This work produced 29 live mice from 210 embryos.
We are now one step closer to same-sex reproduction: Scientists in China have produced healthy offspring from two mother mice.
"Previously, we have derived mice with two female parents with one or two DNA region modifications, but all these mice exhibited significant growth retardation before or after birth", said Zhou Qi, a co-author of the study and a researcher with the Chinese Academy of Sciences.
Although the reality of biological parenthood for same sex couples in humans is still a ways away, the breakthrough is revealing a path to fertility for same sex parents and showing scientists how to overcome the genetic roadblocks that normally prevent such reproduction from working.
Zhou said same-sex reproduction was hard for mammals because of a mechanism called genomic imprinting, while it was much easier for some reptiles, amphibians and fish. The mice were normal, lived to adulthood, and had babies of their own.
Dr Hu said: 'We found in this study that haploid ESCs were more similar to primordial germ cells, the precursors of eggs and sperm.
By deleting these imprinted genes from immature eggs, the researchers produced "bi-maternal" mice - that's mice with two mothers - in the past.
The CAS team did just that, using gene editing to delete and alter imprinted DNA to make babies viable.
"'The genomic imprinting that's found in gametes was 'erased".
They were also able to create 12 live mice with two fathers, but those pups survived only two days, indicating there were greater challenges for successful reproduction using two male parents. Haploid ESCs containing only a male parent's DNA were modified to delete seven key imprinted regions.
The nucleus of the mouse egg was removed, meaning there was no female genetic material left and the fertilised egg was placed in a surrogate mouse.
According to one evolutionary hypothesis, parents' sex cells resolve this difference by shutting off regions of their offspring's DNA that would benefit their partner's desires in a process called genomic imprinting.
She told The Sun: "Animal mothers are subjected to surgical procedures in order to harvest their eggs and implant the engineered embryos, many pregnancies fail, and the animals who are born are commonly forced to live with unintended debilitating conditions, such as missing eyes or ears, skin ulcers, deformed body parts, deafness, seizures, and heart failure, to name just a few". Similar results were achieved in 2011 but using a method that relied on a female intermediary produced from the first father's stem cells to mate with the second father.
"To consider exploring similar technology for human application in the near future is implausible".
A bipaternal mouse pup born to two fathers.
Dr Li says that there are still obstacles to using these methods in other mammals, including the need to identify problematic imprinted genes that are unique to each species and concerns for the offspring that don't survive. They said, however, that they hoped to explore the techniques in other research animals in the future.
"We saw that the defects in bimaternal mice can be eliminated and that bipaternal reproduction barriers in mammals can also be crossed through imprinting modification".
"If the research is reproducible, and also works in humans, it still has to be shown to be safe,"Bob Williamson, the chair of the Board of Stem Cells Australia, said".