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Structure-Activity Studies of Semiochemicals from the Spider Orchid Caladenia plicata for Sexual Deception

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Abstract

Sexually deceptive orchids attract specific pollinators by mimicking insect sex pheromones. Normally this mimicry is very specific and identical compounds have been identified from orchids and matching females of the pollinators. In this study, we conduct a detailed structure-activity investigation on isomers of the semiochemicals involved in the sexual attraction of the male pollinator of the spider orchid Caladenia plicata. This orchid employs an unusual blend of two biosynthetically unrelated compounds, (S)-β-citronellol and 2-hydroxy-6-methylacetophenone, to lure its Zeleboria sp. thynnine wasp pollinator. We show that the blend is barely attractive when (S)-β-citronellol is substituted with its enantiomer, (R)-β-citronellol. Furthermore, none of the nine-possible alternative hydroxy-methylacetophenone regioisomers of the natural semiochemical are active when substituted for the natural 2-hydroxy-6-methylacetophenone. Our results were surprising given the structural similarity between the active compound and some of the analogues tested, and results from previous studies in other sexually deceptive orchid/wasp systems where substitution with analogues was possible. Interestingly, high-level ab initio and density functional theory calculations of the hydroxy-methylacetophenones revealed that the active natural isomer, 2-hydroxy-6-methylacetophenone, has the strongest intramolecular hydrogen bond of all regioisomers, which at least in part may explain the specific activity.

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Acknowledgements

BB, GRF, and RP acknowledge the Australian Research Council for funding (DE160101313, FT110100304, LP130100162 and DP150102762). The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. Alyssa Weinstein is thanked for field and laboratory assistance. RP was hosted as a visiting fellow by UWA while completing this study.

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Authors and Affiliations

  1. School of Molecular Sciences, The University of Western Australia, Crawley, WA, 6009, Australia

    Bjorn Bohman, Amir Karton, Gavin R. Flematti, Adrian Scaffidi & Rod Peakall

  2. Research School of Biology, The Australian National University, Acton, ACT 2601, Australia

    Bjorn Bohman & Rod Peakall

  3. Research School of Chemistry, The Australian National University, Acton, ACT 2601, Australia

    Bjorn Bohman

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  1. Bjorn Bohman
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  2. Amir Karton
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Correspondence to Bjorn Bohman.

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Bohman, B., Karton, A., Flematti, G.R. et al. Structure-Activity Studies of Semiochemicals from the Spider Orchid Caladenia plicata for Sexual Deception. J Chem Ecol 44, 436–443 (2018). https://doi.org/10.1007/s10886-018-0946-0

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  • DOI: https://doi.org/10.1007/s10886-018-0946-0

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