Two years ago there was Dolly, the first mammal cloned from a single adult cell.
Last summer there was Cumulina, the first cloned mouse.
And then, in December, OVI-1 arrived -- the world's first calf to be cloned from an adult cow.
At that point, clone watchers couldn't help but notice: The only animals being cloned were females.
Some people began to wonder. Is there something about males that makes them hard to copy? And if human cloning ever catches on, will men get left behind in the duplicative rush?
Now those concerns can be put to rest. Researchers in Hawaii are reporting today the first documented cloning of an adult male -- a mouse named Fibro, cloned from the tip of a male mouse's tail.
Having proven that it is possible to clone males from tails, the researchers suggest that scientists may want to cut off and freeze the tail tips of various endangered and other precious species, so the animals can be propagated in the future.
University of Hawaii biologists Teruhiko Wakayama and Ryuzo Yanagimachi led the work -- the same scientists who reported last July that they had cloned Cumulina and about 50 other mice. When the scientists made that announcement, they also conceded that their efforts to clone male mice from testicular cells had failed. They promised to keep trying.
In the new experiments, described in today's issue of the journal Nature Genetics, the team started with skin cells, called fibroblasts, taken from the tail tips of male mice. After growing those cells in a dish for awhile, they performed the key step that is the essence of all cloning: In a painstaking procedure repeated 717 times, they isolated the core, or nucleus, of each tail cell -- containing the cell's genes -- and inserted each of those cores into a mouse egg that had its own DNA removed.
As the scientists had hoped, unidentified natural substances inside the eggs "reprogrammed" the tail cell genes so that the eggs behaved as though they were freshly fertilized. After a few initial divisions in a laboratory dish, 274 of the embryos were transferred to the wombs of surrogate mother mice, to grow into clones whose genetic makeup would be identical to the mice whose tails had been snipped.
The method is inefficient. Only three live offspring were born, and two of them died within an hour after birth from breathing problems. The third, Fibro, appears healthy, however, and has sired several litters since arriving by Caesarean section in October.
"He's happy and a good father and has many babies," Yanagimachi said.
All three clones were about normal birth weight. But they also had deformed placentas twice the average size -- a problem that has proven common in many cloned species. Some experts suspect that the abnormal placentas are the result of a molecular glitch caused by the lack of any genetic contribution from a second parent. In some cases, the placental problems seem to be causing other developmental problems, including underdeveloped lungs, which Yanagimachi said may have contributed to the deaths of the two clones.
The lack of cloned males until now was largely a matter of convenience, researchers said. Dolly was cloned from a cow's udder cell, for example, because udder cells were available from other experiments.
Similarly, scientists striving to create bovine clones have focused their efforts more on cows than on bulls because the goal has been to make herds of gene-altered cows that make human medicines in their milk. (Japanese scientists said last year they had cloned a bull, but they have not published their research yet.)
Cumulina and her sisters were cloned from the "cumulus cells" that surround mouse ovaries, which are the mother lode of mouse eggs needed for cloning experiments.
"Basically, they were handy," said George Seidel, a reproductive physiologist at Colorado State University. "They're right where the eggs are, so in the same operation you can get them."