Water bears, known to scientists as tardigrades, are famously adorable microscopic creatures who can survive anything: freezing, total dehydration, radiation bombardment, and even the vacuum of deep space. Now scientists have sequenced a tardigrade genome, and are very surprised by the results.
Yesterday a group of researchers led by University of North Carolina at Chapel Hill biologist Thomas Boothby published their analysis of the tardigrade genome in the Proceedings of the National Academy of Sciences. What they found was that 17.5 per cent of the tardigrade genome actually comes from other organisms, including plants, fungi, bacteria, and viruses. These genes entered tardigrade DNA in a process known as horizontal gene transfer, which is quite common among single-celled organisms but rare among animals. The closest comparison with the tardigrade is a microscopic form of plankton called a rotifer (pictured below, eating), which has about 9 per cent of its DNA from other organisms.
The researchers reached the 17.5 per cent number by isolating non-animal genes in the tardigrade sequence, and then comparing those non-animal genes with those of other sequenced organisms. About 17.5 per cent of tardigrade genes closely resembled genes from non-animal organisms like plants and bacteria. This is the first time scientists have ever found an animal with 1/6 of its genome coming from non-animal sources.
Biologist Bob Goldstein, who worked with Boothby on the paper, told Gizmodo via email that they can't be sure of the exact species that donated DNA to the tardigrade, partly because some of the species' genomes may not have been sequenced yet.
The real question is, how did the tardigrade become such a genetic hodgepodge? Boothby and his colleagues speculate that it has to do with the animal's response to stress. Tardigrades live in wet moss, and one of the common forms of peril they must endure is desiccation, or drying out. When tardigrades are desiccated, their DNA breaks into pieces. Any organisms around them will also suffer the same fate. But when water returns to the tardigrade's environment, they re-hydrate and return to life. As they re-hydrate, their cell walls become porous and leaky, and fragments of DNA from the desiccated organisms around them can flow inside and merge with the animal's rejuvenating DNA.
Boothby explained via email:
So what we speculate is happening is that as tardigrades dry out, their DNA and the DNA of other organisms in their environment breaks. Then as the tardigrades rehydrate their cell membranes become leaky, allowing this environmental DNA to enter their cells and ultimately the nuclei of their cells. Finally, as the tardigrades are stitching their own genomes back together they may incorperate some of this foreign DNA. If this happens in a germcell, the foreign material could be passed on to subsequent generations.
The researchers also found that many of the tardigrade's borrowed genes are actually related to stress tolerance and DNA repair. Boothby continued:
Since we see that many [horizontal gene transfer] genes in tardigrades have known or suspected roles in stress tolerance, it sets up an interesting 'chicken and the egg' scenario. My personal speculation is that tardigrades probably originally had some rudimentary ability to survive drying and have increased their ability to survive extremes through the acquisition of foreign genes. This would also likely setup a positive feedback loop, where the better a tardigrade becomes at surviving drying, the more foreign DNA it can acquire.
Every time a tardigrade survives a life-threatening form of stress, it manages to pick up more genes that can help it recover from that stress. So the more you try to destroy the tardigrade, the stronger it gets.
Read the full scientific paper at PNAS.
Tardigrade portrait via Goldstein Lab