Written by Wang Cong
Edited by Wang Duoyu
Layout by Shui Chengwen
In 1997, the birth of Dolly the cloned sheep sparked global sensation. Shortly thereafter, Japanese scientist Teruhiko Wakayama successfully created the first cloned mouse.
Since then, Wakayama has continuously pushed the boundaries of mouse cloning. He has successfully cloned mice using cells taken from live mice, dead mice, mice frozen for 16 years, frozen stem cells, and even cells extracted from mouse urine. Currently, he is attempting to clone mice using cells derived from mouse feces.
Decades ago, he and his collaborator (also his wife), Sayaka Wakayama, initiated an experiment: how long could a mouse's life be extended solely through cloning technology? In 2013, having successfully continued cloning for 25 generations, they wrote in a paper published in Cell Stem Cell: "Our results show that repeated iterative recloning is possible and suggest that, with adequately efficient techniques, it may be possible to reclone animals indefinitely."
However, trouble soon arrived. Despite their efforts to maintain consistent experimental conditions, cloning mice became increasingly difficult after the 27th generation. By the 58th generation, they were completely unable to clone any more mice.
In 2026, they published these findings in the journal Nature Communications under the title: Limitations of serial cloning in mammals.
Can cloning technology infinitely replicate a mammalian individual? This study provides a clear answer: No. This is the first experimental demonstration that asexual reproduction is unsustainable for mammals. If mammals attempt asexual reproduction, genetic mutations (especially large-scale mutations like chromosomal deletions) accumulate across generations, ultimately leading to the termination of the cloned lineage.
A 20-Year Life "Replication" Experiment
As early as 2005, Teruhiko Wakayama's team at the University of Yamanashi in Japan launched an unprecedented experiment: they obtained somatic cells from a female mouse (the donor) and used nuclear transfer technology to clone the first generation. They then used somatic cells from this cloned mouse to clone the second generation, and so on, like a "relay race" of life.
Initially, the experiment seemed smooth. The cloning success rate even increased slightly with each generation, reaching 15.5% by the 26th generation. This led the research team to optimistically believe that serial cloning might be able to continue indefinitely.
However, a turning point soon emerged. After the 27th generation, the cloning success rate began to decline continuously. By the 57th generation, the average success rate had plummeted to 0.6%. Ultimately, all cloned mice in the 58th generation died on the second day after birth, forcing the termination of the experiment.
Success rate and lifespan of serially cloned mice.
Over 20 years, the research team started from a single donor mouse, went through 58 generations, performed over 30,000 cloning attempts, and successfully bred more than 1,200 cloned mice, finally reaching the limit of serial cloning for a single mouse.
Normal Appearance, but Mutations Silently Accumulating
Surprisingly, for most of their lives, these "replicas" appeared indistinguishable from ordinary mice: normal body weight, normal lifespan (about 2 years), and even the common placental abnormalities seen in cloned mice did not worsen with generations.
But the real crisis hid deep within the DNA. Through whole-genome sequencing of cloned mice from different generations, the research team discovered that harmful genetic mutations were continuously accumulating with each cloning event. From the 1st to the 57th generation, an average of about 69 single nucleotide mutations and 1.4 small fragment insertions/deletions occurred per generation. More lethally, large-scale chromosomal structural variations also appeared, such as the complete loss of an X chromosome, large fragment deletions, or translocations.
A significant portion of these mutations were located in key functional areas of genes. Data shows that from the 23rd to the 57th generation (the period of declining cloning success rates), the proportion of potentially destructive mutations was significantly higher than in earlier generations. This is like repeatedly photocopying a document: the first few copies are only slightly blurry, but as the blurriness叠加s, key information eventually becomes unrecognizable, rendering the entire document useless.
Sexual Reproduction: Life's Built-in "Error Correction Mechanism"
Since serial cloning hits a dead end, can these cloned mice nearing their limit continue their lineage through traditional "sexual reproduction"? The experiment provided a hopeful answer: Yes.
When female cloned mice from the 50th and 55th generations mated with normal male mice, they were able to conceive successfully, but the litter size dropped sharply to an average of 2-3 pups, far below the normal 10. This indicates that their eggs had accumulated a large number of lethal mutations.
However, miraculously, when the offspring of these cloned mice (produced via sexual reproduction) reproduced themselves, their litter sizes returned to near-normal levels (7 pups), and placental size also normalized. This means that sexual reproduction, through meiosis and fertilization, acts as a precise "proofreading and repair system," clearing most of the genetic abnormalities accumulated during cloning and getting life back on track.
Why is "Sexual Reproduction" Irreplaceable?
This study fundamentally explains why mammals in nature almost never use asexual reproduction methods like cloning to sustain their species. For complex mammals, sexual reproduction, although inefficient, is an evolutionarily necessary strategy:
- Clearing Harmful Mutations: Meiosis and genetic recombination shuffle and dilute accumulated errors, preventing them from destroying the entire population like a "snowball effect."
- Maintaining Genetic Diversity: Diversity is the cornerstone for coping with environmental changes and diseases, whereas cloning only produces genetically identical copies.
This study also sounds a warning bell for cloning technology applications. It implies that using cloning for endangered species recovery or large-scale replication of high-quality livestock may face insurmountable generational limits. Cloning can serve as an emergency measure for preserving genes, but it cannot replace natural reproduction to maintain a healthy, sustainable population.
This twenty-year experiment reminds us that mammals cannot achieve immortality through cloning. That winding path of sexual reproduction, filled with random combinations, may seem inefficient, but it is the most robust strategy life has chosen for itself through long evolution to combat genetic errors and adapt to an uncertain future.
Paper Link:
https://www.nature.com/articles/s41467-026-69765-7