A mammal that is the daughter of two female parents has been created for the first time.
Until now such a feat had been considered biologically impossible. But the mouse, called Kaguya, was born without the involvement of any sperm or male cell - only female eggs were needed.
In the same way that the birth of Dolly the sheep in 1997 shattered the dogma that an adult cell could never be reprogrammed to make a new individual, the fact that Kaguya lives challenges another one of long-held rule: that two mammals of the same sex cannot combine their genomes to give rise to viable offspring.
What scientists learn from this remarkable rodent, created in Japan, is likely to have an impact on fields from fundamental embryology to assisted reproduction and even cloning.
However, several experts have already warned against assuming the method could be used in humans to help two women have a biological child, not least because the process is extremely inefficient.
It would also be highly risky and require a very large number of eggs. "To do this kind of experimentation in humans would be outrageous," says fertility specialist Gianpiero Palermo of Cornell University in New York.
Kaguya was created by combining the genetic material of two egg cells. This would not normally work, a fact evidenced by decades of studies into the phenomenon of parthenogenesis, also known as virgin birth.
In parthenogenesis, the egg becomes the sole source of genetic material for the creation of an embryo. It is a mode of reproduction in some species, though not in mammals. In mammals parthenogenesis can begin if an egg is accidentally or experimentally activated as if it had been fertilised - but this parthenote never grows past a few days.
This is because of there a biological phenomenon known as imprinting. During sperm and egg formation in mammals, certain genes necessary for embryo development are shut down with a series of chemical marks, or imprints, some in the sperm, other in the egg. Only when sperm and egg meet are all of the key genes available, allowing proper development.
But Tomohiro Kono and colleagues at the Tokyo University of Agriculture in Tokyo, Japan, circumvented this imprinting barrier by manipulating the nucleus of a female egg to make it more male-like.
This was far from simple. Perhaps the most important of the many steps required was the creation of eggs that produced a protein called IGF-2. This is crucial to embryo growth but is normally only produced by sperm-derived DNA. The researchers achieved the trick by using genetically altered mice to provide the donated eggs.
The nucleus of such an egg was then transferred into a regular egg that, with the genomes of two females, proceeded to grow and divide. However, Kaguya and one sister were the only live animals resulting from 457 reconstructed eggs.
Although attempting to apply such an approach to humans horrifies experts, that does not mean the technique will not have a big impact in studies of human biology. For example, future experiments of the same kind will allow scientists to learn which other genes can be altered to bypass imprinting defects.
This could help optimise fertility techniques, some of which are thought to interfere with imprinting. In addition, the work might provide new hints to make animal cloning more efficient, as many of the failures in cloned animals are thought to come from imprinting defects.
While he agrees the creation of the mouse is a major achievement, imprinting expert Azim Surani, at the University of Cambridge, UK, hopes the work will not be misinterpreted to imply that males are somehow redundant.
"It shows the opposite - clearly IGF-2 is the key gene," he says. "They managed to get around it but to really get to a situation where the procedure would work as well as [fertilisation with] sperm, you would need to mutate a lot more genes."
Journal reference: Nature (vol 428, p 860)
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.