Several orchids (Orchidaceae) make use of floral mimicry. Using sex-based deception, these species imitate female mating signals of certain pollinator species.[6] This results in attempted copulation by males of the pollinator species, facilitating pollen transfer. Bee orchids (Ophrys apifera) and fly orchids (Ophrys insectifera), specifically, utilize flower morphology, coloration, and scent to deceive their respective pollinators. These orchids have evolved traits matching the preferences of specific pollinator niches, leading to adaptive speciation.[7]
The mimicry involves secreting chemicals from glands (osmophores) in the sepals, petals, or labellum, that are indistinguishable from the insect's natural pheromones. The flower attaches a pollinium to the pollinator's body; the insect transfers the pollinium to the stigma of another flower when it makes its next copulation attempt. Pollinators are often bees, wasps, or flies.[8]
The cost to the pollinating insects might be seen as negligible, but pollinators of the Australian orchid Cryptostylis can waste significant amounts of sperm by ejaculating onto the flower. Thus there could be antagonistic coevolution such that pollinators become better at identifying their own species correctly, while orchids become better mimics.[9]
One mechanism in pollination is to use incentives or rewards. These are beneficial offerings to a pollinator, enticing it to engage with the reward and thus transfer pollen. Flowering plants that do not produce such rewards can instead attract pollinators through mimicry — a form of convergent evolution.[3] Such plants are called "deceptive plants" as they mimic the characteristics or rewards of other species without providing any benefit to the pollinator.[10][11]
Although bee and fly orchids are visual mimics of their pollinators, visual traits are not the only (nor the most important) ones mimicked to increase attraction.[12][10] Floral odours have been identified as the most prominent way of attracting pollinators, because these odours imitate the sex pheromones of females of the pollinator species.[6] Male pollinators then track these scents over long distances.[10] The proportions of such odour compounds have been found to be varied in different populations of orchids (in a variety of locations), playing a crucial role in attracting specific pollinators at the population level. The evolution of these interactions between plants and pollinators involves natural selection favoring local adaptation, leading to a more precise imitation of the scents produced by local pollinators.[6]
Chemical compounds (more specifically, alkanes and alkenes), while used for sexual deception, are produced in many species of Ophrys, and likely were preadapted for other functions before being co-opted for mimicry.[3] These orchids increased ancestral levels of alkene production to mimic the female pheromones that attract male pollinators, a form of sensory exploitation called a sensory trap.[13]
Although mimetic plants typically receive fewer interactions with pollinators than truly-rewarding plants do, the evolution of sexual deception appears to be linked to benefits associated with mating behavior. Sex-based mimicry results in pollinator fidelity, the continued revisiting of flowers of the same species by a pollinator, as a result of sexual deception. In support of this, sex-based deception in an Australian orchid results in a higher proportion of pollen reaching stigmas than food-based deception. In another study, deception of male pollinators results in a long-distance dispersal of pollen.[10]
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