A microscopic animal can have chromosomes reduced to dozens of pieces, reproduce despite this damage, and leave its descendants to continue the repairs.
This survivor is called Adineta vaga. It is a bdelloid rotifer, a tiny animal that lives notably in mosses and lichens. Researchers exposed it to high doses of radiation to cause breaks in its DNA. They then tracked its chromosomes and those of its descendants.
DNA fracturing.
Pixabay
The damage was considerable. At the highest dose, 500 grays, scientists estimate around 440 breaks simultaneously affecting both DNA strands. A chromosome could then be fragmented into 30 to 50 pieces. Yet the animals could still produce offspring.
The most surprising part comes next. The chromosome fragments are not all repaired before reproduction. Some pass into the reproductive cells and are then found in the descendants. Restoration then continues over the generations, with missing sections of DNA that can gradually be restored.
To understand this feat, we need to look at the rotifer's unusual chromosome structure. In many animals, a specific region ensures that a chromosome is correctly distributed during cell division. In Adineta vaga, the elements needed for this process are spread across several regions. Even a fragment can therefore retain what it needs to be passed on to a new cell.
The lost DNA must then be rebuilt. The rotifer's chromosomes function in pairs, and their sequences are highly similar. When one end breaks, it can use the corresponding region on the other chromosome as a template. Researchers have named this mechanism BIHER. Several cycles may be necessary before recovering a structure close to the original.
This ability could stem from a problem very different from radiation. Bdelloid rotifers withstand almost complete desiccation when their habitat dries out. Losing that much water can also break DNA. A capacity developed to survive these dry periods could therefore explain their remarkable resistance to ionizing radiation.
Researchers now want to determine precisely which molecules make it possible to stabilize and then repair these fragments.
Another experiment could answer a new question: what happens when both chromosomes in the same pair break at exactly the same location? In that case, the intact template normally used to rebuild the sequence is no longer available. To be continued...