Bärtierchen / Tardigrade

The tiny water-dwelling invertebrates are considered the most robust animals on Earth, as it turns out, they have the most foreign DNA of any species. According to research published in the Proceedings of the National Academy of Sciences, tardigrades have genomes that are nearly one-sixth foreign, which means that they get a massive chunk of their DNA from other creatures other than the animal itself.
The Tardigrade also otherwise known as water bears, are  nearly indestructible, incredibly unique, eight-legged, microscopic worm-like animals that grow to just over 1mm on average and can be found almost anywhere on our planet. They can withstand temperatures of almost absolute zero, survive space radiation, extreme pressure and can live for more than 10 years without food or water.
“We had no idea that an animal genome could be composed of so much foreign DNA,” said co-author of the paper Bob Goldstein from the University of North Carolina at Chapel Hill. “We knew many animals acquire foreign genes, but we had no idea that it happens to this degree.”
So where is the tardigrade getting all its genes from? The foreign DNA comes mainly from bacteria, but also from plants, fungi, and Archaea. And it’s this incredible variety of genes that researchers suggest has allowed the water bear to survive in such extreme conditions.
“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,” said one of the researchers, Thomas Boothby.
In 2007, some tardigrades were rocketed into space on the outside of a satellite. Many of the water bears were still alive when the satellite returned. In fact, some of the females had laid eggs in space, with the young hatching healthily, as though nothing had happened.
The DNA of this animal is very peculiar since 17.5 percent of its DNA is foreign, that has been taken from different bacteria, plants and viruses. That can also be said that it has more than 6,000 foreign genes from different species of whatever the tardigrade took it from. And it can also handle a lot of stress. What is more interesting is that most of tardigrade’s DNA is single celled microorganisms.
In other words, it means that if a tardigrade is taken into a stressful situation, such as desiccation, its DNA will break into small pieces. When the cell rehydrates, the cell’s membrane and nucleus (where the DNA resides) become temporarily “leaky,” such that DNA and other large molecules can pass through easily. Similarly, another example of a ‘stressful’ situation is that, if you put a tardigrade in the freezer for 20 years, within 20 minutes, the animal can thaw itself and live on a normal life, as if nothing has happened.
Boothby said, “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.”
During this process, the water bears not only repair their own damaged DNA, but also stitch in the foreign DNA, creating a mosaic of genes that come from different species.
This would be a random process, but the genes that get passed down would be those that help the animals survive. Further research now needs to be done into exactly how tardigrades are obtaining this foreign DNA, and how often it’s happening. But what’s really exciting is that it provides new insight into exactly how life evolves.
“We think of the tree of life, with genetic material passing vertically from mom and dad, 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,” Boothby said.
“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.
(Kavita Iyer)“