How the tardigrade can resist radiation that would kill us

SAll the mosses in our gardens hide creatures as tiny as they are astonishing. Less than 1 millimeter long, eight clawed legs and a plump body: tardigrades may well have been nicknamed “water bears”, they don't look like much. However, the approximately 1,500 species of the group, present from the bottom of the oceans (– 4,500 meters) to the summits of our mountains (+ 6,000 meters) and from the poles to the equator, hide unequaled capacities for resistance.

The researchers subjected them to the worst abuse, immersing them in liquid helium (–270°C) or in an oven at 150 degrees, the tardigrades survived. They applied phenomenal pressures, several thousand atmospheres… without damage. Depriving them of water does not affect them either: they then enter a vegetative state which can last several decades.

However, what fascinates scientists the most remains the ability of these critters to withstand ionizing rays. For humans, the lethal dose of X-rays is 8 grays, that of gamma rays is 4 grays. The tardigrades receive respective doses of 5,700 and 5,000 grays. A mystery that research has been tracking for decades. A Chinese team has just published in the journal Scienceon October 25, a particularly enlightening article. It shows, in fact, that to defend itself the animal actually has a variety of weapons at its disposal.

“Discovered by chance”

To begin with, Chinese scientists deciphered the genome of a new species of tardigrades, which they named Hypsibius Henanensis. “We discovered her by chance, she happened to be very resistant to radiation and easy to raise, so we chose her as a model”says Lingqiang Zhang, molecular biologist at the National Protein Study Center in Beijing, one of the team coordinators. The researchers subjected the animal to high doses of gamma rays and observed the changes in the genes. They identified 2,801, for which the expression, in other words the production of proteins, was modified.

They then classified these changes and the type of genes involved, and thus identified “three different radiotolerance mechanisms”. Finally, in each of these cases, they highlighted the most active gene, the origin and mode of action of which they detailed. Their most spectacular result is called Doda1. First by its origin: this gene was acquired by horizontal transfer from a bacteria. Upon contact with the micro-organism, the animal would have simply picked up a gene. The event is not exceptional, but it becomes so when it comes to acquiring such an important function.

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