Just ahead of Mother's Day, scientists have found a way to cut dads out of the picture, at least among rodents: They have produced mice with two genetic moms -- and no father.
It is the first time the feat has been accomplished in mammals.
Scientists said the technique could not be used on people, for reasons both technical and ethical. In fact, one of the mouse mothers was a mutant newborn, whose DNA had been altered to make it act like a male's.
But the new work sheds light on why people, mice and other mammals normally need a male's DNA for reproduction, and some experts say it also has implications for the idea of using stem cells to treat disease.
The feat is reported in today's issue of the journal Nature by Tomohiro Kono of the Tokyo University of Agriculture in Japan, with colleagues there and in Korea.
They say they produced two mice, one of which grew to maturity and gave birth. Kono said this mouse, named Kaguya after a Japanese fairy tale character, appears healthy.
Some lizards and other animals reproduce with only maternal genes, but mammals do not.
Experts said ethical concerns and current technology would pose barriers to duplicating the technique in people.
However, the study provides new evidence for the standard explanation for why mammals normally need a male's DNA. Scientists say that in an embryo, some mammal genes behave differently if inherited from the father rather than the mother, and that this paternal activity pattern is needed for normal development.
For the study described in Nature, the researchers got around the need for male-derived DNA by turning to mutant mice. The female mice were missing a chunk of DNA, and as a result, two of their genes would behave in an embryo as if they had come from a male.
That DNA was combined with genes from ordinary female mice to make reconstructed eggs. Only two of 457 such eggs produced living mice.
Marisa Bartolomei, who studies imprinting at the University of Pennsylvania School of Medicine, said she was "stunned" that manipulating just the two genes removed the roadblock to producing live mice.
In fact, an array of other imprinted genes had also somehow taken on their normal levels of activity, as if there had been a standard fertilization. The researchers said they do not know how that happened.
Gerald Schatten, a stem cell researcher at the University of Pittsburgh School of Medicine, said the work showed that scientists need to thoroughly understand imprinting in human embryonic stem cells, which are recovered from early embryos. Otherwise, such cells might behave abnormally if used for treating diseases like diabetes or Parkinson's.
Some scientists hope to produce human stem cells by stimulating unfertilized eggs.