For a science fair project when he was 13 years old, Ben Novak sketched out a recipe for resurrecting the extinct dodo. Many outgrow their wilder teenage passions. Not Novak. The molecular biologist at the University of California, Santa Cruz, is a key player in an effort now sailing into uncharted waters. "It's my job," Novak says, "to bring the passenger pigeon back to life."
A noble mission? A misguided one? Or both? At a TEDx event last month at the National Geographic Society in Washington, D.C., experts explored the technical hurdles and ecological and ethical ramifications of reviving extinct creatures. Proponents say that they are driven, in part, to right historic wrongs. "If it's clear that we exterminated a species, I think we have a moral imperative to do something about it," says Michael Archer, a paleontologist at the University of New South Wales in Sydney, Australia, who is leading efforts to bring back two extinct Australian animals: the thylacine, a hyenalike marsupial, and a frog that brooded young in its stomach (Science, 22 March, p. 1371). The wow factor also plays in, especially when it comes to Ice Age megafauna such as the woolly mammoth. "The boy in me would love to see this majestic creature walking on the permafrost again," says Hendrik Poinar, a molecular evolutionary geneticist at McMaster University in Hamilton, Canada.
Genetic life after death?
A century after the last passenger pigeon died, scientists are embarking on a controversial effort to resurrect the bird's distinctive traits—if not the species itself.
CREDIT: LOUIS AGASSIZ FUERTES/WIKIMEDIA COMMONS
Billions of passenger pigeons darkened North American skies far into the 19th century. But the birds, sold for their meat, proved easy to slaughter; thousands could be captured in a single spring-loaded net. When the last of its kind expired in the Cincinnati Zoo in 1914, the passenger pigeon joined more than 200 bird species snuffed out in the past 2 centuries.
Last year, The Long Now Foundation, a nonprofit organization in San Francisco headed by Stewart Brand of the Whole Earth Catalog fame, held a pair of meetings where scientists sketched out the feasibility of reviving the passenger pigeon. The technical hurdles are formidable. An option pursued for some extinct species—somatic cell cloning—has never worked in living birds, let alone dead ones. Today, says Michael McGrew, an embryologist at The Roslin Institute in Edinburgh, U.K., "We don't have the technology to bring extinct birds back to life."
Long Now is pursuing a radical strategy: Identify the genes underlying key passenger pigeon traits, such as its long tail or orange-colored breast, and splice them into a living relative like the band-tailed pigeon, which is common in the western United States. The first task is to reconstruct the passenger pigeon genome. Recently, Novak sequenced 500 million DNA base pairs—roughly half the genome—from three passenger pigeons stuffed in Troy, New York, in 1860. "We could get the whole genome from just these individuals," says Novak, who is using the rock pigeon's genome as a frame for aligning the sequence. The 1532 passenger pigeon specimens worldwide collectively represent a wealth of genetic variation. Comparing the passenger pigeon and band-tailed pigeon genomes would pinpoint divergent stretches. The next step would be what geneticist George Church of Harvard Medical School in Boston calls "genome editing": splicing passenger pigeon genes into the band-tailed pigeon genome using zinc finger nucleases. Trickier still is producing germ cells carrying the altered genome.
Even if a passenger pigeon began to take shape over successive rounds of genomic editing, there's no telling whether the proxy would thrive. Long Now intends to use passenger pigeon puppets as parental simulacra. But puppets can't teach young pigeons how to migrate. If the team gets that far, Novak says, it will paint the plumage of another migratory species, the homing pigeon, in the colors of their extinct cousins and release tagged passenger pigeons to the surrogate flock.
To Susan Haig, president of the American Ornithologists' Union, the best avian candidates for resurrection are species that recently went extinct, did not migrate, and had a simple mating system, large clutch size, and minimal parental care. The passenger pigeon is not an ideal fit. Haig says she is "dying to see" the ivory-billed woodpecker, a 20th century extinction with enough extant habitat in the southeastern United States to stage a comeback.
The risks of revival are incalculable. "Maybe the passenger pigeon would be a wonderful vector for some nasty disease," says Hank Greely, a bioethicist at Stanford Law School in California (see p. 32). Or its second incarnation could be a devastating invasive species. "No one wants to introduce an avian kudzu into the environment," Greely says. Another worry is that bringing back any extinct species—pigeon, frog, or mammoth—could undercut efforts to protect habitat for endangered species.
Like many scientists on the frontlines of preserving living species, David Ehrenfeld is lukewarm to the idea of bringing back dead ones. The "best expectation" of the passenger pigeon project and other exercises in "recreational conservation," says the zoologist of Rutgers University in New Brunswick, New Jersey, may be to "recover lost traits worthy of study."
Fuente: Science 5 April 2013
No hay comentarios:
Publicar un comentario