What makes a flower produce a flower within a flower? UC San Diego biologists have finally answered this 2,000-year-old question about one of the earliest-recognized botanical abnormalities. It is a mystery that had long perplexed geneticists, who sought to explain the origin of monstrous flowers seen in many roses, camellias, and impatiens. These so-called double flowers, prized within the flower industry for their attractiveness, are effectively sterile because they have lost their reproductive organs.

Plants only rarely produce double flowers in the wild, and scientists have just recently discovered why. Only when each of three virtually identical genes within the plant's cells are mutated, they found, is the result a flower within a flower within a flower, a repetitive process that continues indefinitely-or at least until the smallest organs of the flower cannot be detected. "They endlessly reiterate flower organs," Martin F. Yanofsky, a biology professor who headed the team that made the discovery, said. "They just keep on going and going."

Normal flowers consist of a series of four rings or whorls, the outermost of which is made up of sepals, the green leaflike organ that normally surrounds the flower bud before it opens. Inside the sepals is a ring of petals, then a ring of stamens, the male reproductive structures, and at the center are carpels (often referred to as pistils), the female reproductive structures.

When the three genes found by the UC San Diego scientists are all mutated, the petals, stamens, and carpels are all converted into sepals, resulting in the double flower character. Botanists had long known that all of these structures represent modified leaves, but had been unable to convert leaves into each of the flower organs.

With the knowledge that the trio of genes control the formation of the different flower organs, scientists may now be able to turn on these genes in leaves, where they are normally turned off. This could lead to new plant varieties with colorful petals replacing leaves. More importantly however, the discovery is likely to make an impact on basic science, as geneticists must now rewrite the textbooks on flower development.