The “slow-walking” tardigrade (water bear) Hypsibius dujardini in water with algae. Scientists have now discovered that this tiny creature has acquired the most foreign DNA of any animal species.  

 

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[“Waterbear” by Bob Goldstein and Vicky Madden, UNC Chapel Hill – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons]

Easily one of the weirdest and most fascinating creatures on the planet, the tardigrade or as it is colloquially referred, water bear, is barely visible by the naked eye and can withstand some of the harshest climates on earth and in space. Now, this tiny little creature that lives in wet, mossy environments and whose name in Latin means “slow-walker,” can add a new achievement to its molecular makeup—nearly one-sixth or 17.5% of its genome comes from foreign DNA—the most for any currently known animal species.  

Scientists from the University of North Carolina at Chapel Hill (UNC) recently completed the genome sequence for the hearty tardigrade Hypsibius dujardini and unexpectedly found an unprecedented degree of horizontal gene transfer (HGT) from Eubacteria, Archaea, plant, and fungal species.

“We had no idea that an animal genome could be composed of so much foreign DNA,” remarked senior author Bob Goldstein, Ph.D., professor in the biology department at UNC's College of Arts and Sciences. “We knew many animals acquire foreign genes, but we had no idea that it happens to this degree.”

The research team’s data not only raises the question of whether there is a connection between foreign DNA and the ability to survive extreme environments, but further expands many conventional views of how DNA is inherited and how species evolve.

The findings from this study were published recently in PNAS through an article entitled “Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade.”

The investigators found that tardigrades acquired about 6,000 foreign genes through HGT—which swaps genetic material between species as opposed to inheriting DNA exclusively parental sources. Previously, another microscopic animal called the rotifer was the record-holder for having the most foreign DNA, but it has about half as much as the tardigrade and by comparison, most animals have less than one percent of their genome from foreign DNA.

“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,” explained Thomas Boothby, Ph.D., postdoctoral fellow in Dr. Goldstein's laboratory. “After all, bacteria have survived the Earth's most extreme environments for billions of years.”

The UNC scientists hypothesize that when tardigrades are exposed to extreme conditions such as desiccation, their DNA breaks into tiny pieces. Subsequently, when the cell rehydrates, the cell's membrane and nucleus become temporarily leaky, and DNA and other large molecules can pass through easily. Moreover, tardigrades can repair their damaged DNA as the cell rehydrates and stitch in the foreign DNA in the process, creating a mosaic of genes that come from different species.

“We think of the tree of life, with genetic material passing vertically from mom and dad,” said Dr. Boothby. “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.”