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One of the favorite topics of speculative fiction is a society composed entirely of women. From the Greek legends of the Amazons, to the modern novels of Sheri Tepper and Joanna Russ, we have asked ourselves a series of questions. What would such a society look like? How would its members behave? And most importantly – how would they reproduce?

The answers to the question of reproduction have been as varied as the speculated societies. Most involve men in some form or another – whether captured and enslaved, or merely kept out of the society except for reproduction. Only a few have gone so far as to remove the men – because then the question of how women reproduce on their own comes into play.

Parthenogenesis – the production of offspring from an unfertilized egg – is a frequent contender, but parthenogenesis is unheard of in mammals. Also, it’s a form of asexual reproduction with all the disadvantages that entails. Trying to preserve a form of sexual reproduction with only one sex seems a little odd, but to preserve the genetic variety of the species, it would be necessary. Using two eggs rather than an egg and a sperm would seem a logical solution, but that scenario is never found in nature, and all attempts to produce offspring this way, in any animal, have failed – producing embryos which died early in gestation, if they even survived that long.

At least until recently. Then came Tomohiro Kono, a biologist at the Tokyo University of Agriculture. He and his team of researchers set out to produce a mouse from two eggs. In 2004, he succeeded, producing a mouse named Kaguya, that not only lived to be born, but grew to adulthood, and produced offspring of her own in the more usual manner.

Blastocyst and Unfertilized Ovum
Blastocyst and Unfertilized Ovum

The first necessary part of producing a mouse without sperm was to figure out why the embryos produced so far were not viable. The answer turned out to be in the genes. In a process called imprinting, some genes in a gamete are turned off, while others are turned on. Which genes these are differs in eggs and in sperm, so the zygote created from the combination has a complete, non-duplicated set of active genes. With two eggs, some genes would be turned on in both, while others would be turned off, leaving the egg with no active copy of a potentially vital gene. To try to avoid this problem, the researchers started working with immature eggs taken from newborn mice. These eggs would not have their genetic imprinting yet, and so might substitute for sperm.

The first attempt met with improvement, but not success. The new batch of embryos survived longer than any of the previous batch – more than halfway through gestation. Nonetheless, all the new embryos died before birth. The longer survival was promising though, so Kono and his team began to concentrate on making the genes of the immature egg more sperm-like. They found two genes. One, called IGF2, is required for proper growth in the fetus, and is only active in the sperm. The other, H19, is active in eggs, and aids in deactivating IGF2 in the egg. By removing H19 from the immature eggs, the researchers enabled the IGF2 to activate, as it would in a sperm cell.

Based on results, the researchers believe that activating that one gene may have caused a cascade of gene activations and inactivations throughout the immature egg, causing it to emulate the gene patterning of a sperm cell much more closely than the one manipulation could account for. With paternal imprinting in place, the team could then try again to produce embryos.


Even with the genetically modified eggs, producing a live, healthy mouse proved to be an immense task. Kono and his colleagues produced 457 fertilized eggs, using their modified ova. Of those, 371 survived to the blastocyst stage – far enough along to be implanted in a female mouse. From those 371 pregnancies came 10 live births, and from those ten live births – one mouse, who lived to adulthood and had babies of her own.

That mouse, Kaguya, is the first, and so far only mammal to be born by combining two eggs. After her debut in the April 2004 issue of Nature, the researchers announced that they would be attempting to create a baby pig via the same method, but nothing has been published as of yet. Given the difficulties of producing Kaguya, this hardly seems surprising. It would seem that we are slowly making progress on understanding the complex biology of reproduction. It would also seem that men are going to be a necessary part of the process for some time to come.