I'm going to call it: tardigrades are the weirdest animal on the planet (and beyond).
Also known as water bears, the microscopic eight-legged creatures have been around for hundreds of millions of years, and are best known for being almost indestructible. They can go into a state of suspended animation and survive temperatures way below freezing and well above boiling, go without food and water for years, and have even been known to survive the vacuum and radiation of space. A new study published in PNAS adds another to the list of tardigrades' extraordinary features: their genome contains an unprecedented proportion of foreign DNA. Lead author Thomas Boothby said the finding was "extremely surprising."
The group of researchers based out of the University of North Carolina at Chapel Hill set out to sequence the genome of the tardigrade species Hypsibius dujardini, in the hope of gaining more insight into the unusual creature's biology. What they found was that an unprecedented one-sixth of the tardigrade's genome was not made of tardigrade DNA. It was composed of "foreign" DNA from a large range of completely different organisms—mainly bacteria, but also plants, fungi, and single-celled archaea.
"At first we thought that there had been some mistake made, that there was contamination in our sequencing samples," said Boothby in a phone call. But after testing they were able to confirm that the foreign DNA was actually part of the tardigrade genome.
How does that happen? It's a result of horizontal gene transfer, where genes are not inherited from a direct ancestor but incorporated into the genome from other organisms. "Sometime in the past, probably on many occasions, genes from bacteria and fungi and plants actually crossed over and were incorporated into the genome of tardigrades," explained Boothby.
"You could have foreign DNA incorporated into the genome of germ cells, which would then be passed on to subsequent tardigrade offspring."
The previous record-holder for the animal with the most horizontal gene transfer is the rotifer, another water-dwelling microscopic organism. While a 2012 paper reported a horizontal gene transfer rate of 8-9 percent for the rotifer, the new study puts the tardigrade's at around 17.5 percent.
The authors suggest the tardigrade's aptitude to incorporate foreign DNA into its genes might be down to its unusual ability to completely dry out and then be rehydrated—something that would also explain the prevalence of foreign DNA in the rotifer, which can also survive desiccation. In the paper, they propose that the tardigrade's DNA could be broken up when it is dried out, and that the membrane of the cells could then become temporarily "leaky" when it is rehydrated, allowing larger molecules in their environment such as DNA to pass through. Finally, tardigrades (and rotifers) are known to have robust mechanisms to repair damage to their genome, so they could fix the DNA in place.
"We speculate that desiccation and associated membrane leakiness and DNA breakages might predispose these animals to take up and incorporate foreign material in their genomes," the researchers write.
Watch Motherboard's 2012 doc about tardigrades:
Boothby said the foreign DNA likely built up over millions of years. "If this happens in germ cells, you could have foreign DNA incorporated into the genome of germ cells, which would then be passed on to subsequent tardigrade offspring," he explained.
Interestingly, the researchers also found that many of the foreign genes that were incorporated into the tardigrade have roles in "stress tolerance," i.e. being able to withstand extreme conditions. So in some cases the tardigrade seems to have supplemented its own animal genes with bacterial genes related to stress tolerance; in others, the tardigrade's genes have been replaced.
This, along with their ability to enter a protective dormant phase when desiccated, could help explain the mystery of how tardigrades are able to tolerate conditions that would rapidly kill off other life forms. "There's a good chance that these genes are essential for tardigrades to survive these environmental stresses," Boothby said.
While tardigrades might be even more unusual than we previously knew, there's one thing Boothby is sure of: they're not aliens, despite popular speculation on the matter. "They certainly did not come from outer space; they evolved from animals here on Earth," he said.
They might have foreign DNA, but it's terrestrial.