快猫短视频

IT IS 2050 and only 30 cheetahs are left in the world. Tireless efforts to help the animals reproduce in the wild have failed and the species could soon die out. But there is a lifeline, and it鈥檚 in the freezer. 快猫短视频s turn to thousands of cell samples collected from cheetahs over the years since 2002, and one by one each of these animals is reincarnated with the help of cloning. Welcome to the future of conservation.

This vision is anything but fantastical. If all else fails 鈥� such as habitat preservation or breeding programmes 鈥� cloning could be rolled out as a last resort to save a species on the brink of extinction. The first endangered animal to be cloned was an ox-like beast called a gaur, native to India and parts of Asia, which was born in 2001. A handful of others have followed. 快猫短视频s are even getting close to cloning a species that is already extinct.

But although various projects have already adopted cloning as a tool for saving species, it is still surrounded by doubts and controversy. Fundamental questions about the technique and its effect on the resulting animals remain unanswered. Some of the projects centring on high-profile endangered mammals such as the giant panda are struggling to make progress, while creatures that could reap more benefits from cloning, such as fish or amphibians, are being ignored. Critics worry that cloning could be a distraction from the vital work of protecting habitats so that endangered species can recover naturally in the wild.

Despite the debate, the question is no longer whether cloning endangered species can ever work 鈥� it most likely will for species related to those that have already been successfully cloned, such as cows, goats, sheep, rabbits, cats and horses. The issue is whether it should be done, and if so, for what purpose. 鈥淩eally, what is the conservation utility?鈥� says geneticist Oliver Ryder from the Zoological Society of San Diego. 鈥淭he big answer is we don鈥檛 know yet if cloning can help the conservation efforts.鈥�

As soon as Dolly the sheep, the first mammal to be cloned, was born in 1997, conservationists realised they had a new means of reproducing animals. For those struggling to coax vanishing species to breed in captivity, cloning seemed like a godsend. In theory, you could make a new individual by taking the genetic material of an endangered animal and placing it inside an egg whose own nucleus had been removed. This could then be stimulated to form an embryo, which you could in turn implant into a surrogate mother. A genetic replica of the original animal would then be born.

But if regular cloning was inefficient and difficult, scientists knew that using the technique with endangered species would be even harder. Given cloning鈥檚 dismal success rate (only a few live births from hundreds of implanted embryos), scientists would have to experiment on tens, possibly hundreds, of females. The rarity of endangered species means this is simply not an option. The only possibility would be to find a female from a related but more common species to serve as egg donor and surrogate mother. That would add an extra layer of difficulty to the whole process. Could it ever work?

The answer came with the birth of Noah, the cloned gaur, who had been created using a cow as both egg donor and surrogate. Although he died of a common infection two days after birth, Noah exemplified the power of cloning for conservation. Soon after came announcements of projects to clone the giant panda, the African bongo antelope, the Sumatran tiger (now abandoned) and the cheetah, among others (see Map).

For some of these species though, the obstacles seem insurmountable. Take the quest to clone the giant panda, of which approximately 1000 individuals are left in the wild. The project has been running for six years and has suffered major technical disappointments. In 2002, for example, the Chinese team in charge of the project published results of a series of attempts to implant cloned panda embryos. They used rabbits as a surrogate species, since panda cubs are only a few inches long at birth, about the same size as rabbit kits.

The team introduced DNA from panda cells into rabbit eggs and implanted some 2300 embryos into rabbit females. None resulted in pregnancy. The team then tried using cats as surrogates, and in a rather peculiar series of experiments each of 21 cats was implanted with 10 of the panda-rabbit cloned embryos and 10 cat-rabbit embryos (cat DNA in a rabbit egg). They yielded some early pregnancies, but none lasted past 48 days, compared with about 65 days for a normal cat gestation (Biology of Reproduction, vol 67, p 637).

The leader of the Chinese team, Da-Yuan Chen, says the team has decided to shift gears and look for a better surrogate species 鈥� possibly a bear. But not everyone is as optimistic that this will be the solution. The China Wildlife Protection Association and other panda researchers in China now oppose the project and say that the technique of mixing cells and eggs from different species isn鈥檛 advanced enough in the case of pandas.

The only successful attempts to clone endangered animals have been those using closely related species as egg donors and surrogates. For example, in 2003 the Audubon Nature Institute of New Orleans in Louisiana announced the birth of three male clones of the African wildcat, and five females have just been born. The clones were created using domestic cats as egg donors and surrogate mothers. 鈥淚n one year we will naturally breed them and show they can produce kittens,鈥� says researcher Betsy Dresser.

Similarly, the Zoological Society of San Diego in collaboration with Advanced Cell Technology of Massachusetts 鈥� the same company that created Noah 鈥� announced in 2003 the birth of two clones of a banteng, a wild forest ox from south-east Asia. The egg donor and surrogate species was the Angus cow. One of the clones has recently gone on display at the zoo, says Robert Lanza from Advanced Cell Technology.

But the truth is that nobody really knows how closely related two species need to be for this approach to work 鈥� rather a crucial question when various other high-profile cloning projects involve this kind of scenario. For example, scientists at the Laboratory for the Conservation of the Endangered Species in Hyderabad, India are hoping to clone a cheetah, and are considering the leopard for the surrogate species.

However, some are more concerned with the end result of these efforts, should they succeed. Mixing genetic material from one species with an egg from another means the cloned animal will actually carry genes from both species. During the cloning step, the donor egg retains some of its power-generating mitochondria, and these contain their own DNA. A number of studies suggest that these mitochondria end up in the cloned organism, so an animal cloned in this way won鈥檛 be an exact genetic replica of the original, even if these extra genes aren鈥檛 actually in the nucleus.

