Less charitable critics still call these efforts stunts, designed to attract attention but doing nothing to conserve species teetering on the brink of extinction. “Producing the odd animal here and there, which may be sick, didn’t seem a very sensible thing to do,” says William Holt, a reproductive biologist at the Zoological Society of London, who in a 2004 review paper called the prospect of cloning highly endangered species “hopelessly optimistic”.
Reverse switch
But another scientific breakthrough, on par with the cloning of Dolly 15 years ago, might force sceptics like Holt to someday reconsider. In 2007 Japanese scientists reported, first in mice and later in humans, that an adult cell could be reverted to an embryo-like stem cell. These so-called “induced pluripotent stem cells” (otherwise known as iPS cells) are in turn capable of making every cell in the body. All of which suddenly made Oliver Ryder’s frozen repository of skin samples a cellular gold mine.
“That made us realize that we had probably the largest repository of potential stem cells,” says Ryder. Skin cells could be converted into sperm that could create an animal through in vitro fertilisation (IVF), or even transformed into whole animals. Both feats have been accomplished in mice and they should be possible in other animals, scientists say.
As a first step, Ryder and a team of stem cell scientists have reprogrammed the skin cells from a northern white rhinoceroses named Fatu, one of seven still alive, and from a drill, a monkey species that lives in tiny, dwindling pockets of west Africa. And recently, scientists said they have created iPS cells from a snow leopard. These cells are a long way from saving species, but “it would be the only chance that I can think of that would prevent the extinction of the northern white rhino,” Ryder says.
Ryder shrugs off the suggestion that money devoted towards such ambitious cloning projects would be better spent on preventing habitat destruction and other, simpler conservation measures. Fighting over the best way to save species instead of saving them will, to future generations, look like “fiddling while Rome was burning,” he replies.
Back from the dead?
If this seems like a daunting challenge, scientists face a much tougher task resurrecting extinct species whose cells were not banked before they vanished from the planet. But this is not stopping scientists like George Church from trying, though he does not underestimate the effort required to bring the passenger pigeon back to the skies.
With extinct animals, scientists need to take more involved measures to recover the complete DNA sequence – its genome. Armed with this code, they then need to find a way of engineering a regular pigeon’s stem cells into behaving like a passenger pigeon’s stem cells by mutating the genome. Church says the complete genome of the passenger pigeon from museum specimens will soon be published, and researchers are beginning to alter the genetic make-up of a more familiar bird – the chicken – to practice their techniques. “What you can do for chicken you should be able to do for pigeon, and that can include creating DNA that you haven’t seen alive for a 100 years,” he says.
But even if Church has the passenger pigeon’s full genetic code, which he expects to recreate within a decade, Church admits that bringing it back to life requires a significant improvement in existing genome engineering technologies. To test his idea works, his team is using a similar approach to engineer mice with traits of naked mole rats. The odd-looking rodents live dozens of years instead of a handful like mice. They are impervious to cancer and do not feel pain from acids. To endow ordinary lab mice with these traits, Church will try to partially rewrite the genomes of mouse stem cells. However, he admits that creating a passenger pigeon from the stem cells of an ordinary pigeon would involve a massive scale-up of the same technologies.
Mammoth goal
The favourite candidate for resurrection, though, might lie in nature’s version of Ryder’s Frozen Zoo. Flash-frozen remains of wooly mammoths have been found preserved under the Siberian permafrost, and scientists hope their bones could be a source of DNA-containing marrow cells for cloning.
