"We have made several findings in the past by combining reproduction and regeneration, so we tried to find out whether more normal mice with two female parents, or even mice with two male parents, could be produced using haploid embryonic stem cells with gene deletions."
While some reptiles, amphibians, and fish could reproduce with one parent of the same sex, it was still challenging for mammals to do the same even with the help of fertilisation technology.
In mammals, because certain maternal or paternal genes were shut off during germline development by a mechanism called genomic imprinting, offspring that didn't receive genetic material from both a mother and a father might experience developmental abnormalities or might not be viable.
By deleting these imprinted genes from immature eggs, the researchers produced "bi-maternal" mice – that's mice with two mothers - in the past.
"However, the generated mice still showed defective features, and the method itself is very impractical and hard to use."
To produce their healthy bimaternal mice, Zhou and colleagues used what are called haploid embryonic stem cells (ESCs), which contain half the normal number of chromosomes and DNA from only one parent, and which the researchers believe were the key to their success.
The researchers created the mice with two mothers by deleting three imprinting regions of the genome from haploid ESCs containing a female parent's DNA and injected them into eggs from another female mouse.
They produced 29 live mice from 210 embryos. The mice were normal, lived to adulthood, and had babies of their own.
Twelve live, full-term mice with two genetic fathers were produced using a similar but more complicated procedure.
Haploid ESCs containing only a male parent's DNA were modified to delete seven key imprinted regions.
The edited haploid ESCs were then injected - along with sperm from another male mouse - into an egg cell that had its nucleus, and therefore its female genetic material, removed.
This created an embryo containing only genomic DNA from the two male parents. These embryos were transferred along with placental material to surrogate mothers, who carried them to term.
Dr Tim Hore, of the University of Otago's Department of Anatomy, said while mice with two mothers were created in the early 2000s, this achievement was unique for the range of technology used.
"Yet, the work does fall short of creating mammalian offspring from the same sex in the absence of substantial genetic modification, meaning it is unlikely to be useful in humans – for now," he said.
"In order for same-sex parents to both have genetic contributions to their children in an assisted reproduction setting, it is likely another technological leap will be required.
"One possible approach is using 'epigenetic-editing' on haploid stem cells, essentially reprogramming the DNA of one parent so it looks like that of the opposite sex without altering any genetic sequence."
Dr Teresa Holm, of the University of Auckland's Department of Molecular Medicine and Pathology, said the major impact of this work was the furthering of our fundamental understanding of how imprinting operated in mammals, and how it acted as a barrier to uniparental reproduction.
"In the long-term, this knowledge may help researchers improve assisted reproductive technologies for infertile couples where disturbances in imprinting may contribute to the health of artificially fertilised embryos," she said.
"It may even lead to the development of ways for same sex couples to reproduce healthy children of their own."
However, she noted the current work was carried out in mice and involved a number of genetic modifications in embryonic stem cells.
"Therefore, this kind of approach in humans carries significant ethical and safety concerns that would need to be overcome if it was to move beyond the laboratory."
