Water bears are genetic mash-ups

Water bears may be the ultimate borrowers.

The hardy, microscopic animals also known as moss piglets and technically called tardigrades have scavenged about 17.5 percent of their genes from other creatures. The ability to pick up used genes and spare parts from other organisms’ DNA junkyards may allow tardigrades to survive extreme stress, such as desiccation, radiation and even a trip to space and back, researchers report online November 23 in the Proceedings of the National Academy of Sciences.

Bacteria routinely swap DNA, a process known as horizontal gene transfer. But researchers had thought the practice was unusual in animals. Only strange, asexual animals called bdelloid rotifers were known to grab significant amounts of DNA from other life.

So when biologist Thomas Boothby of the University of North Carolina at Chapel Hill and colleagues discovered bacterial genes mixed in to the DNA of Hypsibius dujardini tardigrades, “our initial thought was, ‘this is actually a mistake. We have contamination in our samples.’” But further testing showed that the tardigrades had incorporated into their own DNA genes from more than 1,300 bacterial species, 40 archaea, 91 species of fungus, 45 plant species and six viruses.

Desiccation may allow tardigrades and bdelloid rotifers to pick up extra DNA, the researchers speculate. When an organism dries out, its DNA breaks up and the membranes that hold in a cell’s guts become leaky, allowing DNA and other big molecules to seep out. For most organisms, that’s the end: Bacteria, plants and other creatures spill their DNA and die. But some tardigrades and rotifers can remain in suspended animation until they encounter water again. Upon rehydration, the microscopic animals might quickly sew their DNA back together, accidentally stitching in foreign DNA discarded when neighboring organisms died.

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Molecular cell biologist Chiara Boschetti of the University of Cambridge and colleagues have uncovered supporting evidence for that hypothesis. Horizontal gene transfer happens more often in bdelloid rotifers that undergo desiccation than in those that live in aquatic environments and can’t stand to be dried out, the researchers report online November 4 in BMC Biology.

Many of the genes that tardigrades have borrowed help them deal with stress. For instance, catalase proteins are antioxidant enzymes that help organisms avoid cell damage from reactive oxygen molecules. Most animals have their own versions of genes that produce catalase enzymes, but all of H. dujardini’s catalases are from foreign genes, Boothby and colleagues found. Alien genes also outnumber tardigrade genes dedicated to repairing damaged DNA.

“How these genes from different species interact is a puzzle of molecular biology,” says David Mark Welch, an evolutionary biologist at the Marine Biological Laboratory in Woods Hole, Mass. Many of the junkyard genes probably don’t work in tardigrades and are thrown away. Researchers are discovering only the ones that have proven useful to the water bears, which suggests tardigrades may swap DNA more often than indicated by the new finding, Mark Welch says.

Rotifers are completely asexual creatures, and some researchers have speculated that horizontal gene transfer allows them to bring in fresh infusions of genetic material that other organisms would get from sexual reproduction. “This ability to take up genes willy-nilly from the environment makes sex redundant,” says Michael Syvanen, a molecular evolutionary geneticist at the University of California, Davis School of Medicine.

Tardigrades, too, are almost all female and reproduce asexually. New tardigrades grow from unfertilized eggs, a process called parthenogenesis. Horizontal gene transfer may allow tardigrades to swap out damaged DNA and acquire new functions without the need for sex, Syvanen says.

As researchers discover more and more animals that visit genetic swap meets, people may need to change the way they think about evolutionary relationships, Boothby says. “Instead of a tree of life where organisms split and never really reconnect, it’s a web of life with frequent exchange of genetic information.”