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本期是关于de-extinction的话题，一不小心就high了~~字数有点超→_→别打脸好么(编辑编的冷汗都出来了~~为毛总有几个不能变底色····ಠ_ಠ)
speaker是关于clone技术让绝种的小鸡小鸭小恐龙们重见天日
speed是以克隆孟买象的de-extinction的例子详实的告诉乃们这项技术面临的可能性和困境是神马
obstacle则是对克隆拯救濒危物种的必要性讨论~~里面举证很多简直开启新世界（有兴趣的小盆友可以去看原文，因为太长了所以删减了很多）
ps··请原谅我偷懒都在SA上面找资料~~等google不抽风我会尽量扩大源头的~~enjoy~~
Part I: Speaker
Extinction May Not Be Forever
De-extinction. What if plants and animal species wiped out of existence could be brought back? That's the novel notion springing from recent advances in synthetic biology.
The idea is simple. Find samples, like the mummified passenger pigeon discovered recently in a museum desk drawer, and collect its DNA. Compare said DNA to that of its closest living relatives to see what specific genes make a passenger pigeon unique. Then splice those crucial genes into the living relative's DNA strands to produce a genetic copy of the extinct animal. Resurrection.
The restoration potential is not limited to plants and animals that we have just recently eliminated. We could also potentially bring back species like woolly mammoths or saber-tooth cats. Not dinosaurs though, since DNA has a half-life of just 521 years or so.
Of course, successfully bringing back the mammoth might also require restoration of its habitat, so it has a home to roam. But even without the reappearance of charismatic megafauna, such techniques will find uses from agriculture to injecting a bit more genetic diversity into dwindling populations of endangered species. The biggest contribution of the new biotechnology may not be de-extinction, but preventing extinction in the first place.
—David Biello
Source: Scientific American
http://www.scientificamerican.com/podcast/episode/extinction-may-not-be-forever-13-03-31/
[Rephrase 1, 01:38]
Part II: Speed Cloning Woolly Mammoths: It’s the Ecology, Stupid
By Jacquelyn Gill | March 18, 2013
![]() Artist's rendering of woolly mammoths.
[warm up]
As an ecologist of ice age giants, I long ago came to terms with the fact that I will never look my study organisms in the eye. I will never observe black-bear-sized beavers through binoculars in their natural habitats, build experimental exclosures to test the effects of mastodons on plants, or even observe a giant ground sloth in a zoo.
As a conservation paleoecologist, I study the natural experiments of the past—like climate change and extinction—to better understand the ecology of a warming, fragmented world. Admitedly, part of the appeal of the ice age past is the challenge of reconstructing long-disappeared landscapes from fragments like pollen, tiny fragments of charcoal, and bits of leaves preserved in lakes. In the absence of mammoths, for example, I rely instead on spores of fungi that once inhabited their dung.
[139 words]
[Time 2]
De-extinction could change that. On Friday, a group of geneticists, conservationists, journalists, and others convened in Washington, D.C. to discuss resurrecting extinct species, including the woolly mammoth. De-extinction sounds like science fiction, but it’s rooted in very real conservation concerns. With the sequencing of the woolly mammoth genome complete and recent advancements in biotechnology, the question of whether to clone extinct species like mastodons, dodos, or the Shasta ground sloth is rapidly becoming more of a question of should, rather than how. The latter isn’t straightforward, and involves the integration of a number of cutting edge disciplines, but I’d like to focus on the former: should we clone woolly mammoths?
A growing problem I’ve had (and one which Brian Switek raises in a recent post at National Geographic) is that the de-extinction proposals are Big Ideas, but they they’re often shallow when it comes to ecology. Even the concept of “de-extinction” itself is misleading. Successfully cloning an animal is one thing; rescuing it from the black hole-like pull of extinction is another. Decades of conservation biology research has tried to determine the careful calculus of how many individuals and how much land are needed for a species to survive without major intervention, accounting for its needs for food, habitat, and other resources.
Mammoths have been extinct on continents for over ten thousand years (though dwarf versions survived into the time of the ancient Egyptians on isolated Arctic islands). Even so, the fossil record has yielded rich clues about ecology. All ethical considerations aside, from a conservation biology standpoint, what does it mean to be a mammoth?
