Scientists in China have created mice with two biological fathers, a milestone that challenges our understanding of mammalian reproduction. While the mice exhibited significant developmental abnormalities and were sterile, the research could one day aid in conserving critically endangered species that are down to just a few specimens (and not that many females).

The achievement builds upon decades of research into a phenomenon known as genomic imprinting, where certain genes are expressed differently depending on whether they are inherited from the mother or the father. This biological quirk has long been seen as a barrier to unisexual reproduction in mammals. But by using CRISPR, a powerful gene-editing tool, researchers have now bypassed some of these barriers — albeit imperfectly.

“This is a notable accomplishment in mammalian genetic engineering,” Zhi-Kun Li, a developmental biologist at the Chinese Academy of Sciences and the study’s first author, told Reuters. Previously, Li’s group had produced the first mice born from two female parents.

CRISPR versus Mammalian Reproduction

Mammals typically require genetic contributions from both a male and a female to produce viable offspring. This is because of imprinted genes, which act like a biological lock, ensuring that embryos develop properly only when they inherit the right “dose” of genes from each parent. Without this balance, development can go awry.

To create mice with two fathers, the researchers targeted 20 of these imprinted genes in male embryonic stem cells, using CRISPR to delete or modify them. They then combined these edited cells with sperm and injected them into egg cells that had their nuclei removed. The resulting embryos were transferred to surrogate mothers, leading to the birth of 134 pups.

But their success was severely limited. Only about 12% of the embryos survived to birth, and more than half of those died before reaching adulthood. The surviving mice were sterile, had craniofacial deformities, and exhibited behavioral abnormalities.

“The bipaternal mice exhibited developmental disorders, including difficulty suckling and a broader facial width-to-length ratio,” said Guan-Zheng Luo, a co-author of the study.

The researchers managed to alleviate some defects by inactivating a specific developmental gene in subsequent attempts. However, the mice still showed signs of abnormal growth and shortened lifespans.

A Step Forward for Science — and Conservation

For some critically endangered species, where finding a mate can be nearly impossible, unisexual reproduction could offer a lifeline. “This technique provides a potential method for producing offspring without relying on complex stem cell differentiation processes,” said Li.

The findings also reveal some novel fundamental insights about genomic imprinting.  Previous research has shown that mice with two mothers (first time achieved in 2018) tend to be smaller and live longer, while the new study shows mice with two fathers grow larger and die sooner. This means that paternal genes may promote growth, while maternal genes restrain it.

“It’s exciting,” Kotaro Sasaki, a developmental biologist at the University of Pennsylvania who was not involved in the study, told MIT Tech Review. “This work helps us better understand the role of imprinting in development and reproduction.”

What does this mean for humans?

These intriguing findings beg the question: could the same be possible in humans? A baby conceived from two male parents? For now, this scenario is destined to fail.

“Editing 20 imprinted genes in humans would not be acceptable,” said Li. “Producing individuals who could not be healthy or viable is simply not an option.”

“I can’t imagine that these experiments could ever be replicated in humans,” added Martin Leeb, a stem cell biologist at the Max Perutz Labs Vienna. “It would be unethical and against the law.”

Still, the researchers in China hope to extend their work to primates, which share more genetic similarities with humans. “We aim to extend the experimental approaches developed in this study to larger animals, including monkeys,” said Li.

The study appeared in the journal Cell Stem Cell.