I’ve long been fascinated by Tardigrades. You’ve gotta love an animal that can survive extreme cold, heat, and pressure, desiccation, starvation (as many as 10 years without food!), exposure to radiation and toxic chemicals, and the vacuum of space. It also doesn’t hurt that they’re nicknamed “water bears,” and are cute little critters that lumber around on 8 clawed legs. For a quick intro to these hardy fellows, check out the video below:
When you have an animal with survival skills as unique as those of the tardigrade, it’s natural to wonder how they are capable of such feats. Well, a bit over a week ago the media (for example, see this article on phys.org and this one on Science Alert) started reporting on a study that suggested that tardigrades have obtained about 1/6 of their genome from foreign sources. A group of scientists led by Thomas Boothby sequenced the tardigrade genome, and based on their results they suggested that tardigrades have obtained large amounts of DNA from other organisms via a process known as horizontal gene transfer (a fancy way of saying “genes transferred from one organism to another”). In the abstract of their study they state:
“…analysis revealed that a large fraction of the H. dujardini genome is derived from diverse bacteria as well as plants, fungi, and Archaea. We estimate that approximately one-sixth of tardigrade genes entered by HGT, nearly double the fraction found in the most extreme cases of HGT into animals known to date.”
Given these results, the study authors suggested that these genes might be related in some way to the ability of tardigrades to survive in extreme environments. Pretty cool, isn’t it!
Well, in a great example of why replication of research is a critical part of the scientific process, a second group of researchers has sequenced the tardigrade genome, and in a pre-print of their results they state: “We compare our assembly to a recently published one for the same species and do not find support for massive horizontal gene transfer.” Rather, it seems that the authors of the second study suspect that the foreign DNA in the original genome sequence is a result of contamination from organisms (bacteria, fungi, etc.) on or in the tardigrade that was sequenced. Buzz kill.
To their credit, co-authors for the original sequence posted a comment on the website where the second study was released. In it they write:
“This paper reports an independent genome for the tardigrade Hypsibius dujardini and raises some reasonable concerns about contamination in our recent paper (1). We thought seriously about the possibility of contamination—it was of course the most likely initial explanation for the large amount of foreign DNA found in our assembly—and much of the analysis in our paper was designed specifically to address this issue. We view the independent data and analysis of Koutsovoulos and coauthors, including their analysis of our data, as valuable toward resolving questions of broad interest. We will work now to try to further resolve the issues that were raised. We plan to refrain from commenting more until we’ve done additional analyses that can shed more light on this issue, and we’ll be happy to share what we learn between groups.
We appreciate that the bioRxiv preprint server is a valuable way to move science like this forward without delay, and we’re grateful to Koutsovoulos and coauthors for making use of it.”
It’s great to see this kind of gracious response from researchers whose work has been called into question, and they deserve credit for being open to further investigation to resolve the issues raised. It will be interesting to see how this plays out, but even if their genome isn’t quite as unusual as suggested, tardigrades are still pretty amazing creatures.
For some additional reads on the conflicting research, read posts by PZ Meyers, T. Ryan Gregory, and The Atlantic.