Sharing of 4-hydroxy-3-(methylthio)benzaldehyde as a pollination semiochemical among distantly related sexually deceptive orchids.

Sexually deceptive orchids mimic female mating signals of their pollinators to achieve pollination. In Australia, multiple orchid genera exploit male thynnine wasps as pollinators, raising the possibility of convergent evolution of floral traits. This study investigates the chemical basis of attraction in Spiculaea ciliata and its pollinator Thynnoturneria abspiculus, uncovering the use of shared chemical attractants. Flowers of S. ciliata were collected in the field and extracted in dichloromethane and methanol. Gas chromatography-mass spectrometry (GC-MS) and field bioassays were used to identify and test candidate floral semiochemicals. Compound identity was confirmed through co-injection and peak enhancement with a synthetic standard, and behavioural responses of pollinators to candidate compounds were assessed with the aid of dummies loaded with candidate compounds. Spiculea ciliata attracts male T. abspiculus using a single semiochemical emitted from its labellum, identified as 4-hydroxy-3-(methylthio)benzaldehyde. This compound alone elicited sexual behaviours in male wasps comparable to those triggered by whole floral extracts. 4-Hydroxy-3-(methylthio)benzaldehyde is also known from distantly related Caladenia orchids and their pollinators, indicating repeated evolution of this unusual aromatic sulfurous semiochemical. These findings demonstrate convergent evolution of chemical signalling systems in sexually deceptive orchids and their thynnine wasp pollinators.