All in the family

Rafflesia, a plant genus native to the jungles of Southeast Asia, is notorious for its flowers, the largest in the world. A Rafflesia flower can be as broad as three feet across and weigh close to twenty pounds. The plant is also completely parasitic. Lacking leaves, roots, and stems of its own, it depends on its host, a grapevine, for nutrients as well as for water. Except when flowering, it lives inside the vine. Rafflesia's most memorable feature, though, is that its enormous blossom stinks of rotting flesh, the better to lure the carrion flies that pollinate it. What on Earth are its evolutionary origins?

That mystery may finally have been solved by Todd J. Barkman, a botanist at Western Michigan University in Kalamazoo, and his colleagues, who have devised a family tree built from ninety-five likely relatives. Their approach was based on an analysis of mitochondrial, rather than nuclear, DNA--the latter changes so rapidly in Rafflesia that it proves of little use to genetic studies. Rafflesia's cousins turn out to be such modest blossoms as the passionflower, the poinsettia, and the tiny, fragrant violet; a few relatives are vines. Perhaps, say the investigators, there's something about the close contact between vines and the plants they entwine that fosters parasitism. ("Mitochondrial DNA sequences reveal the photosynthetic relatives of Rafflesia, the world's largest flower," Proceedings of the National Academy of Sciences 101:787-92, January 20, 2004)

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