Sexual Deception in Orchids

Abstract

Sexual deception has evolved repeatedly within the Orchidaceae. All species exploit the sexual communication system of insects; they rely on a combination of olfactory, visual and tactile signals for tempting mate‐searching males into copulation attempts with their flowers. The exploitation of their pollinator's private communication channel ensures a highly effective pollen transport between conspecific flowers. Despite the elaborate pollination mechanisms deployed by sexually deceptive orchids, their annual reproductive success is notoriously low. This has fuelled intensive research into the evolutionary mechanisms which have given rise to sexual deception. Existing evidence support the hypothesis that these orchids have traded high reproductive output for higher quality seed production. But other factors may also be involved. While their evolution remains shrouded in mystery, their diversification has been revealed to be linked to repeated pollinator shifts mediated by the malleability of their floral traits. Despite decades of research, sexual deception still poses unanswered questions. Key Concepts Sexually deceptive orchids rely on mimicking the chemical, visual and tactile cues used by female insects to attract mate‐searching males. The mimicry is highly accurate, making it difficult for males to distinguish between their conspecific females and the orchid flowers. Sexual deception is a highly specific pollination system; each orchid being usually pollinated by a single pollinator species. The specific pollinator attraction is mediated by the same combination of flower traits in most sexually deceptive species. The sexual pheromone analogue produced by the flowers is thereby the key trait involved in ensuring specific pollinator attraction. Flower colour and morphology can also play a crucial role in pollinator attraction and in ensuring that the pollinators will contact the reproductive structures of the flowers. Despite their elaborate pollination strategies, sexually deceptive orchids are notorious for their low reproductive success. Low fruiting success seems, however, to be compensated by the production of high‐quality seeds through higher outcrossing rates than their food deceptive relatives. It is still debated how and why sexually deceptive orchids evolved from their food‐deceptive ancestors. There are multiple hypotheses which could explain the evolution of sexual deception, but a clear pattern has not yet emerged. Despite their low reproductive success, sexually deceptive orchids seem to be evolutionary highly successful. Their diversification has been linked to repeated pollinator‐shifts driven by often minute changes in their sexual pheromone analogues, followed by subsequent alterations of their flower colour and morphology.