They Shouldn't Have Don't It

Key Points:

• Researchers at the University of Yamanashi conducted a 20-year serial cloning experiment with mice, repeatedly cloning from a single female donor to test if mammal cloning could sustain itself over many generations without biological decline.
• The study found that while the first 25 generations appeared healthy, birth rates began declining from the 27th generation, and cloning failed entirely by the 58th generation, with all mice dying shortly after birth.
• Genome sequencing revealed a gradual accumulation of genetic mutations across generations, including single-nucleotide variants, insertion-deletions, structural changes, and loss of one X chromosome, occurring at a rate three times higher than in naturally bred offspring.
• Fertility in cloned mice dropped significantly by the 50th to 55th generations, with litter sizes shrinking from about 10 pups to fewer than 3, indicating reproductive decline before cloning ultimately failed.
• Notably, offspring produced through sexual reproduction from late-generation clones showed partial recovery in fertility and placental size, suggesting that natural reproduction can help correct some accumulated genetic damage from cloning.