Epsilon Archive for Student Projects

Abstract

Sexually deceptive orchids coax their specific insect pollinators in visiting their flowers by chemical and visual
mimicry of female insects. The question of how this intriguing pollination strategy has independently evolved
multiple times across the world remains unsolved. A good understanding of the chemistry of the sexual attraction
between orchid and pollinator is a cornerstone in solving this mystery. In this study, I investigated the
semiochemicals that are involved in pollinator attraction of three Western Australian sexually deceptive orchids.
Chemical analyses using gas chromatography- mass spectrometry (GC-MS) alongside comprehensive fieldwork
involving diverse bioassays led to the identification of the semiochemical that is emitted from the labellum of S.
ciliata, to lure its pollinator. This compound, 4-hydroxy-3-(methylthio)benzaldehyde, was confirmed by peak
enhancement in the total ion chromatogram of the co-injection of a synthetic standard and the floral extract of S.
ciliata. The full repertoire of sexual behaviour could be elicited by presenting this single compound to the
pollinator. Interestingly, methylthiophenols are only known to be exploited by sexually deceptive orchids that are
not closely related to S. ciliata.
For the second species, Paracaleana nigrita, chemical analyses yielded candidate compounds, but verification in
the field could not be conducted as there were no pollinators available for the duration of this investigation.
For the third species of orchid Cryptostylis ovata, the compound that is responsible for eliciting sexual behaviour
in its pollinator is unknown, despite multiple attempts. By compiling available data including electrophysiological
activity and the presence of compounds in labellum extracts of C. ovata flowers, I present a list of candidate
compounds that may play a role in eliciting sexual behaviour in its pollinator. Additionally, a 3D-model of C.
ovata flowers was designed and printed, to account for the differences in morphology between the flower and the
conventional dressmaker’s pin that is used to present the candidate compounds in field experiments.