Tardigrades—otherwise known as water bears or (gasp) moss piglets—are weird. They can survive in the vacuum of space, for starters. So when researchers at the University of North Carolina announced on Nov. 23 that the minuscule animal had hitherto unheard of ratios of DNA borrowed from plants, fungi and bacteria, many in the scientific community were more than happy to believe the strange findings. Now, a lab at the University of Edinburgh claims the results must be an error.
This might seem like a throwdown, but it’s far from any kind of bitter rivalry: We’re watching science work the way it’s supposed to. The only difference is that the general public, actually get to see it happen—and at digital speeds.
When University of Edinburgh’s Mark Blaxter first read the recent study, he says he instantly suspected something was amiss. The work, led by UNC’s Bob Goldstein, concluded that the tardigrade’s DNA is one-sixth foreign, doubling the previous record for outside DNA contributions. That would mean that it had swapped so many genes with other organisms (through a process called horizontal gene transfer) that it had evolved into a strange mosaic (or Frankenstein) of an animal. Most animals have genomes that are over 99 percent homegrown, so the tardigrade’s massive ream of borrowed genes could theoretically account for its apparent invincibility.
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“Bob Goldstein is a brilliant, brilliant scientist,” Blaxter said. “I use his studies in teaching undergrads all the time, as examples to my students—this is how science is done. So this study coming from his lab, when people already know tardigrades are a little bit weird—to see this was like cherries on the cake.”
“The story, if it were true, is exactly as exciting as they say—but I don’t think it’s true,” Blaxter said.
Blaxter’s lab is no stranger to the tardigrade genome. In fact, they work from the same species culture that UNC does. The labs had shared data frequently, and this new twist came as a complete surprise to Blaxter and his colleagues.
“We just hadn’t seen evidence of that at all,” Blaxter said. The fact that tardigrades have genes they’ve acquired through horizontal gene transfer isn’t up for dispute. But the record-breaking volume didn’t line up with his own lab’s work.
He suspected that the genome had been contaminated with random bacteria from the lab, despite rigorous efforts to avoid exactly that.
“With little things like tardigrades, we can’t scrub each and every one of its bacteria, and just like us they come with bacteria on their skin and in their guts,” he explained. “It’s very hard to get rid of that. Even if you starve the bacteria, they’ll hang out hoping the good days return.
So he did what any suspicious scientist would do: He asked for the data. Once UNC made it public, Blaxter’s lab set out to compare the genome analysis used in the new study with the genome they’d sequenced from their own, presumably identical tardigrade cells. In theory, the genes found in the tardigrade at UNC—including the high volume of bacterial genes—should have shown up in Edinburgh, too.
But they didn’t.
“The simplest answer is that, well, all these genes appearing only in the UNC genome are bacteria-like, so they’re probably bacteria,” Blaxter said. “And the simplest answer is usually the right one.”
That doesn’t mean the jury is out. The initial paper passed peer review—meaning that other scientists signed off on it before it could be published—but the response from Edinburgh was put online before going through this process. Just because it’s a correction doesn’t mean it’s correct.
The University of Iowa’s John Logsdon, who commented favorably on the first paper to The Atlantic, pointed this out.
“I think both groups have been very ernest about their analyses and trying to get a right answer, but it seems to me like neither is entirely right,” Logsdon said.
It isn’t at all unusual for labs to check each other’s work. In fact, that’s kind of the point: A single study is never really “proof” of anything. Doing good science means different scientists in different labs getting the same results over and over again.
On the one hand, the quick appearance of this tardigrade gut check is great, because it shows how quickly science can correct itself in the digital age. Blaxter believes that a few decades ago these two studies would have sat in the literature unnoticed, with no one knowing or caring that one had “corrected” the other. But Logsdon points out that the speed of the Edinburgh response might make it seem like it’s the final word on the subject—which might not be the case.
“This was a unique situation in which that second answer was sort of presumed to be correct without a lot of consideration,” Logsdon said. “The response was sort of, oh, this was more what we expected anyway, so it must be right. It had a more conservative answer, so it struck a chord as, oh, the first one must have been overreaching.”
What it probably comes down to, Logsdon said, is that one lab wasn’t clean enough—but that the other was too clean, in a manner of speaking. It’s possible that Edinburgh was too conservative in removing bacterial genes from its tardigrade sample where UNC was too liberal in counting them. While one group is likely closer to the truth than the other, Logsdon said, it would be a mistake to consider the second lab’s work a definitive takedown of the first study.
Julie Hotopp, a University of Maryland School of Medicine professor who works on horizontal gene transfer, was much more critical of the UNC work in a blog post she wrote on the subject. But Hotopp also pointed out that the truth was likely somewhere between the two datasets.
The authors of the original paper, who declined a request for comment, are keeping quiet while they work on their data. The Edinburgh researchers are continuing to work on their own genome in the hopes of solidifying their findings. But no matter the outcome, one thing is clear: This is all part of how science is supposed to work. Science isn’t a bunch of facts ——it’s a process of finding better and better answers to questions posed by the natural world.
And sometimes that means figuring out whose lab is just clean enough to unlock the secrets of the noble moss piglet.