杏吧原创

DEEP in the Wolong Mountains of south central China lies the last stronghold
of the giant panda. The reserve is home to the largest remaining group of pandas
in the wild鈥攋ust 70 individuals. There are only 1000 giant pandas left in
the wild, and some experts believe they will all be gone within a decade. It鈥檚 a
grim situation, but Chen Dayuan, who works with the Wolong pandas, has not given
up hope. The zoologist from China鈥檚 National Academy of Sciences believes he can
reverse the decline and save the giant panda from extinction.

Chen鈥檚 confidence seems foolhardy. All attempts to introduce more pandas into
wild populations with traditional captive breeding techniques, such as
artificial insemination and in vitro fertilisation, have failed. What鈥檚 his
secret weapon? To sidestep the panda鈥檚 notorious fertility problems, Chen is
using the cloning technology that created Dolly the sheep. And his team has
already taken the first step. In June they reported that they had successfully
produced a cloned panda embryo using cells taken from an adult animal. Chen
hopes that within three years he can find a surrogate mother to carry such an
embryo to term and produce the first cloned giant panda.

Since Dolly鈥檚 debut, biologists have debated the possibilities of cloning
endangered species. Some doubt it will ever come to much because cloning common
animals has proved so difficult, and cloning endangered ones will present far
greater problems. Other researchers say that cloning has a future, provided we
can learn more about the technology and about the reproductive biology of
animals on the brink. 鈥淚 think cloning will provide the technology that may let
us bypass many of the difficulties found in assisted reproduction,鈥� says George
Amato, director of the Wildlife Conservation Society鈥檚 Science Resource Center
in New York City. However, many conservationists fear that this high-tech
approach will divert resources from the real problem of protecting and restoring
the dwindling habitats essential for the survival of endangered animals.

No endangered species has yet been cloned. But for those, like Chen, who put
their faith in this approach, there are some promising signs. Last year,
scientists in New Zealand cloned Lady鈥攖he lone surviving member of a rare
breed of cow. In the mid-1880s, Lady鈥檚 ancestors were brought to Enderby Island,
an isolated, rocky, windswept slice of land in the Auckland Islands chain, as
part of a failed attempt to start a cattle ranch. Though the ranch didn鈥檛 last,
the cows did, and over time the breed developed short bodies and strong legs to
cope with their extreme environment. In 1992, in an effort to restore the
natural balance of the island, the New Zealand government destroyed the herd,
but spared Lady. Her cloning was the fourth reported for of an adult
mammal鈥攁nd the first of a rare animal.

Even so, David Wells plays down the achievement of his team at the Ruakura
Research Centre in Hamilton, New Zealand. 鈥淎ll we were doing was cloning a breed
of cattle within the bovine species,鈥� he says. Like Dolly and the other animal
clones to date, Lady鈥檚 clone was created by transferring genetic material from
an adult animal into an egg cell from a female of the same species. There are
plenty of cows around to act as egg donors, but researchers aiming to clone
endangered species may not be so lucky. Panda eggs, for example, are hard to
come by, so Chen鈥檚 team used the egg of a Japanese white rabbit to create their
embryo. This made their job relatively easy, because the reproductive biology of
white rabbits is much better understood than that of pandas. It remains to be
seen whether using a surrogate egg from another species will have lasting
detrimental effects on a clone.

But generating the embryo is the easy part, says Wells. The real difficulty
lies in finding a suitable surrogate species in which to gestate the cloned
embryo. Lady鈥檚 clone was gestated in another cow. Chen, however, believes that
his best chances of success lie in finding a non-panda surrogate. His team is
currently looking for a 鈥渕other鈥� that is large enough and has reproductive
traits similar to that of a giant panda. Two possible candidates are the black
bear and the sloth bear. But previous research attempting to grow embryos of one
species in the womb of another suggest that Chen鈥檚 task is going to be be
difficult. What鈥檚 more, the fundamental biology of giant pandas is still very
poorly understood. Wells points out that his success with cloning Lady depended
on knowing a great deal about bovine reproductive biology. 鈥淚t鈥檚 extremely
difficult when you don鈥檛 have that knowledge,鈥� he says.

