Phenotypic selection on flower color and floral display size by three bee species

Authors: Brunet, Johanne; FLICK, ANDREW - Oak Ridge Institute For Science And Education (ORISE); BAUER, AUSTIN - University Of Wisconsin

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2020
Publication Date: 1/14/2021
Citation: Brunet, J., Flick, A., Bauer, A. 2021. Phenotypic selection on flower color and floral display size by three bee species. Frontiers in Plant Science. 11. Article 587528. https://doi.org/10.3389/fpls.2020.587528.
DOI: https://doi.org/10.3389/fpls.2020.587528

Interpretive Summary: Many plant species are visited by a diverse array of pollinators, yet few studies have contrasted the impact of these distinct pollinators on the evolution of floral traits. Previous studies have estimated the impact of each pollinator separately, using a distinct set of plants. But plants receive differential visits by distinct pollinators whose abundance and efficiency vary and a methodology that could simultaneously quantify the impact of the major pollinators on floral or other traits would be greatly beneficial. To accomplish this goal, we propose to combine pollinator observations to the measurements of seed set and floral traits typical of phenotypic selection studies. The approach is applied to the study of phenotypic selection on flower color and floral display size by three bee species, the European honey bee, Apis mellifera, the common eastern bumble bee, Bombus impatiens, and the alfalfa leafcutting bee, Megachile rotundata, foraging on Medicago sativa. The selection patterns observed for the combined bee species could be explained by the pattern of selection of the single bee species. Moreover, the trends detected in this study were consistent with previous studies. For example, the three bee species exhibited similar selection patterns for some floral traits but differential patterns of selection for other floral traits. In addition, while all bee species selected on plant-level attractants such as the number of racemes per stem and the number of stems per plant, only two bee species selected on flower-level attractants such as components of flower color. We therefore advocate the approach introduced here for future studies examining the impact of distinct pollinators on floral trait evolution. This approach will strongly benefit scientists and non-scientists interested on the role of distinct pollinators in plant evolution.

Technical Abstract: Plants exhibit a wide array of floral forms and pollinators can act as agents of selection on floral traits. Two trends have emerged from recent reviews of pollinator-mediated selection in plants. First, pollinator-mediated selection on plant-level attractants such as floral display size is stronger than on flower-level attractant such as flower color. Second, when comparing plant species, distinct pollinators can exert different selection patterns on floral traits. In addition, many plant species are visited by a diverse array of pollinators but very few studies have examined selection by multiple, distinct pollinators. In the current study, we examined phenotypic selection on flower color and floral display size by three distinct bee species, the European honey bee, Apis mellifera, the common eastern bumble bee, Bombus impatiens, and the alfalfa leafcutting bee, Megachile rotundata, foraging on Medicago sativa. To estimate phenotypic selection by each bee species and for all bees combined simultaneously and on the same group of plants, we combine pollinator visitation data to seed set and floral trait measurements data typical of phenotypic selection study. When comparing floral traits, all bee species selected on the number of racemes per stem and the number of stems per raceme, two components of floral display size. However, only leafcutting bees selected on hue or flower color and only bumble bees selected on chroma or darkness of flowers. Selection on chroma occurred via correlational selection between chroma and number of flowers per raceme and we examine how correlational selection may facilitate the evolution of flower color in plant populations. Distinct bee species exerted similar selection on some floral traits but different patterns on other floral traits. The trends detected were consistent with previous studies and we advocate the approach introduced here for future studies examining the impact of distinct pollinators on floral trait evolution.