[276 words]
[Time 3]
The woolly mammoth is the ice age species with the best-preserved specimens, and it was the first to have its genome sequenced (though the Neanderthals followed in 2010). As far as de-extinction efforts go, it’s likely to be one of the first successful cloning efforts.
However, not all mammoths were woolly tundra-dwellers; in North America, mammoth remains have been found at elevations ranging from sea level to the mountains of the Colorado Plateau, and from Canada to central Mexico. The largest of these, the Columbian mammoth, dwelled in savannas and grasslands like African elephants today, and the smallest—Pygmy Mammoths—lived on the isolated Channel Islands off the California coast.
While knowing their habitat alone is useful in terms of identifying potential cloned mammoth reserves, we do in fact know quite a lot about what mammoths ate. Based on plant materials found in fossilized dung, the contents of permafrost-preserved stomachs, and isotopes in teeth enamel, we know that most mammoths were grazers, preferring grasses and herbs to woody trees and shrubs.
In this way, mammoths were similar to modern African elephants, though evolutionarily they’re more closely related to the forest-dwelling Asian elephants. Unlike horses and camels, which evolved in North America, mammoths were relatively recent comers, arriving around 1.7 million years ago via the same land bridge that the first humans would later take during the last ice age.
Mammoths likely had elaborate social systems similar to modern elephants, and are thought to have lived in groups of up to twenty individuals. Woolly mammoths males had musth glands, which are important in modern elephant reproduction today. Groupings of mammoth bones at sites where multiple individuals died together show extended family structures. Preserved mammoth tracks show extended families walking side-by-side, as well as a decline in juveniles that indicate populations were in decline due to human hunting.
Just like modern elephants today, these groups were all females, and so it’s likely that mammoths were also matriarchal. Groups of females would typically stay together, and males would have been kicked out of the herd and left to fend for themselves when they reached adolescence.
[359 words]
[Time 4]
![]() Skull of a Columbian mammoth at the Hot Springs Mammoth Site. Photo by Robert Geier
How could we possibly know this? The fossil record shows that mammoth tusks grew rings—just a like a tree, except mammoth tusks can record weeks or even days in a mammoth’s life. From the width of rings and their isotopic makeup, we know that mammoth mothers nursed their young for two or three years. In teenage males, the growth rings in the tusks become suddenly narrow, indicating that the male suddenly had to fend for itself (the equivalent of going from your parents’ home-cooked meals to the macaroni and cheese and ramen diets of your first apartment).
Not all teenaged mammoths survived this dangerous period of isolation; at the Hot Springs Mammoth Site, paleontologists have uncovered a number of single, adolescent male skeletons that fell in the sinkhole and perished, one after the other through time. Broken tusks also reveal that, just like modern elephants, mammoth males fought for mates—there’s even a pair of male skeletons locked in eternal combat, unable to disentangle themselves.
Modern elephants have elaborate communication systems involving touch, sight, chemistry, and sound (including infrasonic and seismic communication across long distances). While fossils cannot recapture the sound of a mammoth’s trumpet call, but we do know from modifications in their hyoid bones, tongue, and voice box that they would have been capable of low frequency communication, too.
The mammoth steppe is just as extinct as its namesake, due to a combination of climate change and the loss of those megaherbivores that were likely “keystones,” ecological engineers of their own habitats. Assuming that parts of modern Siberia or boreal Canada would do, how much land would a woolly mammoth need? The science on this is much less clear. By matching the isotopes in tooth enamel with the isotopes in soils, we know that some species of mammoths and mastodons roamed as much as 500km a year, perhaps migrating to track their habitats.
Calculating the carrying capacity of a mammoth herd is not trivial (trust me—I’m working on it!), and involves a careful consideration of how much forage mammoths would need to consume (modern elephants eat as much as 440 pounds a day), proximity to water (modern elephants drink around 60 gallons daily), and the complex interaction between animals, plants, and the changing climates they experienced as their populations dwindled. Once we know how much land a mammoth herd needs, it’s another matter entirely to determine how many of those herds are necessary to maintain viable populations of woolly mammoths in the wild. Whatever that number may ultimately be, it’s worth pointing out that 14,000 years ago, it only took small bands of spear-wielding humans and a backdrop of changing climates to push mammoths and other ice age megafauna over the brink.
