Media release

From: Cell Press

First mouse with two male parents to reach adulthood

A team of stem cell scientists have successfully used embryonic stem cell engineering to create a bi-paternal mouse—a mouse with two male parents—that lived until adulthood. Their results, publishing on January 28, 2025, in the Cell Press journal Cell Stem Cell, describe how targeting a particular set of genes involved in reproduction allowed the researchers to overcome previously insurmountable challenges in unisexual reproduction in mammals.

Scientists have attempted to create bi-paternal mice before, but the embryos developed only to a certain point and then stopped growing. Here, the investigators, led by corresponding author Wei Li of the Chinese Academy of Sciences (CAS) in Beijing, focused on targeting imprinting genes, which regulate gene expression in a number of ways. “This work will help to address a number of limitations in stem cell and regenerative medicine research,” says Li.

“The unique characteristics of imprinting genes have led scientists to believe that they are a fundamental barrier to unisexual reproduction in mammals,” says co-corresponding author Qi Zhou, also of CAS. “Even when constructing bi-maternal or bi-paternal embryos artificially, they fail to develop properly, and they stall at some point during development due to these genes.”

Earlier attempts to make a bi-paternal mouse used ovarian organoids to derive oocytes from male pluripotent stem cells; those ooctyes were then fertilized with sperm from another male. However, when the homologous chromosomes—the chromosomes that divide during meiosis to create oocytes and sperm—originated from the same sex, imprinting abnormalities arose, leading to severe developmental defects.

In this study, the researchers modified 20 key imprinting genes individually using a number of different techniques, including frameshift mutations, gene deletions, and regulatory region edits. They found that not only did these edits allow the creation of bi-paternal animals that sometimes lived to adulthood, but they also led to stem cells with more stable pluripotency.

“These findings provide strong evidence that imprinting abnormalities are the main barrier to mammalian unisexual reproduction,” says co-corresponding author Guan-Zheng Luo of Sun Yat-sen University in Guangzhou. “This approach can significantly improve the developmental outcomes of embryonic stem cells and cloned animals, paving a promising path for the advancement of regenerative medicine.”

The researchers note several limitations that their work still needs to address. For one thing, only 11.8% of the viable embryos were capable of developing until birth, and not all the pups that were born lived to adulthood due to developmental defects. Most of those that did live to adulthood had altered growth and a shortened lifespan. Also, the mice that lived to adulthood were sterile, although they did exhibit increased cloning efficiency.

“Further modifications to the imprinting genes could potentially facilitate the generation of healthy bi-paternal mice capable of producing viable gametes and lead to new therapeutic strategies for imprinting-related diseases,” says co-corresponding author Zhi-Kun Li of CAS.

The team will continue to study how modifying imprinting genes may lead to embryos with higher developmental potential. They also aim to extend the experimental approaches developed in mice to larger animals, including monkeys. However, they note that this will require considerable time and effort because the imprinting gene combinations in monkeys differ significantly from those in mice. Whether this technology will ultimately be applied towards solving human disease remains unclear. The International Society for Stem Cell Research's ethical guidelines for stem cell research does not allow heritable genome editing for reproductive purposes nor the use of human stem cell-derived gametes for reproduction because they are deemed as currently unsafe.