Almost 1/6 Of Tardigrade DNA Is Foreign

Sinclair Stammers

Tardigrades are arguably some of the most awesome animals in the world. These microscopic organisms, sometimes called water bears or moss piglets, take living life on the edge to the extreme, capable of withstanding the harshest environments on the planet, and even outer space. Full of surprises, it turns out that a lot of their DNA – almost one-sixth – is not actually tardigrade in origin. Thus these organisms might owe some of their unparalleled survival skills to a bit of gene borrowing, according to a new study.

“On a broader scientific scale, this shows us the animal genome can be composed of a much higher proportion of foreign genes than was originally thought possible, or probable,” lead author Thomas Boothby from the University of North Carolina told IFLScience. “It also shows we should think of the ‘tree of life’ as a ‘web of life,’ where you have disparate organisms contributing genetic material to distantly related organisms, not just direct ancestors.”

The tardigrade in question is called Hypsibius dujardini, a freshwater species commonly found in ponds. Unfortunately, this is the only tardigrade whose sequenced genome has been published so far, so the scientists can only speculate at this stage whether this gene exchange is a widespread phenomenon in these animals. Still, what they found was pretty remarkable.

Published in Proceedings of the National Academy of Sciences, the research by the University of North Carolina discovered that this species has acquired about 6,000 foreign genes throughout its history. These have come from a variety of sources, predominantly bacteria but also plants and fungi, through a process known as horizontal gene transfer. That’s basically just a way to describe the movement of DNA from one organism to another that doesn’t involve traditional methods of reproduction. Bacteria do it regularly – it’s largely how they spread antibiotic resistance genes around – and many animals have also acquired genes this way, but something on this scale had never before been documented.

Now you see why they're called "moss piglets." Sebastian Kaulitzki/Shutterstock

Looking at the foreign DNA sequences and comparing them to those found in the organisms from which they were taken, Boothby told IFLScience it looks like a lot of time has elapsed since they first started acquiring these genes.

“It probably wasn’t one single event where tardigrades got all these genes en masse,” Boothby suggests. “It likely went on for a long time, and is probably still going on today. Tardigrades have been around for at least 250 million years, so it doesn’t need to be a frequent event to build up 6,000 genes.”

The team also thinks that these foreign genes contribute to their famous hardiness, helping them withstand environments like extreme dryness, radiation, temperature, and pressure, to name a few. For example, many of the genes have known or expected functions in stress tolerance, such as the catalases which help mop up damaging, reactive particles that build up when the organism is exposed to stressors like extreme dryness or radiation. Normally, animals have their own catalase genes, but Boothby said this isn’t the case for the tardigrade studied, and it seems the entire gene family has been replaced by those from bacteria.

Not all instances of gene transfer were this extreme, though, as there were cases where there was gene supplementation, rather than complete replacement, as seemed to be the situation for the DNA repair enzymes. The tardigrade genome fragments when under extreme stress, such as during desiccation, and it’s thanks to their remarkable ability to stitch it back together that they can survive. The team also thinks this is how they acquired so much foreign DNA, as when the tardigrade cells begin to rehydrate, their membranes are transiently leaky, allowing big molecules such as DNA from the environment to get inside. So when it pieces together its own genome, these get inadvertently added in. 

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