[469 words]
[Time 5]
When we think of cloning woolly mammoths, it’s easy to picture a rolling tundra landscape, the charismatic hulking beasts grazing lazily amongst arctic wildflowers. But what does cloning a woolly mammoth actually mean? What is a woolly mammoth, really?
Is one lonely calf, raised in captivity and without the context of its herd and environment, really a mammoth? Does it matter that there are no mammoth matriarchs to nurse that calf, to inoculate it with necessary gut bacteria, to teach it how to care for itself, how to speak with other mammoths, where the ancestral migration paths are, and how to avoid sinkholes and find water? Does it matter that the permafrost is melting, and that the mammoth steppe is gone? As much as I love mammoths, the ecologist in me can’t help but answer: no.
These are practical considerations as much as they are as philosophical ones. Human activity is pushing the earth system outside of the natural range of climate variability that mammoths of all species—woolly or otherwise—would have experienced during their evolutionary history. Ironically, much of what we know about mammoth ecology comes from the newly-exposed carcasses uncovered from the melting permafrost.
There are compelling ecological reasons to resurrect extinct species. Some have argued for rewilding to maintain certain habitats or to perform important functions like seed dispersal or fire suppression. As I’ve written previously, many plants live today as ecological anachronisms, out of context with their extinct dispersers. Bringing back the passenger pigeon may be an important part of saving the sand cherry (or even the American chestnut).
My research on the ecological consequences of the extinctions of mammoths and other megaherbivores in North America indicates that the loss of mammoths during an interval of rapid climate change led to completely novel communities—a period of ecological upheaval that lasted for two thousand years. Work by others suggests that there may have been cascading effects to the biodiversity of small mammals. Modern elephants are keystone species, helping to maintain the African savanna habitat that many other species rely on.
[351 words]
[Time 6]
![]()
Modern elephants greeting one another. Woolly mammoths are thought to have had similar elaborate communication systems.
Losing species—especially ecosystem engineers, foundation species, or keystone herbivores, can lead to cascading effects that can be difficult to predict. The reverse is also true; adding herbivores to landscapes changes them. Are we—is society—prepared to accept those changes?
I understand the impetus to resurrect the woolly mammoth—it comes from that same sense of wonder and drive for discovery that led me to be a scientist in the first place. When I watched 10,000 BC, I admit that I wept openly at the sight of CGI mammoths on the big screen. I would be the first person on a plane to Siberia if mammoths showed up in Pleistocene Park. Science needs icons—rallying points that capture the public interest. Cloning a woolly mammoth could be the equivalent of the moonwalk for biology, resurrecting not just an extinct species, but also rekindling a child-like sense of excitement for the natural world (though admittedly, cloning’s public opinion record has tended to be more one of fear and admonition that scientists are “playing God”). And yet, as Hannah Waters rightfully points out, cloning extinct species may actually be more about us humans than the wildlife we care about.
Arguments against de-extinction often center around what we don’t know—particularly when it comes to the long-term collateral effects of our actions. The precautionary principle can be unsatisfying in conservation, because taken to its logical extreme it precludes action of any kind. We often don’t have the luxury of waiting to determine how effective an action will be, especially as we race to save species on the brink of extinction.
In the case of mammoths, however, there need be no sense of urgency. Perhaps the best course of action is to first demonstrate that we can effectively manage living rhinos and elephants before resurrecting their woolly counterparts in a warming, fragmented, overpopulated world.
Ultimately, cloning woolly mammoths doesn’t end in the lab. If the goal really is de-extinction and not merely the scientific equivalent of achievement unlocked!, then bringing back the mammoth means sustained effort, intensive management, and a massive commitment of conservation resources. Our track record on this is not reassuring.
In the meantime, the least we can do is be guided by what we do know about woolly mammoths in their ecological context. Before we talk seriously about de-extinction, let’s apply the lessons of the woolly mammoth to help save species in the face of pre-extinction.
[416 words]
Source: Scientific American
http://blogs.scientificamerican.com/guest-blog/2013/03/18/cloning-woolly-mammoths-its-the-ecology-stupid/
Part III: Obstacle Will Cloning Ever Save Endangered Animals?
