Identification of (Z)-8-Heptadecene and n-Pentadecane as Electrophysiologically Active Compounds in Ophrys insectifera and Its Argogorytes Pollinator

Abstract

Sexually deceptive orchids typically depend on specific insect species for pollination, which are lured by sex pheromone mimicry. European Ophrys orchids often exploit specific species of wasps or bees with carboxylic acid derivatives. Here, we identify the specific semiochemicals present in O. insectifera, and in females of one of its pollinator species, Argogorytes fargeii. Headspace volatile samples and solvent extracts were analysed by GC-MS and semiochemicals were structurally elucidated by microderivatisation experiments and synthesis. (Z)-8-Heptadecene and n-pentadecane were confirmed as present in both O. insectifera and A. fargeii female extracts, with both compounds being found to be electrophysiologically active to pollinators. The identified semiochemicals were compared with previously identified Ophrys pollinator attractants, such as (Z)-9 and (Z)-12-C₂₇-C₂₉ alkenes in O. sphegodes and (Z)-9-octadecenal, octadecanal, ethyl linoleate and ethyl oleate in O. speculum, to provide further insights into the biosynthesis of semiochemicals in this genus. We propose that all these currently identified Ophrys semiochemicals can be formed biosynthetically from the same activated carboxylic acid precursors, after a sequence of elongation and decarbonylation reactions in O. sphegodes and O. speculum, while in O. insectifera, possibly by decarbonylation without preceding elongation.

Keywords: Ophrys; fly orchid; pollination; semiochemicals; sexual deception.

Figure 1

(a) Semiochemicals from Ophrys insectifera (1–2; 1 = (Z)-8-heptadecene, 2 = n-pentadecane) and female Argogorytes mystaceus (1–5; 3 = (Z)-6-pentadecene, 4 = (Z)-7-pentadecene, 5 = (Z,Z)-6,9-heptadecadiene). (b) GC-MS total ion chromatograms of female A. fargeii (upper trace) and O. insectifera (lower trace). (c) GC-EAD of SPME extracts of O. insectifera to antenna of A. mystaceus males. Two replicated analyses are shown. (d) GC-EAD of synthetic standards 1–5 to antenna of A. mystaceus. Two replicated analyses are shown.

Figure 2

Proposed biosynthesis of bioactive alkenes in Ophrys sphegodes (from [32]) and O. insectifera.