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Thursday, Dec. 12, 2002

NATURAL SELECTIONS

A rotten trick to play on a fly


Off the coasts of the Mediterranean islands of Sardinia, Corsica and the Balearics, there are a number of smaller islands, studding the sea like olives in a vast focaccia. On these sun-kissed islands there grows a plant with a feature entirely appropriate to the almost mythical setting. When this plant flowers, it releases a scent that seizes the nervous system like no other. It is the stench of a rotting, flyblown carcass.

The flower is aptly named the dead-horse arum. On the first day of flowering, it releases a potent cocktail of oligosulfides: sulfur-containing chemicals that, as reported this week in Nature, are identical to those produced as meat decays.

Marcus Stensmyr and colleagues from the department of crop science at the Swedish University of Agricultural Sciences in Alnarp worked with researchers at Italy's University of Cagliari to determine the function of the singular smelling flower. They wanted to confirm that the foul smell is used to trick blowflies into spreading its pollen. But also they wanted to investigate whether this was a case of genuine mimicry on the part of the plant. If it was, the odor that rises fetidly from the flower should have the same chemical composition as that emitted from a genuine carcass.

Stensmyr's team used gas chromatography to identify the chemicals present in the arum plant and in rotting gulls. They also tested each odor on blowflies. Blowflies are carrion visitors. They are attracted to the smell of rotting meat because it is a source of food for both them and -- as they lay their eggs in it -- their maggots.

The same three types of oligosulfides were found to be present in the arum flower scent and in that of the carcass. Moreover, the blowflies showed identical behavioral responses when presented with each scent, suggesting, say the authors, that the fly cannot distinguish the mimic from the real thing by scent alone.

Arum plants only produce their foul odor on the first day of flowering. The scientists found that flies only visit the flower on that first day. But to confirm that it is the specific oligosulfide mixture that attracts the flies, the number of flies visiting an arum plant on the first day of flowering was measured against the number visiting the second day, when scientists placed oligosulfide-infused cotton balls in the flower. Since on the second day the plant does not produce any odor, if flies were still attracted to it, it would suggest that it is the oligosulfides that were attracting them.

The dead-horse arum has a trap chamber containing female florets, into which it lures flies. The chamber shuts until male florets at the exit begin to produce pollen. Flies dusted with pollen from another plant thus get trapped and fertilize the flower, and when they are released they are coated with fresh pollen to repeat the process.

Stensmyr and colleagues found no difference between the number of flies visiting trap chambers on Day One and Day Two of their experiments, indicating that the oligosulfides are an important part of the mimicry and that their odor attracts flies. The system is comparable, say the authors, to that used by Ophyrs orchids to attract insects. In the orchid's case, it produces a smell that is sexually attractive to pollinators, but the end result is the same: Insects -- fooled by food or sex -- flock to the plants and pollinate them.

The Australian hammer orchid uses a visual stimulus as well as a chemical one to attract pollinators. Its flower has a shiny fake "head" and a furry "body" mimicking a female wasp. It also releases a pheromone similar to that made by real female wasps. Male wasps try to mate with the phony female, but only succeed in getting covered in pollen.

That the arum plant's scent is chemically indistinguishable from that of a carcass is strong evidence, if any were needed, for natural selection. If it were just coincidence, why would the composition be the same?

The same argument applies to edible butterflies that mimic poisonous species. Birds avoid eating both because they have had bad experiences eating the poisonous ones.

Who could keep a straight face and claim -- like Vladimir Nabokov did -- that the similarity between the mimic and the real thing is due to mere coincidence? Nabokov was curator of Harvard University's Museum of Comparative Zoology in the 1940s and his expertise led to several species of butterfly being named after him. His passion for butterflies and his genius as a writer are not in doubt, but perhaps he should have stuck to writing about underage sex rather than trying his hand at rewriting Darwin.

Rowan Hooper welcomes comments at rowhoop@gol.com


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