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For the first time, scientists said Thursday that they had bred mice with two genetic fathers, steering around biological hurdles that would otherwise prevent same-sex parents from having offspring.

The researchers also bred mouse pups with two genetic mothers. Those pups matured into adults and had pups of their own, outpacing previous efforts to create so-called bimaternal mice.

“This research shows us what’s possible,” Wei Li, a senior author of the study, said in a statement. Li conducted the work with colleagues at the Chinese Academy of Sciences.


But for now, the notion of reproducing the experiment in humans is more a matter of science fiction than science. The new study, which appeared in Cell Stem Cell, does not indicate that researchers can now or could anytime soon pull off a similar feat with people.

The cells used to make the mouse embryos were profoundly manipulated. The vast majority of the embryos made did not result in births. And none of the bipaternal mouse pups — those with two genetic fathers — survived to adulthood.


Instead, outside researchers said, the study sheds light on the underlying biology that foils mammals from spinning off offspring without sexual reproduction — unlike some reptiles, fish, and amphibians, which are capable of asexual reproduction.

“It really opens your imagination for what you can do in mammals,” said Dr. Nissim Benvenisty, the director of the Azrieli Center for Stem Cells and Genetic Research at the Hebrew University of Jerusalem, who was not involved in the study.

At issue is “genomic imprinting,” an evolutionary feature found in mammals (and also flowering plants) that researchers believe blocks these species from producing progeny without both maternal and paternal DNA.

In our genomes, there are two copies of each gene — one from mom, one from dad — and both get expressed to make us us. But there are some 100 genes where “imprints” stationed along the genome signal one copy to be active and one to be silent.

“The other copy is there and it’s presented and there’s nothing wrong with the DNA sequence,” said Manus Patten, an evolutionary biologist at Georgetown University, who was not part of the new research. “It’s just turned off.”

Mammals still need both sets, though, to have their full suite of genetic instructions. IGF2, for example, is a gene crucial for growth and development, but only the paternal copy is normally active. If we just inherited DNA maternally then, we wouldn’t grow or develop properly; that gene would simply remain off. On the flip side, there are a number of these genes for which we rely on our mothers.

But scientists started challenging nature’s way a decade and a half ago. The trick was to cajole certain maternal genes to act like paternal genes in terms of their activity, or vice versa.

Adult bimaternal mouse (born to two mothers)
A healthy adult bimaternal mouse (born to two mothers) with offspring of her own. Leyun Wang/Chinese Academy of Sciences

In 2004, a team of Japanese researchers for the first time created mice with two mothers by toying with imprint signals, though only one of the 10 mice born in that study — out of more than 400 embryos — grew to adulthood.

To try to improve on past results, the researchers in the new study manipulated imprint instructions even more extensively.

For the bimaternal mice, they started with embryonic stem cells containing a female mouse’s DNA (mother No. 1). These cells were like egg or sperm in that they were haploid — that is, they had half the number of chromosomes of other cells — but were unlike those sex cells in that they had no imprints ready to silence some genes. The stem cells had been grown in such a way that the DNA markers that normally tell certain genes to turn off had been removed.

The scientists then went a step further and deleted three key imprinted regions from the cells’ genomes using the DNA editor CRISPR. Finally, they injected the cells into sex cells from another female mouse (mother No. 2), simulating something akin to fertilization.

Of the 210 embryos created this way, 29 mice were born — just 14 percent. The mice appeared to be normal and even had regular gene expression patterns. By deleting those imprinted regions, the study indicated, the researchers effectively coaxed certain genes from one of the mothers to act as if they had come from a father in terms of their activity.

“That was really sophisticated methodology,” Benvenisty said.

The steps for breeding the bipaternal mice were even more complicated.

Researchers again started with haploid embryonic stem cells, though this time containing paternal DNA (father No. 1), and deleted seven imprinted regions. They then injected the cells along with sperm from another mouse (father No. 2) into eggs that had had their own DNA removed. Finally, these embryos were transferred into surrogate mothers. (So while there was no maternal genetic information involved, female mice still played a role in forming the embryos and carrying them to term.)

Only 12 bipaternal mice, out of 477 embryos, were born, and only two survived more than two days. Neither of those reached adulthood. The researchers also measured gene activity in the pups and found that some of the imprinted genes were not being expressed as if they had been inherited maternally.

The results were instructive in their own way, outside experts said. The study offered additional evidence that imprinting is what prevents mammals from producing offspring without sexual reproduction. And it showed that even with the intensive cellular changes and genome editing, the research team still could not overcome imprinting and reach regular levels of gene expression.

“We can’t yet make bipaternal mice that are viable,” said Marisa Bartolomei, a professor of cell and developmental biology at the University of Pennsylvania Perelman School of Medicine, who did not work on the new study. “It’s because the imprinting is still messed up in these mice.”

Beyond the technical, legal, and ethical roadblocks that would prevent this type of research in people, experts pointed to another concern. If researchers created, say, a daughter from two mothers or two fathers, and if she were healthy and had children of her own, it is unknown what genetic ramifications might be passed onto the next generation.

In this study, for example, the female mice born with two genetic mothers were later mated with regular males. They produced 22 pups from six litters. Thirteen grew into adults themselves, but nine died soon after birth.