She promises. She doesn't deliver.
POSTED 16 OCT 2003

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A sexually attracted male pollinator Neozeleboria cryptoides, carrying a mass of pollen, departs the orchid Chiloglottis trapeziformis after depositing pollen from another flower. Photo: Courtesy Rod Peakall, School of Botany and Zoology, Australian National University

This wasp is doing the orchid a favor by trying to copulate with it. The wasp will spread the orchid's pollen. And the wasp will get ... zilch from the deal! Photo: Courtesy Florian Paul Schiestl, Swiss National Institute for Technology

An organism has no higher calling than reproduction.

One sneaky orchid. To a male wasp, it smells like a fertile female wasp!

Wasp flares its pollen-laden wings over flower.That's an ancient plot, but it's as modern as the freshest femme fatale film. Empty promises also underlie the reproductive strategy of the Australian orchid, Chiloglottis trapeziformis. "Chilo," as we'll call it, is in a large genus of orchids that use sexual deception to attract pollinating insects.

Pollination -- plant sex -- often uses wind or insects to spread pollen from plant to plant. To attract a pollinating insect, plants typically supply nectar -- sugary fluid. It's not altruism, but simple survival -- an organism has no higher calling than reproduction.

Now we hear that one of these sneaky Australian orchids attracts pollinating male wasps by mimicking a single pheromone -- chemical attractant -- used by the female wasp. The guy's tiny brain (does this sound at all familiar, ladies?) is fooled into thinking that a female is ready and waiting for him, so he engages in copulatory behavior. That spreads pollen among the various Chilo's.

Pheromones are so powerful that the fellas respond like robots to a few micrograms.

And what does the wasp get from the deal? As far as we know, nada. "A plant usually ... produces something that is rewarding for the pollinator," says Wittko Francke, a professor at the Institute of Organic Chemistry at the University of Hamburg. "Many orchids ... save their energy and resources by not producing nectar, and specialize in producing something that cheats the pollinator, attracts them without rewarding them."

Social parasites
The word for this kind of rip-off is parasitism. But don't cry for the wasp Neozeleboria cryptoides . In the Australian eucalyptus forests where these barroom shenanigans take place, the wasp's larvae devour scarab beetles for a living. It's also a parasite, in other words.

While hundreds of orchids use sexual deception, most attract pollinators with a mix of common chemicals. But this week in Science, Francke and colleagues described the first orchid known to fool wasps using a single chemical.

 Black and yellow wasp courts an iridescent flower.

Francke, who has studied chemical communication among plants and insects for 30 years, says orchids that make multiple pheromones, "Are very flexible, they can produce a little more of this, a little less of that compound to make a new bouquet and perhaps attract another pollinator."

It's the opposite story with Chilo, he adds -- the orchid is utterly dependent on the wasp. Being able to study two contrasting systems should help unravel the evolutionary basis for sexual deception, Francke adds. "Now that we have the chemistry in hand, it is very nice to follow up these two different systems, it will facilitate the investigation on the evolution of such systems."

Such a deception probably starts with food deception, says Florian Schiestl, an evolutionary biologist at the Swiss National Institute for Technology, where the orchid appears like food to its pollinator. "Then it's just a minor step to produce specific compounds to attract males as copulators," he says.

Spilling their seed
While such specificity is risky -- if the wasp disappears, the orchid is doomed -- the orchid benefits from its sleight-of-scent. It doesn't need to make nectar to attract pollinators. And it can stretch its precious pollen. "If you produce a sex pheromone, you become very specific in pollination... and you avoid pollen loss," Schiestl says. "They have all their pollen packed into two units, so the whole reproductive effort depends on one pollinator, and you want that individual to go to an orchid of the same species."

If pollinators are attracted to several plant species, pollen can go to waste.

Things are bleaker from the wasps' point of view. "It's clearly parasitism," says Schiestl. Female wasps are wingless, he points out. "When they hatch in the soil, they put out the sex pheromone for the males. If they do it in an orchid patch, the males would have a difficult time finding them."

At issue is the survival of the species -- and of individuals, he adds. "The females depend on the male to pick them up and carry them to a food source, or they will die of starvation." Because males waste time and energy finding females wasps in a Chilo patch, the wasps are under evolutionary pressure to find a pheromone that the plant can't duplicate, Schiestl says.

Wasp perches on dark bead on a thin metal stem.
To test whether they'd made the right sexual attractant, the researchers put some on this bead. The visit of this male wasp confirms the authenticity of their synthetic copy of the female pheromone. One compound, identical in the female and the orchid, lures the male wasps. Some of the pheromonally fooled fellas even tried to copulate with the bead! Photo: Rod Peakall

Success story
Still, Rod Peakall, a researcher at Australian National University, told us via e-mail that sexual baiting is a success story. "Despite the apparent evolutionary risks to both orchid and pollinators, this is a surprisingly common pollination strategy amongst Australian orchids. At least 100 species in nine orchid genera, and perhaps as many as 300 species, are involved."

The victims, he says, include several kinds of wasps, and ants and sawflies.

And while the system exploits males, Francke says it might make their brains evolve. In an orchid patch, he says, the average male wasp is a stupid wasp, and it copulates blindly with whatever smells right. But a few brainy wasps might use visual or other cues to identify females. "They may be clever, and not rely solely on their nose," says Francke.

And guess whose genes would be more common in the next generation? Think about it: Could sexual deception make your offspring smarter?

Why Files to Hollywood: We will ship the screenplay for "I'll do anything for love!" in the morning.

-- David Tenenbaum


Bibliography:
The Chemistry of Sexual Deception in an Orchid-Wasp Pollination System, F.P. Schiestl et al, Science, Oct. 17, 2003.

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