PUBLISHEDYou can’t clone a mouse forever and expect the resulting animals to remain healthy, according to a long-running study that finds sexual reproduction is necessary to prevent large-scale genetic mutations from accumulating in cloned mammals.
Teruhiko Wakayama at the University of Yamanashi in Japan and his colleagues previously reported that serial cloning – where you clone an animal, then clone its clone, and repeat this process again and again – can be done for up to 25 generations without affecting the resulting animals’ health.
After continuing the experiment for a further 22 generations, however, the researchers found mutations to the animals’ DNA accumulated, coinciding with a declining birth rate, and by the 58th generation – reached after the experiment had been running for nearly 20 years – the cloned mice didn't survive after birth.
“The success rate had been gradually increasing up to the 26th generation,” Wakayama told IFLScience. “We believed that if we continued re-cloning beyond the 27th generation, the success rate might improve even further.”
When they saw that it was actually declining, Wakayama said he “nearly lost motivation”. But the team continued their experiment to monitor how the trend progressed and investigate what was causing it. To do so, they sequenced the whole genomes of the cloned mice, finding they had an unusual number of large structural variants, a kind of mutation characterized by major differences in the structure of an organism’s chromosomes.
We have absolutely no idea why clones have three times as many mutations as naturally bred mice.
Teruhiko Wakayama
These mutations had probably been accumulating from the very first clone, but the researchers suspect that prior to the 27th generation, the negative effects had been outweighed by positive selection, as the individuals who most readily produced viable clones would be selected for each generation, somewhat improving the success of the next.
As the generations continued beyond this point, however, the fertility of the cloned mice decreased, and the researchers also observed changes to the structure of the placenta. When late-generation cloned mice were allowed to mate naturally, though, their grand-offspring had improved fertility and their placentas looked very similar to those of uncloned mice, suggesting sexual reproduction is truly important for maintaining these traits.
“We have absolutely no idea why clones have three times as many mutations as naturally bred mice,” said Wakayama, though he suspects it may have something to do with the physical process by which clones are produced. Each time a mouse is cloned, the nucleus of one of its body cells must be extracted using a glass pipette whose diameter is smaller than that of the nucleus itself – meaning it is possible the nucleus becomes physically damaged during the process.
Wakayama said he became interested in cloning because of the idea it could be used to mass-produce more productive livestock, such as cows that make large amounts of milk or cattle with superior meat quality. While this research shows there could be a limit to this process, he doesn’t believe it invalidates the idea, as you could instead store large numbers of the original animal’s cells and use these as a source for new cloned individuals, without needing to approach the 57-generation limit.
Indeed, Wakayama's research group has previously investigated novel ways for storing cells for this purpose by freeze-drying them.
The study is published in Nature Communications.
