Genome sequencing of a tiny organism that can survive freezing, boiling and even the extreme environment of outer space shows the extreme creatures carry around a significant amount of foreign DNA.

Almost a sixth of the genome of the water bear, also known as tardigrades, is foreign, meaning it originated in an animal other than the water bear, researchers from the University of North Carolina found.

Tardigrades, tiny eight-legged micro-creatures almost invisible to the human eye, are seemingly indestructible and have even survived trips into outer space on the outer surface of satellites.

The discovery of foreign DNA in their genome is just one more surprise scientists have encountered in their study of these extreme creatures.

"We had no idea that an animal genome could be composed of so much foreign DNA," says biologist Bob Goldstein, co-author of a study appearing in the Proceedings of the National Academy of Sciences. "We knew many animals acquire foreign genes, but we had no idea that it happens to this degree."

The researchers say they've detected almost 6,000 genes in the water bear genome that are foreign, mostly originating in bacteria but also coming from plants, types of fungi and a number of single-celled microorganisms.

The DNA is not inherited but rather acquired in a process known as horizontal gene transfer, where DNA is swapped between species.

DNA is likely being swapped into the tardigrade genome randomly, but what the creatures are keeping is what allows them to survive even the harshest of environments, the researchers suggest.

"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," says study first author and postdoctoral researcher Thomas Boothby.

Under extreme stress in punishing environments, such as when the tardigrades are dried out, the creatures' DNA may be breaking into small pieces, the researchers suggest.

As their cells rehydrate, their membranes and nuclei, where the DNA is located, can become "leaky" for a period of time, allowing DNA and some other molecules pass in and out easily, they explain.

As this happens, water bears are not only repairing their own DNA but can also collect and include foreign DNA from different species, they note.

We normally think of genetic material being passed down from one generation to the next, but as horizontal gene transfer becomes better understood, it offers a chance to better understand the process of evolution and how genetic material is inherited, Boothby says.

"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," he says. "So it's exciting. We are beginning to adjust our understanding of how evolution works."

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