“Pro-bird” floral traits discourage bumblebee visits to Penstemon gentianoides (Plantaginaceae), a mixed-pollinated herb
The pollination syndrome concept implies that flowers evolved with particular sets of characteristics, such as colors, shapes, orientations, and rewards, as a means of attracting pollinators. However, these traits may have also evolved to deter unwanted visitors. The North American genus Penstemon exhibits a great floral diversity that is mainly associated with bumblebee and hummingbird pollination. Evolutionary shifts from insect pollination to hummingbird pollination have occurred in Penstemon repeatedly, but some species maintain mixed-pollination systems and intermediate floral traits between bee- and hummingbird-pollination modes. The apparently intermediate floral traits of species with mixed-pollination systems might be potentially acting to deter bumblebee foragers. Then, bird-flower traits might be selected with increased hummingbird visitation over evolutionary time might, resulting in specialization to and the evolution of floral traits present in hummingbird-pollinated species. Here, we modified bee-pollination floral traits in Penstemon gentianoides with a mixed pollination system, to resemble hummingbird-pollination traits, and measured the effects of trait modification on bumblebee foraging behavior and plant female reproductive fitness. Our results showed that reduction in the width of the corolla tube and the absence of the corolla lip negatively affects bumblebee visitation and their efficiency as pollinators, and that the synergistic interaction of both traits enhanced the “anti-bee” effect. We conclude that acquisition of floral traits that resemble those of hummingbird-pollination enables Penstemon plant species to deter bumblebee visits.
KeywordsFloral trait modification Hummingbirds Hymenoptera Penstemon Pollen transfer
We thank M. Luna, D. López, I. Morales, V. Mendiola, and L. Salinas for field assistance and the staff of the La Malinche National Park (Coordinación General de Ecología) for allowing the access to their facilities. Four anonymous reviewers provided useful comments on previous versions of the manuscript. Permission to conduct our fieldwork was granted by the Mexican government (SEMARNAT, FAUT-0296). This work constitutes partial fulfillment of LCSA’s doctorate at the Universidad Autónoma de Tlaxcala.
The Consejo Nacional de Ciencia y Tecnología (CONACyT) provided financial support in the form of a scholarship (248118) to the first author.
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