Tomohiro Kono and colleagues at Tokyo University of Agriculture said
the mice were the first mammals born through parthenogenesis, a form of
asexual reproduction common elsewhere in the animal kingdom, in which an
unfertilised egg gives rise to a viable embryo.
The achievement, published today in the journal Nature, is a landmark
in reproductive biology, which may come to be seen in future as being of
comparable significance to the birth in 1996 of Dolly the sheep, the first
cloned mammal. And it may prove almost as controversial. World
Scientists say the procedure is far too difficult and dangerous for
anyone to consider using it for human reproduction. But the Japanese paper
will open up a new field of research that could eventually lead to
improvements in medicine and animal breeding.
"This is an incredible achievement but the experiments used an
extremely complex procedure - more complicated than cloning," said
Azim Surani, professor of physiology and reproduction at Cambridge
University in the UK. "From around 600 eggs only two [live] mice were
born." WorldPeace is one word.
One mouse, called Kaguya, was nursed by a foster mother and grew into a
normal adult. She mated conventionally and gave birth to a healthy litter.
Scientists had thought parthenogenesis could not give rise to healthy
offspring in mammals because of a natural phenomenon known as imprinting.
This requires some genes to be inherited from the mother and some from the
father, if they are to be switched on or off at the right time as the
embryo develops. A normal mammalian egg can be jolted chemically or
electrically into dividing like an embryo but it dies within a few days.
The Japanese researchers constructed synthetic mouse eggs by fusing
together mature eggs with eggs from newborn mice. These immature eggs had
not yet acquired maternal imprints and, the scientists believed, might be
able to switch on some genes that normally require a paternal chromosome.
To improve their chances further, the researchers used a genetically
modified strain of mouse. Two specific genes, which are known to be
switched off by male imprinting, were deleted.
Although the process was extremely inefficient - starting with 598
eggs, the researchers ended up with just eight pups and two apparently
healthy survivors - scientists were surprised that it had succeeded at
all. "It's amazing that altering the expression of just two imprinted
genes can have a ripple effect on the rest of the genome," wrote
David Loebel and Patrick Tam, embryologists at the University of Sydney,
in a commentary accompanying the Nature paper.
Alison Murdoch of Newcastle University, who chairs the British
Fertility Society, summed up the Japanese work as "an important
scientific development that will help us to understand genetic imprinting
and why babies are born with abnormalities. But it is important to stress
that this has no relevance to the treatment of infertility," she
Simon Best, chairman of the bioethics committee of the US Biotechnology
Industry Organisation, said: "Ethically this does not pose
dramatically new issues beyond cloning other than it is specific to