Kamis, 06 Maret 2008

Giant flowered Poinsettia?

Giant flowered Poinsettia?

Reporter: Dr Tim Entwisle

photo of Rafflesia

The rafflesia flower is the largest in the world.[Courtesy Anthanasius Kircher Society]

Is one of the world's biggest, and strangest, flowers a pumped up Poinsettia bloom?

Not quite, but the latest DNA evidence links it to the family of the Poinsettia, our local Cheese Tree, commercial rubber trees, and cassava (the source of tapioca). They are all members of the plant family Euphorbiaceae.

Until now, we had little idea where Rafflesia fitted into the tree of life. As I've mentioned before, it has massive flowers up to 1 metre wide, but no roots and no leaves, just fine threads penetrating the tissue of a rainforest vine.

So what's Rafflesia's closest relative? Normally we look the arrangement of the leaves to get a few clues.

Even DNA analysis failed us – critical genes used to construct the evolutionary tree of flowering plants are found only in the green chloroplasts of plant leaves.

But new genes have been sequenced and compared with other flowering plants. We now know that Rafflesia shares an ancestor with the 'euphorbs', a large and diverse family found all over the world.

Although the flowers of euphorbs are quite variable, often they are so tiny and dull they are not recognized as flowers. Nothing like the giant stinking blooms of Rafflesia, from the jungles of Asia.

Knowing this relationship, we can do fun things like estimate how fast its giant flowers evolved. In a paper published in the journal Science, Charles Davis from Harvard University and his colleagues estimate that it took 50 million years of evolution for the blooms to swell to more than 70 times their ancestral size.

Meanwhile most of the euphorb flowers remained (or perhaps became) small and insignificant.

As Davis puts it, the achievement of the Rafflesia is like humans evolving to 150 metres tall – about the height of the Great Pyramid in Giza.

Maybe we'll get there in a few million years.


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