"Foreign" DNA is not a foreign concept to scientists. Through a process called horizontal gene transfer, any organism can theoretically swap genes with another. It happens among bacteria all the time, which is how antibiotic resistance spreads so quickly. But it's less common in more complex, multicellular organisms. Most animals end up with a genome that's over 99 percent homegrown.
The tardigrade blows these averages — as well as the previous record holder for foreign DNA, the rotifer — out of the water. With a genome that's one-sixth foreign, the water bear has around double the outside contributions of a rotifer.
"We had no idea that an animal genome could be composed of so much foreign DNA," co-author Bob Goldstein of the University of North Carolina Chapel Hill said in a statement. "We knew many animals acquire foreign genes, but we had no idea that it happens to this degree."
All told, the little water bears have about 6,000 genes that come from outside sources — mainly bacteria. Plants, fungi, and Archaea also made appearances in the tardigrade genome.
These bacterial genes may be the secret to the water bear's incredible resilience. I mean, when is the last time you met an animal that could survive being dehydrated for years at a time?
That dehydration may even be the key to all this DNA thievery. The researchers suggest that the process of becoming dehydrated might break apart tardigrade DNA, leaving it open to visitors as the organism later rehydrates. Anything nearby could leave genes behind that would then be incorporated into the animal's DNA, allowing it to pass bacterial and other genes on to its descendants. Tardigrades with the most useful foreigners would be more biologically fit, so those genes would quickly become common in the population.
These tardigrades give us a lot to think about. An animal's genome is far from sacred, and the tree of life is pretty darn tangled up.
"We think of the tree of life, with genetic material passing vertically from mom and dad," first author Thomas Boothby said in a statement. "But with horizontal gene transfer becoming more widely accepted and more well known, at least in certain organisms, it is beginning to change the way we think about evolution and inheritance of genetic material and the stability of genomes. So instead of thinking of the tree of life, we can think about the web of life and genetic material crossing from branch to branch. So it's exciting. We are beginning to adjust our understanding of how evolution works."