Right now, cloning is not a viable conservation strategy. But some researchers remain optimistic that it will help threatened species in the future
Mar 11, 2013 |By Ferris Jabr
![]()
James and snowmanradio, Wikimedia Commons
[warm up]
In 2009 the Brazilian Agricultural Research Corp. (Embrapa) and the Brasilia Zoological Garden began scavenging and freezing blood, sperm and umbilical cord cells from roadkill and other wild animals that had died, mostly in the Cerrado savanna—anincredibly diverse collection of tropical forest and grassland ecosystems home to at least 10,000 plant species and more than 800 species of birds and mammals, some of which live nowhere else in the world. Specimens were collected from the bush dog, collared anteater, bison and gray brocket deer, among other species.
The idea was to preserve the genetic information of Brazil's endangered wildlife. One day, the organizations reasoned, they might be able to use the collected DNA to clone endangered animals and bolster dwindling populations. So far the two institutions have collected at least 420 tissue samples. Now they are collaborating on a related project that will use the DNA in these specimens to improve breeding and cloning techniques. Current cloning techniques have an average success rate of less than 5 percent, even when working with familiar species; cloning wild animals is usually less than 1 percent successful.
[185 words]
[Paraphrase 7]
Many researchers agree that, at present, cloning is not a feasible or effective conservation strategy. First of all, some conservationists point out, cloning does not address the reasons that many animals become endangered in the first place—namely, hunting and habitat destruction. Even if cloning could theoretically help in truly desperate situations, current cloning techniques are simply too ineffective to make much of a difference. Compared with cloning domestic species—particularly cattle, which have been successfully cloned for years to duplicate desirable traits—cloning endangered species is far more difficult for a number of reasons.
Successful cloning generally involves at least three essential components: DNA from the animal to be cloned; a viable egg to receive that DNA; and a mother to gestate the resulting embryo. Often, hundreds of embryos and attempted pregnancies are needed to produce even a few clones. Scientists usually have a poor understanding of endangered animals' reproductive physiology, which makes it too risky to extract a sufficient number of eggs from that species or rely on females of that species to give birth to clones. Legal protections sometimes preclude threatened species from such procedures as well. To compensate, researchers fuse the DNA of an endangered species with eggs from a closely related species and select mothers from the latter. Such hybrid embryos often fail to develop properly.
Although they are keenly aware of these problems, Martins and his colleagues, as well as a few other scientists around the world, think that efforts to archive the genetic information of endangered wildlife are worthwhile. Some researchers remain optimistic that cloning will become a useful tool for conservation in the future. Optimists point to recent successes cloning wild mammals using closely related domestic species, improved techniques for preventing developmental abnormalities in a cloned embryo, better neonatal care for newborn clones and in vitro fertilization made possible by stem cells derived from frozen tissue.
One species that might benefit from cloning is the northern white rhinoceros, which is native to Africa. In 1960 the global northern white rhino population was more than 2,000 strong, but poaching has reduced their numbers to as few as 11 today. By last count, three live in zoos—two in San Diego and one in the Czech Republic—four live in the Ol Pejeta Conservancy in Kenya and as few as four individuals may still live in the wild based on unconfirmed reports, but they have not been spotted in several years. Most of the captive animals are uninterested in mating or infertile, although two rhinos mated in the summer of 2012.
Right now, though, cloning is unlikely to help the white rhino or any other threatened species. To date, the story of cloning endangered animals is one of a few high-profile successes and many, many failures. Since the early 2000s, using the same technique that produced Dolly, researchers have cloned several endangered and even extinct mammals, including a mouflon sheep and a bovine known as a gaur in 2001; a kind of wild cattle called a banteng in 2003; a wild goat known as the Pyrenean ibex in 2009; and wild coyotes in 2012. In each case many more clones died before birth than survived; in most cases none of the clones survived into adulthood.
Mismatched
All those attempted clones of endangered or extinct animals died in different ways for different reasons, but they all shared one fundamental problem—they were not exact replicas of their counterparts. In most cases, researchers have combined DNA from the threatened species with eggs from a related domestic species. Each surrogate mother is often implanted with dozens of hybrid embryos in order to achieve at least a few pregnancies, a strategy that requires extracting hundreds of eggs. Because the reproductive physiology of most endangered animals is so poorly understood, researchers are often unsure when the animals ovulate and how best to acquire their eggs. In some cases legal protections prevent scientists from harvesting eggs from threatened species. For all these reasons, they turn to more familiar domestic species instead.
