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Title: The role of pollinators in maintaining variation in flower color in the Rocky Mountain columbine, Aquilegia coerulea

Author
item THAIRU, MARGARET - UNIVERSITY OF WISCONSIN
item Brunet, Johanne

Submitted to: Annals Of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2015
Publication Date: 3/25/2015
Publication URL: http://handle.nal.usda.gov/10113/61027
Citation: Thairu, M.W., Brunet, J. 2015. The role of pollinators in maintaining variation in flower color in the Rocky Mountain columbine, Aquilegia coerulea. Annals Of Botany. 115(6):971-979.

Interpretive Summary: Flower color varies within and among populations of the Rocky Mountain columbine, Aquilegia coerulea. The abundance of hawkmoths and bumble bees, the two major pollinators of this plant species, also varies among populations. While bumble bees actively forage for pollen because they cannot reach the nectar produced at the bottom of the spur, hawkmoths forage for nectar and only indirectly move pollen from flower to flower during this process. In the current study, we investigated the preference of hawkmoths and bumble bees for flower color in order to assess their role in the maintenance of flower color variation within and among A. coerulea populations. White flowers have been previously associated with the presence of hawkmoths in this plant species and a preference for blue flowers by bumble bees has been observed in other studies. We therefore tested whether hawkmoths preferred white flowers and bumble bees blue flowers. The preference of pollinators for flower color will influence their foraging behavior, thus potentially influencing pollinator movement, gene flow and the foraging range of a pollinator. If hawkmoths preferentially visit white flowers we might expect more gene movement among populations with a majority of white flowers. Moreover, a better understanding of the response of distinct pollinators to flower color would provide useful information to scientists and government officials interested in developing management strategies for the conservation of pollinators. Providing plant species with the proper range of flower color would help attract and maintain the pollinators of interest.

Technical Abstract: Flower color varies within and among populations of the Rocky Mountain columbine, Aquilegia coerulea. The abundance of hawkmoths and bumble bees, the two major pollinators of this plant species, also varies among populations. We investigated the preference of hawkmoths and bumble bees for flower color in order to assess their role in the maintenance of flower color variation within and among A. coerulea populations. Dual choice assays and experimental arrays of blue and white flowers were used to examine the preference of the two pollinators for flower color. We tested whether a differential preference for flower color, with hawkmoths preferring white and bumble bees blue flowers, could explain the polymorphism in flower color observed within populations. The color preference of hawkmoths was examined under both day and dusk light conditions. We determined the impact of previous exposure on choice of flower color and tested whether bees responded to novelty or to flower color frequency. We contrasted the preference of naïve and experienced bees and verified that bees could learn to associate flower color with pollen reward. Contrary to our expectations, both bees and hawkmoths preferred blue flowers and the preference of hawkmoths did not change between day and dusk. Differential flower color preference by the two major pollinators could not explain the variation in flower color observed within and among A. coerulea populations. Naïve bees preferred blue flowers but quickly learned to randomly forage on flowers of the two colors when both colors provided similar rewards. Bees also quickly learned to associate a flower color with a pollen reward. Previous experience affected the choice of color by bees but neither novelty nor frequency dependence could explain bee behaviour. In the absence of a strong association between a flower color and a reward, the two major pollinators of A. coerulea are expected to forage randomly with respect to flower color and are therefore not predicted to exert strong selection pressure on flower color. These results are surprising especially given the strong role attributed to pollinators in explaining the evolution of floral diversity, including the diversification of the North American clade of columbines to which A. coerulea belongs.