Abstract
Orchids pollinated by sexual deception lure their specific male pollinators by sex pheromone mimicry. Despite the growing list of chemically diverse semiochemicals known to be involved, the chemical basis and flexibility of this extreme pollinator specificity are not fully understood. One promising but rarely applied tool is the synthesis and field testing of chemically related variants for investigating the structural specificity of the pheromone mimics. Here, we build on the discovery of the unusual semiochemical blend used by Drakaea micrantha to sexually lure its male Zeleboria thynnine wasp pollinator. This blend consists of a β-ketolactone (drakolide) and two specific hydroxymethylpyrazines, presumably drawn from two distinct biosynthetic pathways. Here, we synthesized and tested the activity of various stereo- and structural isomers of the naturally occurring drakolide. Our study confirmed that in blends with the two pyrazines, both a mixture of stereoisomers, and the specific stereoisomer of the natural drakolide, elicit high rates of landings and attempted copulations. However, in the absence of pyrazines, both the number of responses and the level of sexual attraction were significantly reduced. When structural analogs were substituted for the natural drakolide, attractiveness and degree of sexual behaviour varied but were generally reduced. Based on our findings, and prior knowledge that related hydroxymethylpyrazines are active in other Drakaea spp., we conclude that the dual sex pheromone mimicry of D. micrantha likely evolved via initial changes in just one of the two biosynthetic pathways. Most plausibly, this involved modifications in the drakolides, with the pyrazines as a ‘pre-adaption’ enhancing the sexual response.
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Acknowledgements
It is both an honor and privilege to have been invited by the editorial team to contribute to this special issue. Rod Peakall notes that his collaboration with Florian Schiestl and Wittko Francke, and the associated discovery of chiloglottones, was a pivotal turning point in his career. To this day, chemical ecology continues to feature strongly in his research on the pollination and evolution of orchids.
The authors acknowledge the facilities, and the scientific and technical assistance of the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State, and Commonwealth Governments. They also thank the Australian Research Council for generously funding their research programs.
Funding
This work was supported by the following Australian Research Council grants: LP130100162 to RP and GRF; DP150102762 to RP, DE160101313 to BB.
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All authors contributed to the study conception and design. Chemical syntheses and analyses were performed by B. Bohman, M. M. Y. Tan, and G. R. Flematti. Field bioassays and statistical analysis were performed by R. Peakall. The first draft of the manuscript was written by R. Peakall and B. Bohman, with editorial input from M. M. Y. Tan and G. R. Flematti. All authors read and approved the final manuscript.
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Bohman, B., Tan, M.M.Y., Flematti, G.R. et al. Drakolide Structure-activity Relationships for Sexual Attraction of Zeleboria Wasp Pollinator. J Chem Ecol 48, 323–336 (2022). https://doi.org/10.1007/s10886-021-01324-4
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DOI: https://doi.org/10.1007/s10886-021-01324-4