Tardigrade mysteries

What’s nearly indestructible, cute as a button, and often misunderstood?

Tardigrades.

These little creatures are amazing. They can go into suspended animation, which allows them to survive temperatures as high as 150C and as low as -200C, radiation that would absolutely fry any other organism’s DNA, and pretty much any other extreme environment you can think of (Including, but not limited to, actual space!)

And they’re all around you. They live basically anywhere there’s water to swim in and moss to chew on – trees, buildings, lakes, rivers, ponds… antarctic glaciers.

Some species are parthenogenetic, which means momma tardigrades can make clones of themselves without any help. Other species are hermaphroditic, which is a slightly different form of Do-It-Yourself reproduction, where they fertilize themselves by taking both the male and female roles a the same time.

They are the honey badgers of the microscopic world. Strong, independent creatures that don’t need anybody for anything and don’t care what their environments (or curious scientists) throw at them.

If you follow this blog, you know I love tardigrades. And I’ve written about them before. I even mixed business and pleasure when I pushed for them to feature in the 2015 CHRISTMAS LECTURES.

And right around that time, a controversy started brewing.

A group of researchers at the Unviersity of North Carolina (UNC) published a paper in December 2015 claiming that a full 17.5% of the tardigrade’s DNA was ‘stolen’ from other organisms.

Stolen is a bit of a loaded term there (although it was certainly used in a lot of the reporting around that study). It’s more like finders keepers.

They are referring to horizontal gene transfer (HGT), where an organism picks up snippets of DNA and incorporates them into their own. This is very common in single-celled bacteria and archaea, but much rarer in multicellular animals. It’s not unheard of, but one sixth is a huge proportion of DNA to have come from other creatures.

The theory coming out of the UNC group was that, in the process of rehydrating after suspended animation, tardigrades will let all sorts of molecules, including foreign DNA,  into their cells. This, combined with exceptional DNA repair mechanisms, allows foreign DNA to be knit into the existing genome. And if the foreign DNA helped tardigrades survive, it would persist.

This was a pretty bold claim. And many researchers didn’t believe it. So a group from the University of Edinburgh resequenced the genome and published a paper six months later with basically the same title, with one crucial difference: They added a “No” in front.

A second group from Germany piled on with an analysis of the data, writing “We thus suggest that the published high rate of [horizontal gene transfer] in the genome of [the tardigrade species] H. dujardini is an artifact of sample preparation rather than a biological signal.” That’s as close as it comes to a mic drop moment in biology papers.

Meanwhile, a third group in Japan sequenced the genome of another species of tardigrade (Ramazzottius varieornatus) to get to the bottom of the origin of their superpowers. Their paper, published 9 months after the UNC one, found normal rates of HGT (around 1.5%), a final nail in the coffin of the UNC group’s theory. Not only is the original analysis flawed, a new, more complete analysis of an even more resistant species showed very little HGT.

Tardigrades didn’t pull a Peter Petrelli and absorb their powers from other organisms.

But all of this research has sparked a new theory for the tardigrade’s superhero origin story: Tardigrade Intrisincally Disordered Proteins.

The same Japanese group found an interesting protein that sticks around the DNA of tardigrades and helps them survive radiation. If that same protein was added to human cells in a dish, they improved radiation tolerance by 40%! Further studies (by members of the original UNC group!) found even more proteins that start out jelly-like around water, and turn to glass (vitrify) to protect important cellular machinery while the tardigrade is dried out. These discoveries might have significant consequences for human medicine.

And it wouldn’t be the first time tardigrades inspired medical advances. John Crowe discovered that the sugar trehalose, essential to the tardigrade drying process, can be used to freeze-dry platelets, vaccines or even maybe stem cells. This makes them much more portable and versatile.

All this is good news for understanding the physical mechanisms of their tolerance. But the origin of these incredible powers remains elusive. It’s tricky because a lot of these superpowers are probably byproducts of the same process. The very same things that allow them to survive drying out (probably a fairly common occurrence) allow them to survive cold, heat, vacuum and radiation.

The normal approach – to look at their evolutionary tree – isn’t working. Scientists still can’t figure out how they fit into the tree of life. Are they closer to arthropods or nematodes?

There is so much to marvel at about these incredible creatures. And with much to still be discovered, it’s exciting to watch their public profile rise. They have starring roles in Neil DeGrasse Tyson’s Cosmos series, the latest Star Trek incarnation, and more and more popular science articles are written about them every week. Pretty soon they’ll be a household name.

In the meantime though, I’ll revel in the hipster knowledge that I loved them before they were cool.

Long live the tardigrade.

 

Featured image by Roͬͬ͠͠͡͠͠͠͠͠͠͠͠sͬͬ͠͠͠͠͠͠͠͠͠aͬͬ͠͠͠͠͠͠͠ Menkman