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Bee- to bird-pollination shifts in Penstemon: effects of floral-lip removal and corolla constriction on the preferences of free-foraging bumble bees

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Abstract

Plants might be under selection for both attracting efficient pollinators and deterring wasteful visitors. Particular floral traits can act as exploitation barriers by discouraging the unwelcome visitors. In the genus Penstemon, evolutionary shifts from insect pollination to more efficient hummingbird pollination have occurred repeatedly, resulting in the convergent evolution of floral traits commonly present in hummingbird-pollinated flowers. Two of these traits, a reduced or reflexed lower petal lip and a narrow corolla, were found in a previous flight-cage study to affect floral handling time by bumble bees, therefore potentially acting as “anti-bee” traits affecting preference. To test whether these traits do reduce bumble bee visitation in natural populations, we manipulated these two traits in flowers of bee-pollinated Penstemon strictus to resemble hummingbird-adapted close relatives and measured the preferences of free-foraging bees. Constricted corollas strongly deterred bee visitation in general, and particularly reduced visits by small bumble bees, resulting in immediate specialization to larger, longer-tongued bumble bees. Bees were also deterred—albeit less strongly—by lipless flowers. However, we found no evidence that lip removal and corolla constriction interact to further affect bee preference. We conclude that narrow corolla tubes and reduced lips in hummingbird-pollinated penstemons function as exploitation barriers that reduce bee access to nectaries or increase handling time.

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Acknowledgments

Jennifer Reithel of RMBL helped locate sites and secure permissions. William Clark, the Town of Mt. Crested Butte, and Wita and Gregor Wojtkowski provided access to property. We thank Miguel Rodríguez-Gironés and two anonymous reviewers for their helpful suggestions on the manuscript. Work was supported by an NSERC Discovery Grant to JDT and a Northrop Frye Award (University of Toronto Faculty of Arts and Science) to JLZ.

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Correspondence to Jessica L. Zung.

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Zung, J.L., Forrest, J.R.K., Castellanos, M.C. et al. Bee- to bird-pollination shifts in Penstemon: effects of floral-lip removal and corolla constriction on the preferences of free-foraging bumble bees. Evol Ecol 29, 341–354 (2015). https://doi.org/10.1007/s10682-014-9716-9

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  • DOI: https://doi.org/10.1007/s10682-014-9716-9

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