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

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.

Species-specific male pollinators found for three native New Zealand greenhood orchids (Pterostylis spp.) suggest pollination by sexual deception

Many orchids achieve pollination by deceptive means. Sexually deceptive orchids are pollinated by male insects, which are lured to flowers that mimic the sex pheromones and/or appearance of their female conspecifics. This specialised pollination strategy was recently confirmed for the first time in a Pterostylis species in Australia. We investigated whether this pollination strategy may also be operating in Pterostylis species in New Zealand where generalised plant–insect pollination strategies are most commonly documented. The breeding systems of Pterostylis oliveri Petrie and Pterostylis irsoniana Hatch were investigated in the field with pollination treatments. Sticky traps were set up over flowering P. oliveri, P. irsoniana and Pterostylis venosa Colenso to catch potential pollinators of the flowers. Insects caught carrying orchid pollinia were identified, and the pollinia were identified to plant species with nuclear rDNA internal transcribed spacer (nrDNA ITS) sequences. Both P. oliveri and P. irsoniana were found to be self-compatible, but dependent on insects for pollination. Pollinia from each of the three Pterostylis spp. were found to be carried species-specifically by male fungus gnats (Diptera: Mycetophilidae): only Mycetophila latifascia fungus gnats carried the pollinia of P. oliveri, Morganiella fusca gnats carried the pollinia of P. irsoniana, and Tetragoneura sp. carried the pollinia of P. venosa. The pollinator specificity indicates that each of the male fungus gnat species was attracted to the flowers of a specific Pterostylis orchid. This strongly suggests that each of the orchid species emit a specific floral volatile, most probably resembling the sex pheromones of the female conspecifics, to lure their male pollinators. These are the first documented cases of highly specialised sexually deceptive pollination in New Zealand orchids, which were thought to be predominantly self-pollinating.