In the damp, shifting micro-worlds of mosses and lichens, where conditions swing wildly, lives a tiny animal with an almost unbelievable survival trick. It is not the tardigrade, but its stranger companion: the bdelloid rotifer. Like tardigrades, bdelloid rotifers can dry out almost completely and then revive when water returns. They can also withstand radiation doses hundreds of times higher than what would kill a human, even when their chromosomes are shattered. Now, scientists have found that one bdelloid rotifer can do something never documented in any other organism, tardigrades included: a species called Adineta vaga can reproduce without first reassembling its chromosomes after radiation damage, and its offspring continue repairing that damage over subsequent generations.
A Discovery That Seemed Impossible
“What’s really exceptional in this discovery is that somehow they stabilize the ends of the chromosome; they stabilize these DNA pieces, and then they can transmit it to the next generation and then continue the repair,” evolutionary biologist Karine Van Doninck of the Université Libre de Bruxelles in Belgium told ScienceAlert. “That’s never been shown, that you have this repair across generations.”
The finding emerged from an earlier experiment that produced a signal so unexpected the researchers thought something had gone wrong. The team had exposed A. vaga to powerful radiation, collected an egg laid by one of the animals, and used that egg to establish a clonal population. Because all the clones came from the same egg, the researchers expected to see identical radiation damage across their genomes. Instead, they found large deletions that varied between individuals, and the sequencing data seemed to show the missing DNA being progressively restored.
“It suggested a progressive repair, and we thought that’s impossible,” Van Doninck said.
At first, the team suspected a sequencing error. But when they repeated the analyses, the strange pattern remained. The ragged edges of the deletions offered a clue: they differed between individuals, suggesting that after the chromosomes broke, the exposed DNA had continued to degrade—to different extents in different individuals—before repair began.
Tracking Damage Through Generations
Over the following months, the researchers followed the damaged genomes through successive generations, using DNA sequencing and chromosome counts to understand what was happening. Gradually, they realized that the rotifers were not repairing most of the chromosome damage before reproducing. Broken chromosome fragments were surviving into the next generation, where the repair process could continue.
“Indeed, this was totally unexpected, and it took us some time to figure out what was happening, and we really had to pinch ourselves to believe it,” molecular biologist Bernard Hallet of the Université Catholique de Louvain in Belgium told ScienceAlert.
The damage was far from minor. The researchers exposed A. vaga to proton radiation at doses of 100, 250, and 500 grays, estimated to cause around 88, 220, and 440 double-strand DNA breaks across the genome, respectively. At the highest dose, a single chromosome could be shattered into as many as 30 to 50 pieces.
How the Rotifer Survives Shattered Chromosomes
Ordinarily, such breaks pose a major problem. During cell division, including the meiosis that produces reproductive cells, chromosomes rely on a region called the centromere to pull genetic material into the new cell. Without a centromere, a fragment cannot be delivered to the daughter cell.
A. vaga, however, appears to have found a loophole. “Our results suggest that, in contrast, Bdelloid rotifer chromosomes have a ‘holocentric’ structure with key structural components of the centromere being spread along separate regions across the chromosome—which ensures that chromosome fragments containing these components can be segregated properly,” Hallet explained.
That means when a chromosome shatters, its fragments may still retain what they need to be pulled into a daughter cell, allowing the broken pieces to persist across cell divisions and, ultimately, generations.
The Repair Mechanism: BIHER
But that is only half of the trick. The other half is the repair itself. A. vaga has a diploid genome, meaning its chromosomes come in pairs. The two chromosomes in each pair are homologous—they carry the same genes in the same locations, though the precise versions of those genes can differ. That gives a damaged chromosome a relatively intact ‘twin’ to use as a repair template.
Although the rotifer reproduces asexually, it has retained an unusual form of meiosis in which homologous chromosomes pair up. A broken DNA end can therefore seek out the matching region on its intact homolog and use that sequence to reconstruct what has been lost. The researchers call this mechanism break-induced homologous extension repair, or BIHER.
If the damage is severe enough, the repair takes multiple rounds—successive generations of rotifers all chipping away at the problem. The findings suggest that, eventually, the damaged chromosome can be completely repaired.
“Our results indeed show that reiterative BIHER can potentially restore the original structure of a broken chromosome, with some deletions being completely eliminated after a certain number of generations,” Hallet said. “In other words, given sufficient time, the missing sequence can be progressively restored.”
Why a Moss-Dweller Needs Radiation Defense
At this point, one might wonder what A. vaga does in its spare time that requires such a potent defense against ionizing radiation. The answer lies in its mossy habitat. Mosses and lichens can dry out for long periods before becoming wet again, and their tiny inhabitants have had to evolve ways to survive those extremes. Bdelloid rotifers, like tardigrades, do this by entering a state of almost complete desiccation.
Drying out, however, is extraordinarily hard on DNA. It can cause the same kinds of physical damage as ionizing radiation, including the double-strand breaks that shatter chromosomes. So A. vaga probably did not evolve its repair abilities in response to radiation at all. Its radiation resistance appears to be a fortuitous side effect of adapting to survive repeated bouts of desiccation.
“The physical damage of desiccation is by chance the same as radiation, and so they became radiation resistant,” Van Doninck explained. “That’s the beauty of nature.”
Implications for Mars and Cancer Research
The ability to survive while its genome is in disarray could make A. vaga useful for understanding other situations in which chromosomes undergo catastrophic damage. On Mars, for example, human explorers will likely experience radiation doses considered dangerous for humans. And cancer cells can experience a phenomenon called chromothripsis, in which chromosomes shatter and are stitched back together in dramatically rearranged forms.
“Sometimes I say rotifer is like a living cancer, so it’s a very interesting model system for all this research,” Van Doninck said.
There are still questions to answer. The researchers now want to better understand the molecular machinery that allows A. vaga to pull off its extraordinary repairs, and whether similar strategies might be hiding elsewhere in nature. Van Doninck also noted that the team is investigating what happens when both homologous chromosomes are broken at the same location, leaving the rotifer without the intact template BIHER normally relies on.
This tiny microscopic creature is probably living in your backyard—and yet it has a remarkable self-repair superpower that could teach us something about survival in the most extreme circumstances.
“The concept of transgenerational transmission and repair of broken chromosomes was totally unexpected,” Hallet said. “I consider it as one of the nicest discoveries of my career.”
The findings have been published in Science Advances.
