Phenotypic Selection on Flower Color and Floral Display Size by Three Bee Species

Abstract

Plants exhibit a wide array of floral forms and pollinators can act as agent 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 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 introduce a new method that combines 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 plant, 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 open flowers per raceme and we examine how correlational selection may facilitate the evolution of flower color in plant populations. When comparing bee species, the three bee species exerted similar selection pattern on some floral traits but different patterns on other floral traits and differences in selection patterns were observed between flower-level and plant-level attractants. 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.

Flower Color Evolution and the Evidence of Pollinator-Mediated Selection

“…In Gentiana lutea , which varies in flower color from yellow to orange, Veiga et al (2015) found selection for increased yellowness as yellow flowers received higher pollinator visitation than flowers of alternative colors. In the study of Medicago sativa by Brunet et al (2021) alfalfa leafcutting bees, Megachile rotundata , exerted stabilizing selection on hue, but neither Apis mellifera nor Bombus impatiens showed preferences for flower hue resulting in non-significant selection when summed over all bees.…”

“…Moreover, of these studies, only six used a hand-pollination treatment to estimate selection gradients associated with pollinator interactions ( Caruso et al, 2010 ; Parachnowitsch and Kessler, 2010 ; Lavi and Sapir, 2015 ; Sletvold et al, 2016 ; Zhang et al, 2017 ; Souto-Vilarósa et al, 2018 ; Supplementary Table 3 ). Some other studies used modeling approaches linking the flower color phenotype - fitness relationship to pollinator visitation data to determine the contribution of pollinators to observed total selection (e.g., Veiga et al, 2015 ; Rodriguez-Castañeda et al, 2020 ; Brunet et al, 2021 ; Supplementary Table 3 ).…”

“…In addition, the study by Sletvold et al (2016) confirmed that pollinators accounted for 100% of observed net selection among open-pollinated plants. In addition, Brunet et al (2021) found, in a population of Medicago sativa , that bumblebees ( Bombus impatiens ) preferred darker flowers but this was due to correlational selection with flower number. That bumblebees use achromatic flower color information (i.e., green contrast) and preferentially chose flowers with increased brightness resulting in significant directional selection, when modeled through the bees visual system, was also demonstrated by Renoult et al (2013) in an experimental greenhouse setup, where bumblebees were freely foraging on potted cornflower, Centaurea cyanus .…”

“…For example, experimenting may be through manipulation of the pollination environment (i.e., a hand-pollination treatment to remove variation in fitness that is not associated with variation in the interaction with pollinators; Caruso et al, 2018 ; Sletvold, 2019 ). On the other hand, statistical estimation approaches may be used with structural equation modeling estimation (i.e., estimating the causal relationship of pollinator interactions with plant fitness; Souto-Vilarósa et al, 2018 ; Rodriguez-Castañeda et al, 2020 ; Brunet et al, 2021 ).…”

Self-Fertilization, Inbreeding, and Yield in Alfalfa Seed Production

“…This pattern will occur in screened polycrosses ( Riday et al, 2017 ) as well as in large multi-acre pollination (this study) as long as bees are used for seed production. Potential metrics to describe floral display size in alfalfa include the number of racemes per flowering stem, or the total number of open flowers per flowering stem ( Brunet et al, 2021 ).…”