If cloning is to save the giant panda it will have to be quick. But for
researchers planning to clone less endangered animals there is more time to iron
out technical problems such as the high rate of failed versus successful cloning
attempts and the lack of knowledge concerning interspecies cloning. Many
researchers are confident that these hurdles will be overcome for at least some
endangered species. If so, cloning offers a way to bring more genetic diversity
into populations that are down to only a few individuals. The key to success in
the future will be to put the remaining diversity on ice now. And that鈥檚 where
鈥渇rozen zoos鈥� come in.

In 1975, Kurt Benirschke, who is now vice president at the San Diego Zoo,
began a programme to preserve cells from endangered species so that researchers
could study the genetic similarities among animals. The Center for Reproduction
of Endangered Species at the zoo now has cells from over 4000 individual
animals, comprising over 400 species and subspecies within 14 mammalian orders.
They are stored in liquid nitrogen, which preserves DNA structure and genetic
viability.

The centre鈥檚 director, Oliver Ryder, points out that his team is not
currently using cloning technology. 鈥淲e鈥檙e more concerned with the fact that
while people are figuring out the best way to save endangered species, they鈥檙e
losing the opportunity to collect samples that might be valuable in the future,鈥�
he says. His view is shared by Betsy Dresser, director of the Audubon Center for
Research of Endangered Species in New Orleans, who is creating her own frozen
zoo. 鈥淚 don鈥檛 know what `in the future鈥� is going to mean. I鈥檓 not sure what
technologies there will be,鈥� she says. 鈥淏ut I do know we should be preserving
cells and give our children the option of thawing them out.鈥�

If cloning does take off, then frozen zoos will be an invaluable source of
genetic material for creating new animals. But successful conservation is about
much more than producing animals, as David Wildt, a reproductive physiologist
with the Smithsonian Institution鈥檚 Conservation and Research Center at the
National Zoological Park in Washington DC, points out. 鈥淐onservation is this
huge jigsaw puzzle with a lot of different pieces that have to do with things
such as habitat conservation, reproduction, behaviour, nutrition and disease,鈥�
he says. 鈥淵ou really have to consider each piece and shape them and put them
together in just the right way to actually have conservation.鈥�

One especially tricky piece of the puzzle is captive breeding. The genetic
diversity contained in frozen zoos will only strengthen populations of
endangered animals if biologists can reintroduce clones into the wild. And it鈥檚
a big 鈥渋f鈥�. In 1994, Benjamin Beck from the National Zoo and his colleagues
looked at 145 reintroduction programmes of animals bred through traditional
captive-breeding methods. They found only 16 cases where wild populations had
been successfully established. The main problems faced by captive-bred animals
in the wild are disease and difficulties with foraging, avoiding prey and
interacting socially with wild animals. Also, animals born in captivity find it
hard to pick up learnt behaviours, as Noel Snyder and his colleagues from
Wildlife Preservation Trust International, based in Philadelphia, have
discovered.

Snyder believes that captive breeding could be more successful if captivity
was more like the natural environment. Another improvement would be to decrease
the time animals spend in captivity. Wildt cites the black-footed ferret
programme as evidence that captive breeding can work. The ferret, which was
found throughout the American plains, started to decline in the 1960s due to a
government-sponsored programme to eradicate prairie dogs, which are an
agricultural pest but the ferret鈥檚 main food source. In 1978, the species was
declared extinct. However, a few years later, a small population was discovered
and in 1986 the last 18 individuals were brought into a captive-breeding
programme. Over the past 13 years, more than 2500 black-footed ferrets had been
born in captivity and today there are 200 individuals in the wild.

Bold experiment

But there may be an alternative to captive breeding. Dresser is trying a bold
experiment that may help the reintroduction of some endangered animals into the
wild. She is trying to transfer the embryo of a threatened species of antelope,
the bongo, into a wild eland antelope. As the bongo and eland are closely
related species, she hopes the eland will deliver and raise the bongo in the
wild, so overcoming the problems of captive breeding. She is trying this in
Kenya, where there is a small population of wild bongos which she hopes the
bongo clone will eventually join.

