BioEd Online

With blooms that stink of rotting flesh and span up to a metre across, a flowering Rafflesia arnoldii is hard to miss in the tropical forests where it grows. But it has taken taxonomists nearly 200 years since when the odd plant was first described to find its place in the family tree.

Knowing the plant's closest relatives not only solves a centuries-old mystery, but could also help scientists to work out how floral gigantism evolved.

The plant's taxonomic position, along with that of other species in the Rafflesiaceae group, has been hard to pin down because of a lack of clues in the morphological and genetic data, says Charles Davis of Harvard University Herbaria in Cambridge, Massachusetts.

The plants are parasitic, using fungal-like threads to extract nutrients and water from host tissues. As a result, they have no leaves or stems the morphological features normally used to identify plants.

Even DNA has proved unhelpful in classifying this group. DNA from food-producing chloroplasts is commonly used to identify plant relationships because it is easy to extract and evolves at a rate that allows scientists to track evolutionary changes (fast enough so that variation can be easily detected, but not so fast that later changes conceal earlier ones). But in parasitic plants, the genes for food production are redundant, and so are either missing or are severely truncated.

Family roots

Davis and his collaborators tackled the taxonomic challenge by instead looking at mitochondrial DNA.

Broad-scale studies using mitochondrial DNA had already shown that Rafflesiaceae are members of the Malpighiales order¹. But by analysing about 11,500 base pairs of DNA from more than 100 species representing all families of the Malpighiales, Davis and his team show that Rafflesiaceae are nestled within the Euphorbiaceae, or spurge family.

The spurge family includes more than 6,000 species among them the tropical cassava - which mostly have small flowers measuring just millimetres across.

"To find Rafflesiaceae being bedfellows with a small-flowered family blew us away," says Daniel Nickrent of Southern Illinois University in Carbondale, a co-author of the study, which was published in this week's Science².

"The scale of change is unheard of in the plant kingdom and is even pushing anything in the biological world," says Nickrent. "It's like comparing man to the Great Pyramids of Giza."

They must be giants

All the Rafflesiaceae plants have giant flowers that smell of rotting meat, with R. arnoldii being the queen stinker. The plants probably experienced an evolutionary pressure to generate larger and larger flowers to broadcast their powerful stench to distant pollinating flies in the damp, still conditions of a tropical forest, explains Nikrent.

By looking at the fossil record of Euphorbiaceae, the researchers showed that the enormous flower size evolved mainly during a 46-million-year period after Rafflesiacea diverged from the rest of the Euphorbiaceae, but before it diversified into multiple lines. Unfortunately, however, there are no Rafflesiaceae fossils. So it is impossible to determine whether the dramatic shift in flower size occurred during a tiny period of time within this window, or in some slower or more stepwise fashion, points out Doug Soltis, a botanist as the University of Florida in Gainesville who was not involved in the study.

Knowing the taxonomic status of Rafflesiaceae could still, however, prove an important first step in understanding what developmental genes are involved in floral gigantism. "If you know Rafflesia's closest relatives, you can ask more specific questions from a developmental-genetics viewpoint, as you know what to compare it with," says Soltis. "This research is a springboard for the next studies."

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