Biological Limit Discovered for Serial Mammal Cloning

A two-decade study by Japanese researchers has established a biological limit to the serial cloning of mammals, revealing that repeated cloning leads to the accumulation of fatal genetic mutations. The research, conducted at the University of Yamanashi, demonstrated that a lineage of cloned mice could only be sustained for 58 generations before all offspring died shortly after birth.

"A lineage of cloned mice could only be sustained for 58 generations before all offspring died shortly after birth, challenging previous assumptions about the indefinite potential of current cloning technologies."

Study Overview and Methodology

Initiated in 2005, the study involved serially cloning a single female mouse for 58 generations, producing over 1,200 mice from more than 30,000 cloning attempts. Researchers employed Somatic Cell Nuclear Transfer (SCNT), the same technique used to clone Dolly the sheep in 1996 and Cumulina the mouse in 1998. This process involves transferring the nucleus from a donor cumulus cell into an egg cell whose own nucleus has been removed. The findings were published in the journal Nature Communications.

Initial Optimism to Revised Reality

Early in the study, preliminary results published in 2013, covering the first 25 generations, suggested that the re-cloned mice appeared healthy, had normal lifespans, and that cloning could potentially continue indefinitely. However, 13 additional years of research and genetic sequencing led to a revised conclusion. Teruhiko Wakayama, a developmental biologist at the University of Yamanashi and senior author of the study, noted that genetic sequences were not examined in the earlier research.

The Stealthy Accumulation of Genetic Mutations

While no outward physical abnormalities were observed through the first 25 generations, genetic analysis revealed significant degradation over time. Key genetic issues included:

• The accumulation of approximately 70 new mutations in each subsequent generation.
• A loss of the X chromosome, which became a prominent issue after the 25th generation.
• The frequency of deleterious mutations nearly doubled by the 57th generation.
• Catastrophic chromosomal damage, including inversions and incorrect attachments, became evident starting around the 27th generation.
• Mutations were found to occur at a rate three times higher than in offspring born through natural mating.

Researchers described the process as similar to repeatedly copying a picture, where quality deteriorates with each subsequent duplication.

Collapsing Success Rates and Inevitable Demise

Initially, cloning success rates improved. However, beyond the 27th generation, the birth rate of clones began to collapse. By the 57th generation, the survival rate of cloned embryos was 0.6 percent. Ultimately, all mice of the 58th generation died within a day of birth, marking the end of the serial cloning lineage. Despite the fatal genetic issues, the 58th-generation clones did not exhibit visible physical abnormalities.

Declining Fertility and the Power of Sexual Reproduction

The study also observed a decline in fertility among the cloned mice. Clones up to the 20th generation produced litter sizes similar to naturally reproducing female mice (approximately 10 babies per litter). However, subsequent generations began having significantly smaller litters, reflecting the impact of accumulating mutations.

Interestingly, when female mice from the 50th and 55th generations were bred with normal male mice, the subsequent generations produced through sexual reproduction showed increased litter sizes, suggesting a recovery in fertility through genetic recombination.

Theoretical Alignment and Future Implications

The study's findings align with Muller's ratchet theory, which predicts that deleterious mutations will accumulate in asexual lineages due to the absence of a mechanism, such as genetic recombination in sexual reproduction, to remove them. In mammals, sexual reproduction plays a crucial role in countering and filtering out harmful genetic mutations.

The research concludes that current cloning technology cannot create an infinite number of mammalian copies. Wakayama stated, "This is the first time the limits of repeated re-cloning have been discovered." He added that mammals, unlike plants and lower animals, cannot sustain their species through cloning due to this accumulation of mutations, and that fundamental improvements to nuclear transfer technology are needed to overcome this limitation.

For applications such as preserving superior livestock, scientists suggested that storing a large number of somatic cells might be more advisable than relying on repeated serial cloning across generations.

The study underscores the risks and limitations inherent in current cloning technology and emphasizes the critical role of sexual reproduction in maintaining the genetic health and long-term survival of mammalian species.