Injecting the DNA of one species into the egg of another species—even a closely related one—creates an unusual hybrid embryo that often fails to develop properly in the womb of a surrogate mother. Hybrid embryos have the nuclear DNA of the cloned species and the mitochondrial (mtDNA) DNA of the donor egg. This mismatch becomes problematic as the embryo develops. Nuclear DNA and mtDNA work together; they both contain genetic recipes for proteins with which cells extract energy from food. In a hybrid embryo these proteins do not always fit together properly, which leaves cells starved for energy. Complicating matters further, the surrogate mother often rejects the hybrid embryo because she recognizes some of the embryo's tissues, particularly the placenta, as foreign.
Another problem—and the most intractable so far—is that a hybrid embryo created via nuclear transfer is not a genetic blank slate like most embryos. All vertebrates begin life as hollow balls of embryonic stem cells, which can become almost any type of adult cell. Each of those stem cells contains a copy of the exact same genome packaged into chromosomes—tight bundles of DNA and histone proteins. As the embryo develops, the stem cells begin to take on their adult forms: some become skin cells, others heart cells and so on. Different types of cells begin to express different patterns of genes. Inside each cell an assortment of molecules and enzymes interacts with DNA and histones to change gene expression. Some molecules, such as methyl groups, physically block cellular machinery from reading the genetic instructions in certain segments of DNA; some enzymes loosen the bonds between histones and DNA, making particular genes more accessible. Eventually, each cell type—skin cell, liver cell, brain cell—has the same genome, but a different epigenome: a unique pattern of genes that are actively expressed or effectively silenced. Over time, an adult cell's epigenome can change even further, depending on the animal's life experiences.
So when researchers inject an adult cell's nucleus into an empty egg, the nucleus brings its unique epigenome with it. As Gurdon's early experiments in the 1950s and subsequent studies have shown, an egg is capable of erasing the epigenome of introduced nuclear DNA, wiping the slate clean—to some extent. This process of "nuclear reprogramming" is poorly understood, and the egg often fails to complete it properly, especially when the egg is from one species and the nuclear DNA from another. Incomplete nuclear reprogramming is one of the main reasons, scientists think, for the many developmental abnormalities that kill clones before birth and for the medical issues common to many survivors, such as extremely high birth weight and organ failure.
Some researchers see ways around these problems. Pasqualino Loi of the University of Teramo in Italy was part of a team that successfully cloned endangered mouflon sheep in the early 2000s; the clones died within six months of birth. Loi and his colleagues think they can increase the chances of a hybrid embryo surviving in a surrogate mother's womb. First, they propose, researchers could nurture a hybrid embryo for a short time in the lab until it develops into what is known as a blastocyst—the ball-shaped beginnings of a vertebrate composed of an outer circle of cells, the trophoblast, surrounding a clump of rapidly dividing stem cells known as the inner cell mass. Eventually, the trophoblast becomes the placenta. Researchers could scoop out the inner cell mass from the hybrid blastocyst, Loi suggests, and transplant it into an empty trophoblast derived from the same species as the surrogate mother. Because the surrogate mother is far less likely to reject a trophoblast from her own species, the developing embryo within has a much better chance of surviving.
Scientists have also figured out how to encourage nuclear reprogramming by bathing the egg in certain compounds and chemicals, such as trichostatin A, which stimulate or inhibit the enzymes that determine a cell's epigenome. Most recently, Teruhiko Wakayama of the RIKEN Center for Developmental Biology in Kobe, Japan and his colleagues produced 581 cloned mice from a single donor mouse over 25 generations, using trichostatin A to achieve success rates as high as 25 percent in some but not all generations. To solve the mismatch of mtDNA and nuclear DNA, Loi suggests simply removing the egg's native mtDNA and replacing it with mtDNA from the species to be cloned—something that researchers tried in the 1970s and '80s, but have not attempted recently for reasons that are unclear.
Some of the most successful attempts to clone endangered animals in recent years have involved two of the most beloved domestic species—cats and dogs.
[1448 words]
Source: Scientific American
http://www.scientificamerican.com/article/cloning-endangered-animals/
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发表于 2014-6-16 21:59:02
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