Even as researchers make strides that may help to reintroduce endangered
animals into the wild, many conservationists feel that cloning will not play an
important role in this process. 鈥淚 believe that cloning will have little if any
application to true conservation of endangered species鈥攊ncluding the
cloning of giant pandas,鈥� says Wildt. 鈥淧eople can鈥檛 separate the production of
one or two giant pandas by cloning from real conservation. In this context, I
have to side with the conservation biology side of my training, which is
habitat, habitat, habitat.鈥�

Most endangered species have become imperilled because development has either
destroyed or fragmented their natural environment. A growing number of
conservationists are worried that cloning will take the focus away from what
they see as the more important goal of protecting and restoring habitats.

Pan Wenshi, the director of the Panda Research Centre at Beijing University,
has been one of the most vocal opponents of cloning, saying that instead money
and resources should be used to save and restore habitats. But Dresser believes
reproductive and conservation biologists are not competing for the same funding
sources. She explains that many organisations that fund biotechnology research
are not going to put resources into habitat preservation. She adds: 鈥淢any people
think [cloning] is a shot in the dark because it鈥檚 not very successful yet.
Well, of course things never are in the beginning. But I think over time, we鈥檙e
going to see a rise in cloning technologies, helped along by many things we
haven鈥檛 even thought of yet.鈥�

One man who intends to be at the forefront of any developments is Wells. He
has plans to follow up his cloning of Lady with a more ambitious project.
Recently he was sent tissue samples from two Mesopotamian fallow deer that had
died. This species, which was once found in large numbers throughout the
Mediterranean, is now down to only 200 individuals worldwide鈥攅ight of
which live in New Zealand. Wells has now cultured the tissue and preserved the
resulting cells. He believes enough is known about the reproductive biology of
fallow deer to warrant an attempt to clone the species and he plans to use the
closely related European fallow deer as a surrogate. 鈥淚 don鈥檛 like the term
`bringing them back from the dead鈥�,鈥� says Wells, 鈥渂ut that鈥檚 literally what we
could do, while at the same time reintroducing those genes back into the
辫辞辫耻濒补迟颈辞苍.鈥�

NOT satisfied with merely cloning endangered species, a few labs have chosen
to channel their resources in an even more extraordinary direction. Taking a
page straight out of Michael Crichton鈥檚 Jurassic Park, these researchers are
trying to clone extinct species.

Michael Archer, a palaeontologist and director of the Australian Museum,
recently helped to start a foundation that will focus its resources on cloning a
thylacine鈥攁lso known as a Tasmanian tiger or wolf. Thylacines became
extinct on mainland Australia around 1936, says Archer, in part because they
were subject to a government bounty for attacking sheep. 鈥淭he animal wasn鈥檛
protected until two months after the last one died,鈥� he explains. 鈥淪o there was
a serious bit of mismanagement there.鈥�

Archer wants to clone a thylacine from specimens that have been stored in
ethanol, a DNA preservative. 鈥淲e鈥檝e had the thylacine sitting here like a time
capsule, waiting for the technology to catch up, and it occurred to us that that
time might be now,鈥� he says. His team is trying to create a genetic library of
the thylacine鈥檚 DNA, which would be stored in bacterial elements such as viruses
or plasmids. If the chromosomes are badly fragmented, they will have a
painstaking task reconstructing them. If not, they may be able to start cloning
experiments soon.

Meanwhile, this year researchers in New Zealand have been exploring the
possibility of cloning the huia鈥攁 bird that was prized for its feathers
and declared extinct in the 1920s. The cloning effort began when students from
Hastings Boys鈥� High School on North Island organised a conference and invited
experts on cloning to discuss the possibility of reviving their school mascot.
Diana Hill, a biochemist at the University of Otago, is leading the team of
scientists who want to help. She is currently searching for a pristine cell from
a well-preserved specimen so that she can begin to reconstruct the bird鈥檚
DNA.