For 20 years, researchers copied one mouse from the cells of the last, preserving the same sex and the same look. Each new birth seemed to promise continuity, until it didn’t anymore.
The colony kept its familiar appearance long after hidden damage had begun to build. In this long-running mouse experiment, each round of serial nuclear transfer reproduced the agouti coat color, while fertility fell and losses mounted, until the 58th generation delivered the last pups born alive. Then nothing.
How one mouse line was copied for two decades
The project began with cells from one female mouse and kept going for 20 years. At the University of Yamanashi, researchers built a cloned female line that stayed strikingly consistent across generations, with female sex and the same agouti coat color appearing again and again.
Each round followed the same rhythm. After a mouse matured, the team used donor cell transfer and repeated nuclear transfer in a steady laboratory breeding cycle, producing about three to four generations a year and, over time, more than 1,200 cloned mice from the original donor.
When the births began to fail, the warning was no longer subtle
For a long stretch, the line looked stable enough to suggest the process could keep going. That impression faded after generation 27, when a declining birth rate started to appear and each new round of cloning delivered fewer live animals than the one before.
The endpoint was blunt. In the generation 58 collapse, the last mice were born alive but died the next day, meaning cloned pups survival had failed outright. At that stage, the experiment no longer hinted at strain. It showed a biological ceiling.
No one has ever continued re-cloning for this long before. As a result, this is the first time we’ve discovered that repeated re-cloning eventually reaches its limits.
Teruhiko Wakayama, developmental biologist at the University of Yamanashi
Mutations accumulated quietly while the mice still looked normal
Outwardly, many of the animals seemed ordinary for years. Yet the paper’s genome sequencing data showed damage building from generation 1 to generation 57, including about 3,700 single-nucleotide variants and 80 indels, or roughly 69.4 SNVs and 1.4 indels per generation.
The pattern went beyond tiny spelling changes in DNA. Researchers also found structural DNA changes and cases of X chromosome loss, signs that the line was drifting genetically even while the mice still looked healthy, fertile enough to breed, and broadly normal inside the animal room.
Fertility dropped long before the line reached its end
Breeding tests exposed the slowdown before the final cloning failure arrived. When late-generation females were paired with normal males, generation 20 still produced about 10 pups per litter, a figure close to the control mice and easy to read as reassuring.
That reading did not last. The study recorded a sharp litter size decline, falling to 2.8 pups in generation 50 and 2.2 in generation 55. After sexual reproduction, the next offspring improved to about 7.0 pups, with placentas resembling those from fertilized embryos more closely.
What this mouse study says about the limits of repeated cloning in mammals
After 20 years and 58 generations, the paper’s message is plain. Serial cloning could keep a mouse line going for a remarkably long time, yet it also stacked accumulated genetic defects that gradually weakened birth outcomes, fertility, and long-term continuity.
The partial sexual reproduction recovery sharpened that contrast. Meiosis and fertilization corrected part of the damage, while serial nuclear transfer did not, pointing to clear mammalian reproduction limits when cloning is repeated again and again from the same lineage without a natural genetic reset.