鈥淚t is a big issue,鈥� says conservationist William Holt of the Zoological society of London. 鈥淲hat you get out is not what you are trying to conserve.鈥� Dresser agrees in part, but adds that nobody knows whether this small number of genes will make a noticeable difference to the animal. 鈥淚f the other choice is extinction I know what I am going to choose,鈥� she says.

But even as scientists work to solve the technical challenges of cloning, one crucial question looms: how do you decide which animals to clone? More than 970 animal species, including 184 mammals, are believed to be at extremely high risk of extinction in the wild. Thousands more are threatened. Dresser says that one of the hardest things as a conservationist is accepting that logic is not always the deciding factor when making that choice. 鈥淚t鈥檚 all about funding. The public would rather fund a project with a tiger than a little brown frog,鈥� she says.

The trouble is that the most popular species could turn out to be the worst candidates for cloning. And some have dwindled to such low numbers that cloning might not be much help anyway, says Paul Bartels, head of the Wildlife Biological Resource Centre of the Endangered Wildlife Trust in Pretoria, South Africa. Once a species becomes too inbred, the animals become weaker and more prone to inherited genetic disease, though the issue of how low is too low is highly contentious. There have been cases in which successful breeding programmes have reintroduced a species to the wild after the population had gone down to only tens of individuals, argues Dresser.

Holt also believes that conservation鈥檚 cloning efforts have overlooked the potential benefits for non-mammalian species 鈥� ironic considering that the very first cloning experiments, done in the 1950s and 60s, were performed on amphibians. Among vertebrates, mammals present the most challenges for cloning because it interferes with a genetic mechanism called imprinting, which only mammals rely on to develop properly. Mammals also require additional steps during embryo culture and implantation, as well as monitoring during pregnancy.

If the goal of conservation is to save as many species as possible with limited resources, wouldn鈥檛 it make more sense to focus on amphibians or fish? After all, more of these species are disappearing, and assisted reproduction has been of little help. 鈥淚t might be simpler to just go straight to cloning,鈥� says Holt. 快猫短视频s wouldn鈥檛 face the imprinting issue or have to develop ways of implanting embryos, since fertilisation and development happen outside of the body, he adds.

Whatever the outcome of this debate, conservationists agree on one thing: cloning should not be seen as conservation鈥檚 panacea. The technique is still in its early stages, so plans to clone an animal should only take place alongside other efforts to save a species. For example, Dresser says that during her team鈥檚 work with the African wildcat, they also devised techniques for IVF, embryo freezing and embryo transfer. 鈥淲e have developed kittens with all of these,鈥� she says.

But there is one area where cloning could play a key role once the technology has matured: maintaining the genetic diversity of rare species. Tissue from animals could be collected and frozen to be defrosted later and cloned to revitalise the gene pool. Hundreds of cell lines from threatened populations are already being stored in these 鈥渇rozen zoos鈥�, a kind of insurance policy for the future. 鈥淚f you have collected live cells from a population and suddenly they die out, you can bring back diversity,鈥� says Bartels.

South Africa鈥檚 frozen zoo, BioBank SA, started collecting samples about two years ago and so far has a cache of cell lines from about 60 wild endangered species. And the Audubon Nature Institute鈥檚 frozen zoo has collected cells from over 5000 individuals representing about 150 species, says Dresser. 鈥淎ny time we get our hands on any animal at the zoo we take a skin biopsy and grow a cell line,鈥� she adds.

But the advantage of tissue banking reaches far beyond cloning. It can provide scientists with a way to monitor the genetic fitness of a declining population in order to decide when to take action, says Bartels. 鈥淵ou can go back to a sample from 10, 20, 30 years ago to redefine how your population is changing,鈥� he says. Banking also essentially buys time while scientists perfect cloning technology, and importantly, make it cheaper and more suitable for conservation (see How now cloned cow). 鈥淚t would be an incredible gift for the future,鈥� says Ryder.

The billion-dollar question is, of course: can cloning help resurrect a species that has gone extinct? Japanese and Australian teams are aspiring to the ambitious goal of cloning extinct animals such as the woolly mammoth or the Tasmanian tiger, which died out in 1930. Most scientists agree these projects have little to do with conservation. The groups are not even close to achieving their goal, since they don鈥檛 have cells to work with and so must first piece together fragmented DNA from remains.

However, Jos茅 Folch and colleagues at the Agricultural Research Service in Zaragoza, Spain, have come close to recreating a bucardo, a type of mountain goat that became extinct in 2000 after the last member of the species was killed in an accident. The team is trying to clone the animal from cells taken while it was alive, using domestic goats as surrogates. Folch told 快猫短视频 the team has achieved two pregnancies 鈥� one lasted 45 days and the other two months.Now the team is now fine-tuning the technology, though Folch says he can鈥檛 forecast whether they will succeed.

Even if they do, it is unlikely they could ever resurrect the species. For one thing, scientists would have to create a male bucardo to mate with the cloned female. For another, the problems of inbreeding make re-establishing a viable population from just two individuals well nigh impossible. For these and other reasons, experts believe the projects like these can鈥檛 be considered conservation. 鈥淭hey are entertainment,鈥� says Holt. Although cloning could help us conserve endangered species, it cannot salve humanity鈥檚 conscience by raising the dead. Extinction is still forever.