Cloning: Can it resurrect extinct species? A century ago, vast flocks of passenger pigeons covered the North american skies. Hundreds of millions, even billions, stretched across the horizon in every direction. A flyover could last an entire day.""It was this beautiful super-organism, like a dragon going through the skies,  says Harvard Medical school geneticist George Church. And then European settlers arrived. Mass hunting and habitat loss rapidly reduced their numbers until on 1 september 1914, Martha, the last known passenger pigeon, died in Cincinnati Zoo. But if Church has his way, this majestic sight could one day return to our skies.  Armed with new reproductive biology and genome engineering technologies, he and other scientists are dreaming up ambitious plans to resurrect long-dead animals from pigeons to Tasmanian tigers and wooly mammoths.  The same technologies could also prevent endangered species from going the way of the dodo oe or the passenger pigeon. A key part of this grand ambition lies in the lines of test tubes frozen in liquid nitrogen in a Californian zoo. In 1972, Oliver Ryder, a geneticist at the San diego Zoo, had the visionary idea of freezing skin samples from endangered animals in the hope they might help protect these species in the future. Many captive animals suffer from genetic abnormalities and inbreeding, and Ryder imagined that his repository of animal cells could be used long after their donors died to help zoo veterinarians manage captive populations. He never viewed his"Frozen Zoo  oe which now stores cells from 9 000 individual vertebrates belonging to more than 1, 000 different species oe as a way of producing new animals. As far as he and other scientists knew,  only stem cells found in embryos had the ability to transform into the building blocks of  any part of the body, whether a cell in the liver or the eye. Ryder's collection of skin cells were thought just that scientists-they could not possibly switch identity.""I believed like the vast majority of the scientific community that cells made commitments and those commitments were irrevocable,  says Ryder.""Then Dolly happened.  Ëoecloning stunts'On 22 february 1997, Ian Wilmut and his team at the University of Edinburgh's Roslin Institute unveiled a 7-month-old baby to a stunned world oe Dolly the sheep. The team had found a way of replacing the nucleus of a sheep's egg cell with a nucleus from an adult ewe's cell, and yet still trick the egg into dividing as normal. After the hybrid egg cell began dividing, it was implanted into the uterus of another animal, which carried the clone to birth.""Dolly is derived from a mammary gland cell and we couldn't think of a more impressive pair of glands than Dolly parton's,  said Wilmut at the time. From dogs to cows, scientists rushed to clone a menagerie of animals using Wilmut's technique, known as somatic cell nuclear transfer (SCNT. Within a few years, attempts to clone endangered species began to surface. The first, Noah, a threatened species of ox from Southeast asia called a guar, was born seemingly healthy on 8 january 2001. Scientists at the American biotechnology company Advanced Cell Technologies who had helped pioneer cloning in cattle, shuttled nuclei from gaur skin cells into cow eggs and then implanted the embryos into cows. However, Noah died two days later of an infection unrelated to the cloning process. And like the guar, other attempts to clone endangered species through somatic cell nuclear transfer tended to be one-offs. Many animals were born unhealthy and the cloning process was inefficient, with success rates of around 1%.Less charitable critics still call these efforts stunts, designed to attract attention but doing nothing to conserve species teetering on the brink of extinction.""Producing the odd animal here and there, which may be sick, didn't seem a very sensible thing to do,  says William Holt, a reproductive biologist at the Zoological Society of London, who in a 2004 review paper called the prospect of cloning highly endangered species"hopelessly optimistic Â. Reverse switch But another scientific breakthrough, on par with the cloning of Dolly 15 years ago, might force sceptics like Holt to someday reconsider. In 2007 Japanese scientists reported, first in mice and later in humans, that an adult cell could be reverted to an embryo-like stem cell. These so-called"induced pluripotent stem cells  (otherwise known as ips cells) are in turn capable of making every cell in the body. All of which suddenly made Oliver Ryder's frozen repository of skin samples a cellular gold mine.""That made us realize that we had probably the largest repository of potential stem cells,  says Ryder. Skin cells could be converted into sperm that could create an animal through in vitro fertilisation (IVF), or even transformed into whole animals. Both feats have been accomplished in mice and they should be possible in other animals, scientists say. As a first step, Ryder and a team of stem cell scientists have reprogrammed the skin cells from a northern white rhinoceroses named Fatu, one of seven still alive, and from a drill, a monkey species that lives in tiny, dwindling pockets of west Africa. And recently, scientists said they have created ips cells from a snow leopard. These cells are a long way from saving species, but"it would be the only chance that I can think of that would prevent the extinction of the northern white rhino,  Ryder says. Ryder shrugs off the suggestion that money devoted towards such ambitious cloning projects would be spent better on preventing habitat destruction and other, simpler conservation measures. Fighting over the best way to save species instead of saving them will, to future generations, look like"fiddling while Rome was burning,  he replies. Back from the dead? If this seems like a daunting challenge, scientists face a much tougher task resurrecting extinct species whose cells were banked not before they vanished from the planet. But this is not stopping scientists like George Church from trying, though he does not underestimate the effort required to bring the passenger pigeon back to the skies. With extinct animals, scientists need to take more involved measures to recover the complete DNA sequence oe its genome. Armed with this code, they then need to find a way of engineering a regular pigeon's stem cells into behaving like a passenger pigeon's stem cells