A team of Beijing-based biologists have bred healthy mice from same-sex parents for the first time. The female-female reproduction produced healthy pups, but male-male reproduction resulted in offspring with a range of health issues.

"We were interested in the question of why mammals can only undergo sexual reproduction,” Qi Zhou, a biologist at Chinese Academy of Sciences Institute of Zoology, said in a statement. “We tried to find out whether more normal mice with two female parents, or even mice with two male parents, could be produced.”

Although asexual reproduction is found in many animals, it doesn’t ever occur naturally in mammals. When a male and a female mouse reproduce, each contributes 20 chromosomes in a sperm and an egg, respectively, and the cells of the resulting baby mouse have a total of 40 chromosomes.

The offspring receives two copies of some genes—one from the mother and one from the father.

Image: Leyun Wang

Yet for a select few genes, the offspring only gets a single copy from either the egg or sperm in what’s known as “genomic imprinting.” It’s the reason that same-sex reproduction is difficult to induce in mammals since offspring that don’t receive the genes unique to a sperm or an egg cell may end up developing abnormally.

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Although researchers have bred mice from female-female parents in the past, the pups always had developmental abnormalities. In these cases, researchers had deleted imprinted genes from immature eggs with the full 46 chromosomes.

As detailed in a paper published Thursday in Cell Stem Cell, the Chinese researchers avoided the abnormalities in female-female breeding by using haploid embryonic stem cells (HSC) to induce same-sex reproduction. HSCs are a type of cell that only carry half of the normal number of chromosomes.

In one experiment, the researchers generated HSCs using material from a female mouse, and deleted the main imprinting regions in the cell’s DNA were deleted. Then, they injected the HSC into a mouse egg, effectively replacing the role of sperm.

Out of 210 embryos produced by female-female parents, the researchers produced 29 healthy mice that lived to adulthood and had pups of their own.

Things didn’t go quite so well for the mice produced by male-male breeding. The researchers had to delete more imprinting regions from the HSCs generated from male mice before injecting it into sperm from another mouse. The sperm was then injected into an egg cell that had all its genetic material removed.

Out of 1,023 embryos produced by male-male parents, only 12 pups were delivered. These were short lived, however. All died within 48 hours, were swollen, and had abnormally large tongues.

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