Abstract
Pollinator-mediated selection has been suggested to play a major role for the origin and maintenance of the species diversity in orchids. Sexually deceptive orchids are one of the prime examples for rapid, pollinator-mediated plant radiations, with many species showing little genetic differentiation, lack of postzygotic barriers, but strong prezygotic reproductive isolation. These orchids mimic mating signals of female insects and employ male insects as pollinators. This kind of sexual mimicry leads to highly specialised pollination and provides a good system for investigating the process of pollinator-driven speciation. Here, we summarise the knowledge of key processes of speciation in this group of orchids and conduct a meta-analysis on traits that contribute to species differentiation, and thus potentially to speciation. Our study suggests that pollinator shift through changes in floral scent is predominant among closely related species in sexually deceptive orchids. Such shifts can provide a mechanism for pollinator-driven speciation in plants, if the resulting floral isolation is strong. Furthermore, changes in floral scent in these orchids are likely controlled by few genes. Together these factors suggest speciation in sexually deceptive orchids may happen rapidly and even in sympatry, which may explain the remarkable species diversity observed in this plant group.
Highlights
The species diversity in the orchid family is extraordinary
Our study suggests that pollinator shift through changes in floral scent is predominant among closely related species in sexually deceptive orchids
Using sexually deceptive orchids as examples, we systematically review what is known about the process of pollinator-driven speciation, examine the key factors that are essential for speciation processes, and discuss the possible modes of speciation in this orchid pollination system
Summary
The species diversity in the orchid family is extraordinary. Orchid species richness and the spectacular diversity of plantpollinator interactions provide an exceptional opportunity for the study of pollinator-driven speciation. There is more and more evidence suggesting that pollinators play a major role in the generation of novel floral forms and the maintenance of species diversity in orchids [2, 3]. Deceptive orchids are suitable for investigating pollinatordriven speciation, because of their specialised pollination system. For reasons detailed below and discussed elsewhere [2], nonadaptive speciation is unlikely to play a major role in the diversification of the well-investigated genera of sexually deceptive orchids, Ophrys and Chiloglottis. We focus on these three factors in sexually deceptive orchids
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