Thursday, July 23, 2009

World's Largest Flower Mystery Solved

The world's largest flower, called Rafflesia, can have a diameter up to one meter and can weigh up to 10 kilograms. It also smells like rotting flesh. Discovery News tells us that its genetic roots have been uncovered and that this plant that smells so bad is related to delicate flowers such as poinsettias or violets.

The flower, which is one of 20 species collectively called Rafflesia, is related to poinsettias, violets, passionflowers, and other members of the order Malpighiales, according to a paper published in the current Proceedings of the National Academy of Sciences (PNAS).
Researchers also have some intriguing theories as to why Rafflesia smells so awful, why its flowers bloom only once a year and live for five to seven days, why the plant is a parasite, and why it likes to attract flies that normally go for mounds of dead flesh.

Before going further, here is a photo showing a man surrounding a rafflesia (Credit: Bruce Coleman Collection, photo by Allain Compost).

A rafflesia in one man's hands

How did the researchers proceed?

Rafflesia is a parasitic plant that lacks roots, stems, and leaves. Usually botanists trace plant orders using chloroplast DNA, but since Rafflesia does not have chloroplast genes, Todd Barkman, assistant professor of biological sciences at Western Michigan University, and his team instead analyzed the Rafflesia's mitochondrial DNA (mtDNA), which corresponds to chloroplast DNA.
The mtDNA studies revealed that the large, stinky flower is related to more normal-sized, pleasantly fragrant posies. Despite its enormity, Rafflesia does resemble passionflowers, according to Barkman. Both have their stamens and pistils fused together in a central column, and both produce a corona, or crown, in the shape of a ring.

Here is another photo showing details of this flower (Credit: Todd Barkman).

Details of a rafflesia
Rafflesia did not begin its life as a parasite, but likely evolved this lifestyle in Southeast Asia as it began to depend upon a variety of grape, Tetrastigma. Now Rafflesia lives inside of the grape plant and only reveals itself once a year when it produces the smelly blooms.
"By human standards, the large, lumpy, mottled, stinky flowers could be interpreted as mimicking rotting flesh," Barkman told Discovery News. "The fact that flies are highly attracted to the flowers suggests that they think the flowers look and smell like rotting flesh too!"

For more information about the rafflesia, you can look at this definition on Wikipedia and at this Rafflesia gallery.

Finally, here is the abstract of the paper published by PNAS, "Mitochondrial DNA sequences reveal the photosynthetic relatives of Rafflesia, the world�s largest flower."

All parasites are thought to have evolved from free-living ancestors. However, the ancestral conditions facilitating the shift to parasitism are unclear, particularly in plants because the phylogenetic position of many parasites is unknown. This is especially true for Rafflesia, an endophytic holoparasite that produces the largest flowers in the world and has defied confident phylogenetic placement since its discovery >180 years ago. Here we present results of a phylogenetic analysis of 95 species of seed plants designed to infer the position of Rafflesia in an evolutionary context using the mitochondrial gene matR (1,806 aligned base pairs). Overall, the estimated phylogenetic tree is highly congruent with independent analyses and provides a strongly supported placement of Rafflesia with the order Malpighiales, which includes poinsettias, violets, and passionflowers.

Source: Jennifer Viegas, Discovery News, January 12, 2004; and various websites

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