雅思阅读动植物类真题：塔斯马尼亚虎

　　Tasmanian tiger Extinction Is Forever?“Danger, “ says the sign on the doorof a laboratory at the Australian Museum in Sydney: “Tasmanian Tiger,Trespassers will be eaten!” The joke is that the Tasmanian tiger—a belovedsymbol of the island state that appears on its license² plate—has been extinctfor nearly seven decades. But researchers behind that door are working to bringthe animal back to life by cloning it, using DNA⁴ extracted from specimenspreserved decades ago. Among other things, the work raises questions about thenature of extinction itself.

　　A

　　The Tasmanian tiger’s Latin designation, Thylacinus cynocephalus, or“dogheaded pouched-dog,” makes it redundantly⁷ clear that the marsupial’s felinenickname is a misnomer⁸. Yet its striped coat was cat-like, which runs nearlyshoulder to tail. The animal had large, powerful jaws⁹, which secured thepredator a place atop the local food chain. Females carried their young inbackward-facing pouches¹⁰. Thylacines, once spread throughout mainland Australiaand as far north as New Guinea, were probably outcompeted for food by thedingoes ( 猎狗) that humans introduced to the area some 4,000 years ago, saysAustralian Museum director Mike Archer³, founder¹¹ of the cloning project.Eventually, thylacines remained only on the dingo-free island of Tasmania, southof the mainland. But with the arrival of European settlers in the 1800s, themarsupial’s days were numbered. Blamed (often wrongly) for killing¹² livestock,the animals were hunted indiscriminately. The government made thylacines aprotected species in 1936, but it was too late; It was a frigid¹³ winter night in1936. A lone¹⁴ Tasmanian tiger huddled¹⁵ in his— or her— open enclosure at HobartZoo. With nowhere to shelter from the cold and no keepers to care, thedelicately striped animal died. When this solitary¹⁶ animal—whose sex was not evenrecorded because of lack of interest—died, so did an entire species, the lastspecimen reportedly died in captivity¹⁷ the same year. What’s more, with thepassing into extinction of the Tasmanian tiger, Thylacinus cynocephalus, it wasthe end of the line for an entire family of marsupials thathad lived inAustralia for millions of years.

　　B

　　The Australian researchers set out to bring the animal back partly to atonefor humanity’s role in its extinction, Archer says. The idea took root 15 yearsago when he saw a pickled thylacine pup in the museum’s collection. “It jarredme and started me thinking,” recalls the 58-year-old paleontologist andzoologist, who received his undergraduate degree from Princeton University andhis doctorate¹⁸ from the University of Western Australia. “DNA is there cipe formaking a creature. So if there is DNA preserved in the specimen⁵, why shouldn’twe begin to use technology to read that information, and then in some way usethat information to reconstruct the animal? I raised the issue with ageneticist. The response was derisive²⁰ laughter.”

　　C

　　Then, in 1996, Dolly the sheep burst onto the scene and, suddenly, Archersays,“cloning wasn’t just a madman’s dream.” Dolly proved that DNA from anordinary animal cell— in her case, a ewe’s udder— could generate a virtuallyidentical copy, or clone, of the animal after the DNA was inserted into atreated egg, which was implanted in a womb and carried to term. Archer’s goal iseven more ambitious: cloning an animal with DNA from long-dead cells,reminiscent of the sci-fi novel and movie Jurassic Park. The challenge? The DNAthat makes up the chromosomes²³ in which genes²⁴ are bundled falls apart after acell dies.

　　D

　　Researchers working with Don Colgan, head of the museum’s evolutionarybiology department, extracted DNA from a thylacine pup preserved in alcohol in1866, and biologist Karen Firestone obtained additional thylacine DNA from atooth and a bone. Then, using a technique called polymerase chain reaction, theresearchers found that the thylacine DNA fragments could be copied. Thescientists next have to collect millions of DNA bits and pieces and create a“library” of the possibly tens of thousands of thylacine genes— a gargantuantask, they concede. Still, an even greater obstacle looms²⁵, that of stitching allthose DNA fragments together properly into functioning chromosomes; thescientists don’t know how many chromosomes a thylacine had, but suspect that,like related marsupials, it had 14. But no scientist has ever synthesized amammalian chromosome²² from scratch. If the Aussie scientists accomplish thosefeats, they may try to generate a thylacine by placing the synthetic²⁶ chromosomesinto a treated egg cell of a related species— say, a Tasmanian devil, anothercarnivorous marsupial—and implant²¹ the egg in a surrogate mother.

　　E

　　Such cross-species cloning, as the procedure is called, is no longerfantasy. In 2001, Advanced Cell Technology (ACT) of Worcester, Massachusetts,succeeded in cloning, for the first time, an endangered animal, a rare wild oxcalled a gaur. This past April, scientists from ACT, Trans Ova Genetics of SiouxCenter, Iowa, and the Zoological Society of San Diego announced they had cloneda banteng, an endangered wild bovine²⁷ species native to Southeast Asia, using adomesticated cow as a surrogate mother. Meanwhile, researchers in Spain aretrying to clone an extinct mountain goat, called a bucardo, using cellscollected and frozen before the species’ last member died in 2000. Otherscientists hope to clone a woolly mammoth²⁸ from 20,000-year-old specimens⁶ foundin Siberian permafrost.

　　F

　　Many scientists are skeptical²⁹ of the thylacine project. Ian Lewis,technology development manager at Genetics Australia Cooperative Ltd., inBacchus Marsh³⁰, Victoria, Australia, says the chances of cloning an animalfrom“snippets” of DNA are “fanciful.” Robert Lanza, ACTs medical director andvice president, says cloning a thylacine is beyond existing science. But itmaybe within reach in several years, he adds: “This area of genetics is movingforward at an exponential rate.”

　　G

　　In Australia, critics say the millions of dollars that the thylacineproject will cost would be better spent trying to save endangered species anddisappearing habitats. One opponent, Tasmanian senator and former AustraliaWilderness Society Director Bob Brown, says people might become blase³¹ aboutconservation if they’re lulled³² into thinking a lost species can always beresurrected. The research “feeds the mind-set that science will fix everything,”he says. Another concern touches on the great nature-nurture quandary³³: Would acloned thylacine truly represent the species, given that it would not have hadthe chance to learn key behaviors from other thylacines? For some carnivores,says University of Louisville behavioral ecologist Lee Dugatkin, “it’s clearthat young individuals learn various hunting strategies from parents.” And afosterparent might not fill the gap. Dugatkin asks whether a cloned Tasmaniantiger raised by a surrogate Tasmanian devil would just be a devil in tiger’sclothing.

　　H

　　But Archer says, in effect, a thylacine is a thylacine, however its DNAblueprint is obtained, because much animal behavior, including that ofmarsupials, is genetically³⁴ hardwired or instinctual. We take kittens and raisethem with humans, but they still behave like cats,” he points out. And Archer,who envisions nature preserves populated by cloned thylacines and theiroffspring, says the project is actually a boon³⁵ to conservation: it shows what ittakes just to contemplate³⁶ resurrecting a vanished species. For now, Archer andcoworkers are trying to piece together the thylacine’s exact genetic¹⁹ makeup³⁷.

分享到：