Sexual deceit of orchids uncovered

ANU researchers have discovered how an Australian orchid species hoodwinks male wasps into pollinating its flowers by emitting sex pheromones.

Florian Schiestl, Rod Peakall and Jim Mant, from the ANU School of Botany and Zoology have found that the orchid Chiloglottis trapeziformis attracts males of its pollinator species, the thynnine wasp Neozeleboria cryptoides, by emitting a unique volatile compound, 2-ethyl-5-propylcyclohexan-1,3-dione - which is also produced as a sex pheromone by female wasps to attract males.

Dr Peakall said pollination by sexual deception was known only in orchids and the exact replication of a complex volatile compound in this way was previously unknown.

The findings are published in the latest edition of Science, in a paper produced in collaboration with staff from the Institute of Organic Chemistry at the University of Hamburg in Germany.

“In most cases where pollinator-attracting substances have been identified, they proved to be mixtures of rather common compounds,” the researchers wrote.

“The Australian orchid genus Chiloglottis, relies exclusively on sexual deception for pollination.”

By comparing extracts from the orchid and from the heads of female wasps, the researchers were able to identify a single, identical, biologically-active ingredient that stimulates receptors in the antennae of male wasps and sexually attracts them.

The chemical identity of the compound was determined by co-authors Dr Wittko Franke and his research team in Hamburg.  Back in Australia a synthetic copy of the compound was placed on black beads and significantly more male wasps attempted to mate with the decoys than with odourless - beads – which were not attractive to wasps at all.

Dr Peakall was this week awarded a $390,000 five year Australian Research Council grant to further investigate sexually deceptive orchids, and the evolution of the extraordinarily close relationship between insect and flower.

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