Biggest Bloom: Superflower changes branch on family tree

Plants with buds the size of basketballs, which open flowers up to a meter across, must be reclassified as relatives of poinsettias, say researchers who’ve examined the DNA of the world’s largest known flowers.

HUGE CHANGE. Rafflesia arnoldii ranks as the species with the largest known individual bloom. Smelling of rotting flesh, the meter-wide flower attracts carrion-loving insects for pollination. J. Holden
All 50 species of rafflesias, including some small-flowered ones, live as parasites on plants in the grape family in Southeast Asia. J. Holden

For almost 2 centuries, botanists have debated where rafflesia plants, with their odd flowers, sit on the plant family tree. Early observers asked whether they were flowering plants or fungi. Later, botanists disagreed about the plants’ nearest relatives. Some pointed to passionflowers, with their elaborate collars and fused sex organs, while others argued for pipevines, with their big, meat-colored flowers.

Now, after analyzing eight genes, Charles C. Davis of Harvard University and his colleagues put the rafflesias in other company. The closest relatives of rafflesias lie in Euphorbiaceae, the family of poinsettias and castor beans, the researchers say in the March 30 Science.

Although holiday decorators may think of poinsettias as big flowers, botanists see all that red fandangle as bracts, or modified leaves, that surround tiny true flowers. The Euphorbiaceae family includes plenty of other tiny flowers, and the species within it that Davis and his colleagues have identified as the nearest relatives of rafflesias have blooms only a few millimeters across.

Davis calculates that some little dot of an ancestor started a 79-fold size increase during the past 46 million years to yield the modern champ Rafflesia arnoldii.

Rafflesias have also evolved into parasites without true roots or leaves, and as such provide a huge challenge for gardeners trying to grow them (SN: 9/11/99, p. 172). Davis says that he knows of only four botanic gardens that have successfully grown any rafflesias.

People sometimes confuse rafflesias with the big, smelly Amorphophallus corpse lily, Davis says. However, the corpse lily isn’t a single flower but instead a blooming spike a meter or so long covered with hundreds of tiny flowers.

Davis’ new assertion is a surprise, says Todd Barkman of Western Michigan University in Kalamazoo. He does agree that rafflesias belong on the big evolutionary tree branch occupied by the order Malpighiales, which includes the family Euphorbiaceae. He and his colleagues published that conclusion, based on a DNA analysis, in 2004. However, Barkman says that flower structures don’t suggest to him that rafflesias are close to poinsettias and castor beans.

“No botanist in their right mind would have accepted a bet that among the 38 families of Malpighiales, it would be the Euphorbiaceae,” says Susanne Renner, the systematist who directs the Munich Botanical Garden.

It’s not clear whether the new results will lead to renaming the storied family Rafflesiaceae. “That would really bristle some people,” says Davis. “They’re our charismatic megaflora.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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