Tardigrades, already made of indestructible win, have shown up again in the scientific weirdness Hall of Fame this week, thanks to a new study that sequenced the first tardigrade genome and found that 17.5% of it came from other species. Otherwise known as water bears, tardigrades are actually a large group of related species which have a key trait in common: They’re impossible to kill. Tardigrades are the only species ever observed to survive outside Earth’s sheltering atmosphere. Now scientists are speculating that horizontal gene transfer, the phenomenon identified by a team of researchers at UNC as the reason for the unprecedented proportion of foreign DNA discovered in the tardigrade genome, may also be responsible for some of the tardigrade’s famous durability.
If the idea of fully a sixth of an animal’s genome being of foreign origin seems far-fetched, you’re in good company. Most organisms have a maximum of 1% foreign DNA. Creatures like Elysia chlorotica, which literally consumes a steady diet of other organisms to acquire their powers (of photosynthesis), have been known to science for many years – until now the rotifer, distant cousin to the water bear, was the most extreme example in its class for having acquired about 10% of its DNA from other species via horizontal gene transfer. Even the alarming phenomenon of increasing resistance to antibiotics has its roots in the fact that some single-celled organisms are very good at shaking down other microbes for their spare plasmids. But the proportion of foreign DNA in this clearly successfully adapted organism surprised even the researchers who did the experiment. Bob Goldstein, one of the co-authors of the study, said “We had no idea that an animal genome could be composed of so much foreign DNA. … We knew many animals acquire foreign genes, but we had no idea that it happens to this degree.”
The method used to sequence the DNA in this experiment is crucial to the assertion that the DNA found in the samples wasn’t just coincidental contamination by nucleic acid residue from other species. Traditional (Sanger) sequencing breaks DNA molecules into fragments, and then relies on further intervention to reassemble the fragments by matching up overlapping regions. In this study the researchers used a technique called single-molecule real-time (SMRT) sequencing, created by Pacific Biosciences, which can process and sequence an entire DNA molecule without breaking it into fragments. Like having intact negatives to a photo, the intact molecule provides a master copy for comparison: The physical associations between genetic loci can be used to parse out evolutionary history from the array of genes present in the living creature. Using this technique, the researchers were able to state with confidence that the genes had been acquired by the species within its evolutionary timeline, and not introduced by experimental error.
How did tardigrades get so good at acquiring that much foreign DNA? Thomas Boothby, lead author, believes it’s related to the tardigrade’s multifaceted approach to adversity. Under dehydration stress, for example, water bears can actually dry themselves out, a process that breaks up their DNA into small pieces, much like the relatively stable form the genetic material takes during most of the cell’s growth and replication cycle. When moisture returns, they rehydrate by making their cells leaky, which admits water – but also molecules from the environment, even macromolecules like DNA. And their robust DNA repair process is fault-tolerant enough to handle such tremendous disruption. That very fault tolerance may make the tardigrade more likely to assimilate DNA molecules from different species, since the ligation enzyme easily associates DNA fragments with complementary sticky ends.
Perhaps even more interesting is the fact that the team discovered that under stressful conditions, their tardigrades could switch on and off some of that wide array of aftermarket genes – genes related, in their original hosts, to stress tolerance. It’s vaguely reminiscent of that scene from Independence Day where Will Smith and Jeff Goldblum manage to hack an alien computer with a MacBook, except this is real life and the hack actually works.
(Top photo credit: AMNH)