by mutating the genome. Church says the complete genome of the passenger pigeon from museum specimens will soon be published and researchers are beginning to alter the genetic make-up of a more familiar bird oe the chicken oe to practice their techniques.""What you can do for chicken you should be able to do for pigeon, and that can include creating DNA that you haven't seen alive for a 100 years,  he says. But even if Church has the passenger pigeon's full genetic code, which he expects to recreate within a decade, Church admits that bringing it back to life requires a significant improvement in existing genome engineering technologies. To test his idea works, his team is using a similar approach to engineer mice with traits of naked mole rats. The odd-looking rodents live dozens of years instead of a handful like mice. They are impervious to cancer and do not feel pain from acids. To endow ordinary lab mice with these traits Church will try to partially rewrite the genomes of mouse stem cells. However, he admits that creating a passenger pigeon from the stem cells of an ordinary pigeon would involve a massive scale up of the same technologies. Mammoth goalthe favourite candidate for resurrection, though, might lie in nature's version of Ryder's Frozen Zoo. Flash-frozen remains of wooly mammoths have been found preserved under the Siberian permafrost, and scientists hope their bones could be a source of DNA-containing marrow cells for cloning. Last year, Japanese and Russian scientists announced they had found just such a bone and predicted they would be able to clone a mammoth within 5 years. They hope to insert nuclei from the mammoth cells into egg cells from its closest living relative, the elephant, and carry the mammoth embryo in an elephant's womb. However, some scientists have cast doubt about whether this is possible. Hendrik Poinar, a palaeo-geneticist at Mcmaster University in Hamilton, Canada, and his team have uncovered similarly well-preserved mammoth bones and never found viable cells or nuclei.""The likelihood of finding an intact cell that can be rejigged to life oe it's not that it's an impossibility oe but the chances are very, very slim,  he says.""The other hope has been to find frozen testicles and use that for insemination, which is a complete pipe dream.  Poinar says that genome engineering offers a more realistic shot at resurrecting woolly mammoths and other long-extinct species. Ten thousand-year-old cells and their nuclei may be degraded too to be used in cloning, but they still contain the animal's genetic code. This genome is shredded into short fragments, but DNA sequencing machines can read these shards and powerful computers can stitch them into a genome sequence. Scientists published a 80%complete version of the mammoth genome in 2008 and more ancient animal genomes are on the way, such as the Tasmanian tiger. These genomes exist in the form of computerised data, but they could serve as a blueprint for altering the DNA of a cell from a closely related species. For instance, the code of a woolly mammoth's genome differs from an African elephant's by roughly 240,000 DNA letters out of a total of 4 billion, though most of these changes are not likely to have a biological effect. An elephant ips cell engineered to contain those mutations would theoretically be capable of producing woolly mammoth sperm. Better yet the woolly mammoth stem cells could be implanted besides an elephant embryo early in development, producing a chimera animal with some tissues made from elephant cells and others from mammoths. In some individuals the mammoth cells would contribute to sperm or eggs, and these cells be used to create a genuine mammoth through IVF. In the absence of a living mammoth, scientists are reconstructing some of its most vital components from DNA fragments to discover how it adapted to life at subzero temperatures. Scientists have recreated already haemoglobin, the oxygen-carrying protein in red blood cells, and found that it is optimised to catch and release oxygen molecules in the cold. Scientists could go one step further and test woolly mammoth red blood cells made from ips cells, Poinar says. Reality check If the idea of mammoths roaming the Earth still sounds a bit farfetched, it should. Resurrecting a mammoth or indeed any extinct species would require a dizzying list of technological leaps in genome engineering, reproductive biology, and veterinary medicine, and that is just to surmount the challenges that scientists know about. What is more, the technologies that scientists are hoping to use have mostly been developed for use in laboratory animals and valuable livestock only. Basic genetic principles may carry over to more exotic animals but many steps will not, particularly those involving reproduction and development. For instance, a recent study found that making a chimeric rhesus monkey oe a process needed to resurrect a monkey species from frozen cells oe is much trickier than a mouse.""I think we are going to encounter numerous problems as we try to figure this out for different species,  says Robert Lanza, chief scientific officer for Advanced Cell Technology, the firm that has tried to clone endangered species . But Hendrik Poinar believes it will be a matter of when, not if, humans resurrect an extinct animal. A wealthy American investor approached him several years ago and asked Poinar to quit his academic job and work full time on bringing back woolly mammoths. Poinar declined, but he expects someone will eventually take on a similar project.""The technology clearly moves at lighting speed and faster than we can predict. Saying it won't happen is silly.  Harvard's Church says his goal is not necessarily to fill the planet with mammoths or provide an easy solution to the global extinction crisis. Rather, he wants to give conservationists the option to save species this way. There could even be an upside for other animals on the brink of extinction.""If there's enough people enthusiastic about bringing an extinct species like a mammoth or passenger pigeon,  says Church, "maybe there will be interest in maintaining the species we still have.  Ewen Callaway writes about biology and medicine for Nature Newsif you would like to comment on this story or anything else you have seen on Future, head over to our Facebook page or message us on Twitter
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