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An extinct species of Australian frog has been cloned by scientists who implanted a dead cell nucleus into an egg from another breed of frog.
The gastric brooding frog, or Rheobatrachus silus, swallowed its eggs, brooded its young in its stomach and then gave birth through its mouth.
It became extinct in 1983 but researchers at the University of New South Wales managed to recover cell nuclei from tissue samples collected in the 1970s. These samples were kept for 40 years in a deep freeze.
Scientists on the Lazarus Project have been trying to bring the species back from extinction by using the cell tissue.
Over five years, the researchers used a technique called somatic cell nuclear transfer, which involved taking fresh donor eggs from the great barred frog, a distant relative of the extinct species.
They inactivated the egg nuclei and replaced them with the nuclei from the gastric brooding frog.
Some of the eggs began to divide and grow to early embryo stage but none have yet survived beyond a few days. Genetic tests have confirmed the embryos are that of the extinct species.
Mike Archer, research team leader, said: "We are watching Lazarus arise from the dead, step by exciting step.
"We've reactivated dead cells into living ones and revived the extinct frog's genome in the process. Now we have fresh cryo-preserved cells of the extinct frog to use in future cloning experiments.
"We're increasingly confident that the hurdles ahead are technological and not biological and that we will succeed. Importantly, we've demonstrated already the great promise this technology has as a conservation tool when hundreds of the world's amphibian species are in catastrophic decline."
Archer spoke about his research at the TEDx DeExtinction event in Washington DC, which was hosted by Revive and Restore and the National Geographic Society.
Scientists highlighted plans to bring a number of extinct species back to life, including the Tasmanian tiger, the sabre-toothed tiger, the woolly mammoth and the North American passenger pigeon.
They also confirmed that it would not be possible to bring back dinosaurs for a real-life Jurassic Park scenario as they have been extinct for too long.