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Critter Chemistry

October 31, 2003

The Chemistry of Sexual Deception

An Australian orchid uses a single simple diketone to attract wasps that pollinate it

Bethany Halford

The Australian orchid Chiloglottis trapeziformis' very existence hinges upon there being some truth to the old adage that love is blind. Unlike other flowers, which lure pollinators with nectar, C. trapeziformis attracts its pollinator, the thynnine wasp Neozeleboria cryptoides, by producing the same pheromone that the insects use to attract the opposite sex--a practice known as sexual deception. Fooled by the pheromone, the male wasp will try to mate with the orchid. But ultimately it leaves the encounter with nothing more than a fine coat of pollen that it carries to its next liaison with a sexually deceptive orchid.

DECEPTIVE LIAISONS A simple diketone emitted by the Australian orchid Chiloglottis trapeziformis elicits the amorous attention of the thynnine wasp Neozeleboria cryptoides. ©Science 2003

When a team of researchers from the University of Hamburg, Germany, and Australian National University, Canberra, set out to elucidate the chemistry of this sexual deception, they were surprised to find that one simple, cyclic diketone accounted for all of the wasp's amorous behavior [Science, 302, 437 (2003)]. "I was almost shocked," says principal investigator Wittko Francke, an organic chemistry professor at the University of Hamburg. Francke, who has studied insect-plant interactions for nearly 30 years, explains that relying on a lone chemical attractant leaves the orchid with limited evolutionary flexibility. Its procreation depends completely upon the wasp.

Sexually deceptive European orchids that Francke and the paper's coauthor Florian P. Schiestl, now at ETH's Geobotanical Institute in Zürich, have studied use mixtures of ubiquitous alkenes and alkanes in specific proportions to attract pollinators [Nature, 399, 421 (1999); C&EN, June 7, 1999, page 24]. From an evolutionary standpoint, this may offer the orchids increased flexibility. Francke says when a plant uses many compounds, it can vary the proportions of those chemicals to create an infinite number of attractants. But when a plant uses just one compound, then it either lures a pollinator or it doesn't. That strategy, Francke says, is "very unusual."

Thomas Eisner, a biology professor at Cornell University, describes Francke and Schiestl's work as elegant and agrees that the finding is unexpected. "I would not have predicted that. One would think of the system as being too inflexible," he tells C&EN. "Plants evidently have many a secret worth discovering."

The researchers identified and isolated the lone--and previously unreported--pheromone that they dubbed "chiloglottone" from extracts of both the orchid's labella and female wasps' heads.

Although chiloglottone is a relatively simple molecule (2-ethyl-5-propylcyclohexan-1,3-dione), determining its structure proved challenging. The orchids and wasps contain only nanogram amounts of the compound, and the team was not able to extract enough of it for nuclear magnetic resonance spectroscopy. The diketone's reactivity further limited the researchers, because it readily polymerizes at room temperature. Ultimately, they used gas chromatography-mass spectroscopy, infrared spectroscopy, and chemical derivatization to figure out chiloglottone's structure.

"I think it's a beautiful piece of work," says Cornell University chemistry professor Jerrold Meinwald. "It also shows the crucial role of organic synthesis in determining structure."

Francke and Scheistl's group also synthesized chiloglottone in a one-pot procedure that they speculate may be similar to the biosynthesis of the compound. In field studies, the synthesized pheromone proved to be equally as attractive to male wasps as the orchid itself, orchid extract, extract from female wasps' heads, and even a pheromone-emitting female wasp.

Francke jokes that while the male wasp clearly gets nothing out of the orchid's sexual deception, perhaps it helps the female wasps determine which males would make the most clever mates.

Chemical & Engineering News
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