago. Since then, it seems the species has evolved with unique superpowers. 

In the recent study, researchers revealed tardigrades acquire roughly 6,000 foreign genes mainly from bacteria, but also from plants, fungi and Archaea through a process known as horizontal gene transfer. Compared to inheritance - when genes are simply passed down from mom and dad to offspring - horizontal gene transfer occurs when genetic material is swapped between species. (Scroll to read more...)

This simply means that when a tardigrade is subjected to a harsh environment, its DNA reforms and adapts. For example, in a state of extreme dryness, or desiccation, researchers believe the animal's DNA breaks into tiny pieces. As the cell rehydrates, its membrane and nucleus - where DNA is stored - becomes temporarily "leaky," during which time genetic information and other large molecules can pass through easily. Tardigrades then begin to repair their own damaged DNA and incorporate useful genes from other species within their environment. 

"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."

Thier findings were recently published in the journal Proceeding of the National Academy of Sciences,

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