Tardigrades Stir Up Controversy and Showcase Science in Action
Rival scientists quickly challenge recently published research about water bears’ mysterious genomes
There’s no doubt that the microscopic aquatic invertebrates known as tardigrades, moss piglets or water bears are weird. The oddly-adorable critters can survive the vacuum of space, endure desiccation by drought and emerge alive after freezing, among other feats. A recent study suggests that tardigrades are also gene stealers—that their genomes hold more DNA from unrelated plants, bacteria and other microbes than any other animal on Earth, Rachel Nuwer recently reported for Smithsonian.com. That ability might even explain why tardigrades are so resilient, the researchers suggest.
But about a week after the announcement of that surprising discovery, another group has challenged the findings, reports Ed Yong for The Atlantic. The quick back-and-forth between the two teams demonstrates how responsive and potentially self-correcting science can be in the digital age.
The first study, from a team based at the University of North Carolina, reported that one tardigrade species carries a whopping 6,600 genes that originally belonged to organisms from other kingdoms—mostly bacteria but also fungi, plants, viruses and archaea, and acquired through a process called horizontal gene transfer. Those findings meant that Hypsibius dujardini's genome is composed of about 17.5 percent foreign DNA, the team reported in Proceedings of the National Academy of Sciences. Because some of the foreign genes are involved in other organisms' stress responses, the researchers suspect that they might have bestowed tardigrades with some of their own unique abilities to weather extreme conditions.
However, a second group from the University of Edinburgh also sequenced the same tardigrade species and got very different results. Those researchers found the critter has between 36 and 500 horizontally transferred genes—a much smaller percentage. When the UNC paper came out, the Edinburgh results were still unpublished, but the researchers scrambled to published their findings on the preprint server bioRxiv.org—a website where manuscripts can be published before they are submitted to a journal and the peer review typical of scientific research.
The Edinburgh group suspects that the UNC group may have "sequenced DNA from bacteria that were living alongside the tardigrades and, despite their best efforts, had mistaken the genes of those microbes for genuine tardigrade genes" writes Yong. But how could the first group have been mistaken, if indeed they are? Yong explains that the technology to sequence genomes involves chopping DNA up into small pieces and then reassembling the bits into a coherent picture. Some contamination can easily slip in, so scientists make an effort to figure out which bits belong to the organism they are studying, rather than microbes nearby.
The Edinburgh group realized that some of the DNA fragments were rare, compared to other fragments. "There is no way, biologically, these can be part of the same genome," the team’s leader, Mark Blaxter, tells Yong. The group concluded that there are 500 genes in the tardigrade’s genome that could have come from other sources, but they only have strong evidence for 36 of them. That gives the tardigrade a much more typical proportion of foreign DNA for an animal.
In a comment on the newest paper, Thomas Boothby and Bob Goldstein from UNC point out that they were aware of this potential problem and worked hard to ensure it didn't affect their results—but that there is always possibility for error. They add that they will do additional analysis before making further comment. The researchers also said they appreciated their rivals' decision to publish their contradictory results so quickly on bioRxiv.org.
The feelings were mutual. Sujai Kumar from the the Edinburgh team told Yong that if the UNC group hadn’t released their data promptly, it would have been more challenging for their group. "What is evident is the amazing new ability of science to self correct rapidly," Blaxter tells Yong. What took just nine days in this case would have taken months in the past.
For the time being, the jury is still out on how much of the tardigrade's genome comes from foreign sources. But even if those tiny creatures aren’t gene stealers, they may be headline stealers for a while longer.
Editors' note: This article was edited by Rachel Nuwer, who originally reported on the tardigrade research for Smithsonian.com.