快猫短视频

BIRDS do it. Bees do it. Even aquatic fleas do it. Yes, they have sex. But in these creatures, females can also produce offspring without any male contribution. Until the birth of Kaguya the mouse was announced in Nature this week, this was deemed an impossible feat in mammals. Now we know that it can be done.

Kaguya is the 鈥渄aughter鈥� of two female mice. No sperm nor any male cell was involved in her creation. Female cells alone were used. Just as the birth of Dolly the sheep in 1997 shattered the dogma that an adult cell could never give rise to a new individual, so Kaguya鈥檚 birth demolishes the notion that the genomes of two mammals of the same sex cannot be combined to create healthy offspring.

Dolly鈥檚 birth led to the creation of cloned human embryos and even attempts by maverick scientists to create cloned babies. So could babies be created from the eggs of two women, in the same way as Kaguya? Could two men have children too?

Speculation about same-sex couples having babies last hit the headlines nearly three years ago (快猫短视频, 14 July 2001, p 3). At that time two teams reported that it was possible to get normal cells to spit out one of their two sets of chromosomes, a process called haploidisation, leaving them with just one set, like eggs and sperm cells. This might yet allow artificial sperm or eggs to be derived from any individual.

But the speculation missed the point. If producing cells with one set of chromosomes is all that is needed to allow same-sex couples to have children, then simply combining the eggs from two women, or the sperm from two men, should produce normal babies. In fact, decades of experiments show this simply does not work in mammals. Embryos with two female-derived sets of chromosomes, or two male ones, usually grow only for a short time, and never result in the birth of healthy offspring.

Some of these experiments were inspired by the phenomenon of virgin births, or parthenogenesis, in which an egg develops into a new individual without being fertilised. Many animals and plants, from insects such as aphids to birds such as turkeys, resort to this alternative to sexual reproduction. Some lizards reproduce exclusively by parthenogenesis.

In mammals, parthenogenesis occasionally happens when an egg is accidentally (or deliberately) activated as if it had been fertilised. The egg then retains an extra set of chromosomes 鈥� in the form of the polar body, which is normally expelled after fertilisation 鈥� and begins to divide as if it were an embryo. But such parthenotes, as they are known, either die or turn into cancers called teratomas, which can have features such as teeth and hair.

The reason why combining two eggs or creating parthenogenic embryos does not result in viable offspring is thought to be the result of a mechanism known as imprinting. During sperm formation, certain key genes are permanently switched off by chemical modification of the DNA encoding them. When eggs form, a different set of key genes is switched off. Around 30 genes are imprinted in mammals.

This means that if two sets of chromosomes with female imprinting are combined, both copies of a few genes necessary for embryonic development will be switched off, while both copies of other genes will be switched on, resulting in the overproduction of some proteins. If two male-imprinted sets of chromosomes are combined, there is a similar mismatch.

While researchers have proposed ways of overcoming the imprinting barrier (快猫短视频, 19 December 1998, p 36), no one had ever come close to achieving it. But now Tomohiro Kono鈥檚 team at Tokyo University of Agriculture in Japan has managed what was once thought impossible.

The team鈥檚 trick was to bypass the need for a male-imprinted set of chromosomes by manipulating the genome of a female egg to make it behave more like a male-imprinted sperm genome. This was no simple task, though. The making of Kaguya involved numerous steps.

The team started with mice genetically modified so they lack a gene called H19 that is imprinted, or shut down, in sperm. When H19 is switched on, it suppresses the expression of the gene for the growth factor IGF-2, which is crucial to embryo growth. The H19 mutation, which promotes IGF-2 expression, gives the egg鈥檚 chromosomes a key characteristic of a male-imprinted genome.

The second key step was obtaining immature eggs from the modified mice. Normal cells have one set of female-imprinted chromosomes and one set of male-imprinted chromosomes. It is thought that during the formation of eggs all imprints are erased before the female imprints are established. Kono harvested immature eggs in the hope that their chromosomes would not yet have acquired female imprints, although the team did not do any studies to prove this was the case.

After a series of complex intermediate steps, the researchers fused eggs containing the modified, immature nucleus with normal, mature eggs. The fused eggs were chemically activated to mimic fertilisation, and the growing embryos were implanted in the wombs of mice. The end result of the experiments was the birth of Kaguya, named after a character in a Japanese fairytale. The only other healthy pup was sacrificed at birth. Kaguya has since given birth to offspring of her own, fathered in the conventional way.

So is there any chance of women producing babies in a similar way? 鈥淭o do this kind of experimentation in humans would be outrageous,鈥� says fertility specialist Gianpiero Palermo of Cornell University in New York, who leads one of the teams studying haploidisation.

For starters, the method requires even more eggs than cloning, and normal human eggs are in very short supply. Immature eggs would also have to be taken directly from the ovary.

Next, genetically modifying a woman to knock out the H19 gene in her eggs would be ethically unacceptable, even if it were feasible. There might be other ways to switch on the IGF-2 gene in the egg, though, possibly by temporily adding the gene to an egg.

This would be highly risky, warns Azim Surani of the University of Cambridge, who has done many of the pioneering studies on parthenogenesis, and whose team coined the term imprinting. The level of expression of IGF-2 needs to be just right, he points out: too little prevents growth but too much leads to abnormalities.

Kono鈥檚 results suggest the risk of abnormalities could be very high. From the hundreds of eggs implanted, only 28 pups resulted. Of these 18 were stillborn and eight had abnormalities (see Table).

Men are certainly not about to become redundant as a result of Kono鈥檚 work, Surani stresses. 鈥淚t shows the opposite. It shows males are relevant,鈥� he says. 鈥淭o really get to a situation where the procedure would work as well as fertilisation with sperm, you would have to mutate a lot more genes,鈥� he says.

Nevertheless, experts agree that overcoming the imprinting barrier is a huge achievement. What has surprised them is that manipulating H19, and thus IGF-2, seems to be enough to restore relatively normal expression of other imprinted genes, and of many other genes as well. 鈥淚t鈥檚 amazing that altering the expression of just two imprinted genes can have a ripple effect on the rest of the genome,鈥� David Loebel and Patrick Tam of the University of Sydney, Australia, write in a commentary in Nature (vol 428, p 809 and p 860).

Even if the technique is never used to create babies directly, it will have a big impact on human studies, Palermo adds. For example, future experiments in animals will reveal which other genes have to be altered or manipulated to ensure normal development. This could help optimise fertility treatments, some of which are thought to interfere with imprinting. 鈥淚t confirms that manipulating the nucleus and cytoplasm is only part of the work, and that what is important is manipulating genes,鈥� says Palermo.

In addition, the work might hint at ways to make animal cloning more efficient. Many of the failures in cloned animals are thought to come from imprinting defects. Fixing these problems might be simpler than we thought, although Surani points out the situation in cloning, where an adult cell is transferred to an egg, is very different.

But like cloning, the 鈥渉omoparental鈥� technique might find practical uses too. In 1957 biologist Richard Beatty of the University of Edinburgh, UK, suggested that breeding farm animals would be more efficient without males. He proposed creating lines of parthenogenic cows, pigs and sheep.

Kono鈥檚 method would allow two females to be 鈥渕ated鈥�, which could be useful in dairy farming. And by taking two eggs from the same cow, a kind of clone could be created, as occurs in natural parthenogenesis. Nearly half a century on, Beatty鈥檚 vision of cutting out the bull may finally be feasible.