The trick with cloning is to take the genetic material from the animal to be cloned—which is sequestered in the nucleus of the cell—and transferring it into an unfertilized egg from which the nucleus had been removed. The chemicals in the egg have special properties that allow the genes in the transplanted nucleus to guide the development of a complete organism.
Loi's team recovered cells from two mouflon ewes that had died the previous day in a Sardinian pasture. Using a very fine needle Loi removed the nucleus from the unfertilized egg of a common domesticated sheep and replaced it with a single mouflon cell.
With a nucleus containing mouflon DNA—the genetic instruction manual to make a mouflon lamb—the egg started to divide as if it had been fertilized. After a few days it formed a small ball of cells that was transplanted into the surrogate mother—in this case, also a domesticated sheep.
Of the seven embryos that were surgically implanted into four surrogate mothers, one resulted in the birth of a clone.
Since Dolly, cattle, pigs, mice and more sheep have been cloned. While these species may appear dull material for a technique as powerful as cloning, the techniques currently available have been fine-tuned to these domestic species.
"We know a lot about the reproductive cycles of cattle and sheep that we don't know for species like the black rhino and the Sumatran tiger," said Ryder, "which makes exotic species more difficult to clone."
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