Scientists have known for centuries about the tardigrades’ ability to dry themselves out. But a new study suggests that this ability might have contributed to their superlative endurance in a strange and roundabout way. It makes them uniquely suited to absorbing foreign genes from bacteria and other organisms—genes that now pepper their genomes to a degree unheard of for animals.
Thomas Boothby from the University of North Carolina at Chapel Hill made this discovery after sequencing the first ever tardigrade genome, to better understand how they have evolved. Of the 700 species, his team focused on Hypsibius dujardini, one of the few tardigrades that’s easy to grow and breed in a lab.
At first, Boothby thought his team had done a poor job of assembling the tardigrade’s genome. The resulting data was full of genes that seemed to belong to bacteria and other organisms, not animals. “All of us thought that these were contaminants,” he says. Perhaps microbes had snuck into the samples and their DNA was intermingled with the tardigrade’s own.
But the team soon realized that these sequences are bona fide parts of the tardigrade’s genome.
That wouldn't be unusual for bacteria, which can trade genes with each other as easily as humans might swap emails. But these “horizontal gene transfers” (HGT) are supposedly rare among animals. For the longest time, scientists believed that they didn't happen at all, and reported cases of HGT were met with extreme skepticism.
Recently, more and more examples have emerged. Ticks have antibiotic-making genes that came from bacteria. Aphids stole color genes from fungi. Wasps have turned virus genes into biological weapons. Mealybugs use genes from many different microbes to supplement their diets. A beetle kills coffee plants with a borrowed bacterial gene. Some fruit flies have entire bacterial genomes embedded in their own. And one group of genes, evocatively called Space Invaders, has repeatedly jumped between lizards, frogs, rodents, and more. But in all of these cases, it's usually one or two genes that have jumped across. At most, the immigrants make up 1 percent or so of their new native genome.
But Boothby found that foreign genes make up 17.5 percent of the tardigrade's genome—a full sixth. More than 90 percent of these come from bacteria, but others come from archaea (a distinct group of microbes), fungi, and even plants. “The number of them is pretty staggering,” he says.
Claims like these have been debunked before, so the team took extra care to confirm that the sequences did indeed come from outside sources.
For a start, they re-sequenced the genome using PacBio—a system that decodes single unbroken strands of DNA without first breaking them into smaller fragments. This revealed that the foreign genes are physically linked to the tardigrade’s native ones. They are all part of the same DNA strands, which means they couldn't have come from other contaminating microbes. They have also gained several features that are characteristic of animal genes, like an animal gloss over their fundamental bacterial character. John Logsdon from the University of Iowa, who studies genome evolution, is certainly convinced. “It’s a very interesting and technically robust paper,” he says.