The Water Bears and Their Controversial Genes

Time to combine two of my favourite things: scientific controversy and water bears. Not a common combination, I grant you, but both are great and the chance to cover a scientific controversy about water bears is too good to pass up.

Water bears, also known as moss piglets, proper name: tardigrades, are really quite incredible. They are tiny little creatures about half a millimetre in length, they have eight legs and will eat anything from plants and bacteria to other tardigrades dependent upon the species. They prefer moist environments but can survive pretty much anywhere, and I mean that literally. They exist naturally at either pole and around the equator, they live at the top of the Himalayas and at the bottom of oceanic trenches. They can be frozen to nearly absolute zero or dried out till they have lost 97% of their water but they will still come back to life once reheated and rehydrated. They have even been shown to be able to survive the vacuum of space. So, you know, they’re awesome.

Back in November there was a paper published in the Proceedings of the National Academy of Sciences that marked them out as even more amazing. It published a first draft of the tardigrade genome and showed that about one sixth of the genes present were actually from other organisms. Horizontal gene transfer is very common in bacteria and other asexual, microscopic organisms but is thought to be rare amongst animals so it was a shock to see so much foreign DNA in the tardigrade specimens. Indeed, the authors said that they originally thought that their samples must have been contaminated but they looked further and concluded that the alien DNA had actually been deliberately incorporated into the genome.

They found 6000 genes from other species, most of them bacterial but also from animals, fungi, archaea and plants. This finding would mean that tardigrades have acquired twice as much of their genome as the next highest animal known to use horizontal gene transfer. They even suggested a mechanism for how this might happen: tardigrades are known for their ability to survive extreme desiccation, normally this results in the death of an organism, the breakdown of its DNA and the membranes that normally hold it in place. At this point the molecular mish mash just sort of oozes over any tardigrades that might happen to be in the area and, once conditions become a bit more favourable to life, the tardigrades are able to reanimate themselves and repair their own genomes whilst simultaneously incorporating the new genes from other organisms.

Pretty amazing. At least it would be if it were true. This is where the controversy comes in. About a week after the paper came out a different one was published by a rival group claiming that this was broadly untrue and that the methods of the original group were flawed, that their samples really were contaminated and that in reality only about 54 genes had been taken from other species instead of more than 6000. The new paper is currently only in BioRxiv and is yet to be peer reviewed so it is by no means the final word in this story. There are significant differences in the findings between the two groups, though, and they can’t both be right.

In fairness the original team has taken it on the chin. The project leader, Bob Goldstein, has commented:

“This paper reports an independent genome for the tardigrade and raises some reasonable concerns about contamination. 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, 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.”

This is a very mature, measured and magnanimous response considering that someone has just come along and potentially just trashed your last couple of years of work. Importantly it is, however, a great example of how science works. Science, when done well, is self-correcting. There is always someone out there who will be checking up on what you’re publishing and they won’t hesitate to point out in excruciating detail where you went wrong.

I often here criticisms of science that it is stuck in it’s ways, dogmatic, unwilling to change and accept new ideas and criticisms. Nothing could be further from the truth. Science is a living, breathing body of knowledge that is constantly being corrected, refined and added to. There are no absolutes in science, every claim has an error bar, every fact is open to refutation by new data. Sometimes scientists know they have got something wrong but they can’t see where and so they throw it open to the community for them to pick over, this is what happened with the faster than light neutrinos hubbub a few years ago.

In this instance the scientists involved, potentially, simply made an honest mistake. The author of the second paper thinks that they did the right tests but that they didn’t use them in enough examples, they extrapolated too much and needed to be more thorough. A few months from now the real answer, as near as we’ll ever have it, will emerge. Who knows, maybe they’ll publish together? In any case, here we have an almost real time example of how science and scientists, far from being stubbornly wedded to their pet theories, are able to challenge the established ideas, to propose new ones and to, if necessary, admit defeat. As a clever German bloke once said:

“No amount of experimentation can ever prove me right; but a single experiment can prove me wrong.”

Cute little critters, aren’t they? Image courtesy of the AMNH

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