Singapore: The genome of an extinct Australian frog has been revived and reactivated by a team of scientists using sophisticated cloning technology to implant a "dead" cell nucleus into a fresh egg from another frog species.

The bizarre gastric-brooding frog, Rheobatrachus silus - which uniquely swallowed its eggs, brooded its young in its stomach and gave birth through its mouth - became extinct in 1983. But the Lazarus Project team has been able to recover cell nuclei from tissues collected in the 1970s and kept for 40 years in a conventional deep freezer. The "de-extinction" project aims to bring the frog back to life.

In repeated experiments over five years, the researchers used a laboratory technique known as somatic cell nuclear transfer. They took fresh donor eggs from the distantly related Great Barred Frog, Mixophyes fasciolatus, inactivated the egg nuclei and replaced them with dead nuclei from the extinct frog. Some of the eggs spontaneously began to divide and grow to early embryo stage - a tiny ball of many living cells.

Although none of the embryos survived beyond a few days, genetic tests confirmed that the dividing cells contain the genetic material from the extinct frog. The results of the research are yet to be published.

"We are watching Lazarus arise from the dead, step by exciting step," says the leader of the Lazarus Project team, Professor Mike Archer, of the University of New South Wales, in Sydney. "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."