LONDON — Dolly the cloned sheep was put to death Friday, after premature aging and disease marred her short existence and raised questions about the practicality of copying life.
The decision to end Dolly's life at age 6 — about half the life expectancy of her breed — was made because a veterinarian confirmed she had a progressive lung disease, according to the Roslin Institute, the Scottish lab where she was created and lived.
"We must wait for the results of the post-mortem on Dolly in order to assess whether her relatively premature death was in any way connected with the fact that she was a clone," said Richard Gardner, a professor of zoology at Oxford University and chair of the Royal Society working group on stem cell research and therapeutic cloning.
"If there is a link, it will provide further evidence of the dangers inherent in reproductive cloning and the irresponsibility of anybody who is trying to extend such work to humans."
Ian Wilmut, the leader of the team that created Dolly, said it was unlikely her illness was attributable to being a clone.
"The most likely thing is an infection which causes a slow progressive illness and for which there isn't an effective treatment," he said. "Sadly, we have had that in some of the sheep on the farm, so that's the most likely explanation, but we don't know."
Wilmut declined to name the disease but said it was a common respiratory infection that had been diagnosed in another of the sheep Dolly was housed with.
"The most likely thing is she caught it from that sheep and it's an unfortunate result of having to be housed in order to give her security and so that we could observe her," Wilmut said.
"Clearly, the whole group are very upset and sad."
Griffin said that Dolly had been coughing for about a week before the vet came Friday afternoon.
She was born July 5, 1996, in a research compound of the Scottish institute, and the achievement of her creation — announced Feb. 23, 1997 — created an international sensation.
Researchers had previously cloned sheep from fetal and embryonic cells, but until Dolly, it was unknown whether an adult cell could reprogram itself to develop into a new being.
The Dolly breakthrough heightened speculation that human cloning inevitably would become possible.
But one of the biggest fears was that Dolly might have been born prematurely old.
It was feared that using adult genetic material to make a clone might produce an animal whose cells were already aged. On the other hand, scientists hoped the genetic clock might be "wound back" to its starting point.
Dolly, a Finn Dorset sheep named after the singer Dolly Parton, bred normally on two occasions with a Welsh mountain ram called David, first giving birth to Bonnie in April 1998 and then to three more lambs in 1999.
The births were good news, showing that clones can reproduce.
But in 1999, scientists noticed that the cells in Dolly's body — cloned from the breast cell of a 6-year-old adult ewe — had started to show signs of wear more typical of an older animal.
Then in January 2002, her creators announced she had developed arthritis at the relatively early age of 5 1/2 years, stirring debate over whether cloning procedures might be flawed.
Some geneticists said the finding showed that researchers could not manufacture copies of animals without the original genetic blueprint eventually wearing out.
There are now hundreds of animal clones around the world, including cows, pigs, mice and goats, many of them appearing robust and healthy.
But many attempts to clone animals have ended in failure. Deformed fetuses have died in the womb with oversized organs, while others were born dead. Others died days after being born, some twice as large as they should have been.
"It's important to remember just what she did contribute," Wilmut said. "She made biologists think totally differently about the way cells develop for all of the different tissues. The experiments that led to her birth are one of the things that are making people think very differently about how to produce cells to treat Parkinson's disease and other unpleasant diseases."
Dolly's body has been promised to the National Museum of Scotland and will eventually be put on display in Edinburgh, the Roslin Institute said.
On the Net:
Roslin Institute, http://www.roslin.ac.uk/news/