Countless scientists and enough sci-fi comedies over the last three decades have speculated on the potentially degrading effect of making clones from other clones, endlessly. The same year that researchers first cloned Dolly the sheep in 1996, the Michael Keaton comedy “Multiplicity” compared the process to the visual “noise” that occurs when the Xerox copier in the office makes copy after copy after copy, Gizmodo recalls.
Now, biologists in Japan have established what they believe may be a real limit to the number of successful, viable clones that can be created from successive generations of clones.
Drawing on 20 years of their own research into serial cloning of mice, the team hoped to be able to produce an infinite number of clones of clones with the help of a promising additive called trichostatin A, which helps suppress genetic mutations during the cloning process.
“We initially concluded that serial cloning could continue indefinitely because the success rate increased slightly with each successive generation,” noted Teruhiko Wakayama and colleagues in the new study published Tuesday in the journal Nature Communications.
Everything went according to their expectations until the researchers reached generations 25-27 of mice. By the 58th generation, the mice no longer survived more than a day.
Thousands of healthy clones
Scientists have traditionally used trichostatin A as an antifungal antibiotic, but it can also inhibit the function of certain enzymes in mammals, such as mice and humans. For Wakayama and his team's purposes, however, the compound also acts as what they call an “epigenetic modification agent.” Essentially, it suppresses unwanted DNA transcription factors, that is, proteins that activate segments of potentially harmful mutations in a mouse's genetic code during cloning.
“More than 1,200 cloned mice were produced from a single original donor,” explained Wakayama, a developmental biologist at Yamanashi University's Center for Advanced Biotechnology, and the other members of the research team. And surprisingly, most of the clones did well before the decline.
The researchers found that these clones mostly developed with healthy and intact reproductive organs, “raising the possibility that subsequent generations could be produced through sexual reproduction.”
Their study suggests that further experiments with reagents such as trichostatin A could further extend cloning over successive generations.
A steep decline in clone viability
However, Wakayama's team also measured some hard facts about the number of natural mutations that occurred between each successive generation of clones. Each new round of cloned mice accumulated about 70 small “single nucleotide variants” and about 1.5 additional, more substantial “structural variants” in their genetic code.
Although this rate was not unusual, these structural variations accumulated over several recloning cycles.
Over time, they found, “the accumulation of deleterious variants appears to have overwhelmed adaptive effects,” in the absence of chromosomal recombination specific to sexual reproduction, which could filter out large and potentially harmful genetic variation.
And here, evidence emerged that even a partial return to sexual reproduction could correct these problems. For example, cloned mice in later generations were born with visible placental abnormalities, but when these mice were mated naturally, their offspring's placentas returned to normal.
There are many theories as to why mammals and other organisms evolved to reproduce sexually, including the idea that this type of reproduction helped early species protect themselves from parasites by providing robust genetic diversity.
Wakayama and his team's journey to the extreme limits of genetic recloning confirms that mammals need mating to protect their genomes from degradation over time.
