A surprising tale of a frog's tail.

The tadpole's metamorphosis into a frog has intrigued biologists for more than a century. Scientists have long known that a hormone produced by the thyroid gland sets the dramatic event into motion, but they've had less insight into the influences of other molecules. Donald D. Brown of the Carnegie Institution of Washington in Baltimore and his colleagues have begun to address that issue.

The researchers have focused on the way that the tail of a tadpole disappears, or is resorbed, during metamorphosis. More than a dozen genes increase their activity during the tail resorption, said Brown last month in Charlottesville, Va., at the annual meeting of the Society for Developmental Biology.

Brown's group has also genetically altered frogs to overproduce certain hormones thought to act during metamorphosis. These hormones haven't proven as important as expected. Frogs engineered to make lots of growth hormone, for instance, become so fat that they die within 9 months after hatching, but they still undergo seemingly normal metamorphosis. Tadpoles that overproduce the hormone prolactin also metamorphose as usual--with one interesting exception.

"They do everything perfectly well, except they can't resorb their tails. They turn into frogs with extremely long tails," says Brown. The leftover tails lose all their muscles during metamorphosis, so they are like scar tissue, he adds.