Botanists have long been puzzled by the Rafflesia plant. Found only in the rainforests of Southeast Asia, it produces the largest flower in the world. But the lack of most organs normally used to classify plants has made it almost impossible to determine where it sits on the plant family tree. Now, a report to be published online this week in the Proceedings of the National Academy of Sciences reveals all.
As plants go, Rafflesia is an oddball genus. Parasites of tropical vines, the 20 or so species lack roots, stems, or leaves. Usually they live quietly inside the tissues of their hosts. Only when it is time to reproduce sexually do they produce flowers. And what flowers they are! Up to 1 meter wide and 7 kilograms in mass, they emit foul odors to attract pollinating carrion flies. Then, in a few days, they collapse to a black pulp. They are among the strangest things found on the Southeast Asian rainforest floor.
Since its discovery in 1822 by the founder of Singapore, Sir Stamford Raffles, Rafflesia's family background has eluded botanists, says plant taxonomist Todd Barkman of Western Michigan University in Kalamazoo. To solve the puzzle, he and Malaysian colleagues collected bits of the flower of Rafflesia keithii from Borneo and analyzed the DNA sequence of a gene called MatR. Other genes commonly used in plant classification had garbled DNA sequences in Rafflesia, presumably having lost their function, says Barkman. But Rafflesia's MatR appeared intact, and when the researchers compared its sequence to that of almost 100 other plant species, they found a close relationship with the Malpighiales, the plant order that includes passionflowers and violets.
Other Rafflesia experts are elated that its evolutionary lineage has finally been revealed. John Beaman of the Royal Botanic Gardens in Kew, U.K., exclaims, "I was totally surprised!" For nearly 200 years, he says, many botanists believed Rafflesia belonged to the Aristolochia family, also known as pipe vines. He adds that the study is timely, as the forests on which Rafflesia depend are close to extermination.