Zombie cells in bacteria and the cloning limit in mice: genome transplantation and generation boundaries

Two Nature Briefing stories examine genome engineering in bacteria by transplanting genomes into inactivated recipients to test genome transfer, and reveal a cloning limit in mice after 58 generations due to mutation accumulation. The discussions highlight the potential of synthetic biology as a platform for testing genome engineering while cautioning about genetic instability in cloning across generations.

Overview: genome engineering stories from Nature Briefing

The podcast presents two linked stories about the transfer and testing of genomes in living systems. The first centers on zombie cells, where a dead bacterial cell is used as a host to receive a genome from a sister species, to demonstrate a proof-of-concept for genome transplantation and synthetic biology. The second story shifts to mice and the limits of cloning, detailing how generations of cloned mice accumulate mutations and eventually reach a generation cap.

Zombie cells and bacterial genome transplantation

Historically, researchers reported the creation of the first synthetic cell by chemically synthesizing a full genome, 1.1 million base pairs, of Mycoplasma mycoides and transplanting it into a related species, Mycoplasma capricolum. The resulting organism carried a synthetic genome and an antibiotic resistance marker used to identify successful transplantation. The current study builds on this by using a dead recipient cell as the host, inactivating its genome so it cannot replicate. This approach prevents issues from natural DNA uptake mechanisms that can yield false positives in tests for genome transfer. As one author described, the cell is destined to die, but we give it life because of the genome transfer, illustrating a methodological framework that can be adapted to other bacteria and synthetic DNA work.

"The cell is destined to die, but we give it life" - Nature Podcast

In this setup, the recipient cell’s genome is inactivated, and a genome from the sister species is introduced. Because the cell cannot replicate, concerns about homologous recombination or environmental DNA uptake are minimized, creating a cleaner test bed for genome engineering. The developers emphasize that this is a proof of concept with potential to serve as a general base platform for cross-species chassis testing, possibly with tools like CRISPR to ensure efficient uptake of new genes.

Looking ahead, the hosts discuss how this approach could be extended beyond Mycoplasma to other bacteria, enabling researchers to explore which cellular chassis combinations work best and to study bacterial evolution at the genome level. The conversation also touches on how such platforms could accelerate the development of advanced synthetic biology techniques while highlighting the need for careful design to avoid unintended gene uptake or recombination events.

Cloning limits in mice: a generational boundary

The second story revisits cloning in mammals, focusing on mice and a long-running effort to push cloning to ever greater generations. Since the first cloned mouse in 1997, researchers have sought to determine how many times a cloned lineage can be continued. In 2013, the team reported success for 25 generations, suggesting cloning could be indefinite. More recent work, however, shows a different picture: after 58 generations, cloned mice reach a point where genetic mayhem—an accumulation of mutations and large-scale genome instability—renders new clones nonviable. Mutation rates in these clones were estimated to be about three times higher than in normal mice, with missing or rearranged DNA sections, and, eventually, loss of the X chromosome, contributing to the cessation of cloning.

One of the podcast’s guests notes that some organisms may cope with mutation accumulation through alternative reproductive strategies, such as asexually reproducing fish, hinting at potential workarounds. The discussion also notes practical implications for animal breeding and preservation, suggesting that storing a large collection of cells from the original animal could be more reliable for future cloning than continuing to clone the clones themselves.

In closing, the hosts invite listeners to share insights and perspectives via social media or email, and remind audiences that the content links to the Nature Briefing story whenever they want more detail.

Key takeaways and future directions

The two stories illustrate how genome engineering is increasingly tested using innovative strategies to reduce false positives and to probe the boundaries of mammalian cloning. They underscore the potential of genome transplantation to serve as a versatile platform for testing cross-species genome transfers and for advancing synthetic biology, while also highlighting the genetic stability challenges that limit cloning over generations. Together, they reflect ongoing questions at the intersection of genetics, microbiology, and biotechnology, and raise important considerations for research, policy, and the responsible use of powerful genome manipulation technologies.

"There may be ways to circumvent it" - Nature Podcast

"Genetic mayhem is the name of the game" - Nature Podcast

"store a large number of cells from the original animal to then clone rather than cloning the clone" - Nature Podcast