Floral Evolution: Breeding systems, Pollinators, and Beyond

Authors: SAPIR, YUVAL - Tel Aviv University; Brunet, Johanne; BYERS, DIANE - Illinois State University; IMBERT, ERIC - National Council For Scientific Research-Cnrs; SCHONENBERGER, JURG - University Of Vienna; STAEDLER, YANNICK - University Of Vienna

Submitted to: International Journal of Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2019
Publication Date: 10/15/2019
Citation: Sapir, Y., Brunet, J., Byers, D., Imbert, E., Schonenberger, J., Staedler, Y. 2019. Floral evolution: Breeding systems, pollinators and beyond. International Journal of Plant Science. 180(9):929-933. https://doi.org/10.1086/706240.
DOI: https://doi.org/10.1086/706240

Interpretive Summary: Plants exhibit a wide variety of floral shapes, colors, sizes, and scents. It has been commonly believed that flowers represent organs to attract pollinators. Moreover, pollinators have been considered the primary agents of selection on floral traits, in other words they influence the shape, color, size and scent of flowers. This view is supported by associations between pollinators and specific floral trait combinations, such as the association between red tubular flowers that produce large quantities of nectar and hummingbird pollination and between white, strongly scented flowers opening at dusk and hawkmoth pollination. However, evidence accumulated over the past fifteen years have brought into question the sole and primary role of pollinators in shaping floral traits. Low values or values not statistically different from zero have been estimated for phenotypic selection on floral traits by pollinators in many studies. The role of abiotic factors such as water availability and non-pollinator biotic factors including herbivores and pathogens as selective agents of floral traits have been demonstrated. It is therefore time to reexamine the notion that pollinators are the major agents of selection on floral traits. The goal of this special issue on floral evolution is to expand the perception of flowers beyond mere organs of pollinator attraction and to broaden the scope of floral evolution research beyond plant-pollinator interactions. Studies in this special issue support the role of abiotic factors in the evolution of floral traits and introduce methodology to disentangle the contribution of pollinator-mediated selection relative to selection by abiotic or non-pollinator biotic factors on floral traits. Some studies point out the potential role of phenotypic plasticity, where the expression of a genotype changes with the environment, and sexual selection via pollen competition, on the evolution of floral traits. One study indicates how plant physiology can affect selection on floral traits and another reexamines a well-accepted although weakly supported hypothesis on whether competition for pollinators selects for staggered flowering. Understanding the factors beyond pollinators that affect the evolution of floral traits is important. In agriculture, breeders select for floral traits and need to consider genotype X environment interactions (phenotypic plasticity). Moreover, in insect-pollinated crops any selection that affect floral traits can lower pollinator attraction and yield. The results presented in this issue are relevant to plant breeders and scientists and the general public interested in understanding the mechanisms responsible for the floral diversity prevalent in the world today.

Technical Abstract: Flowering plants exhibit an amazing diversity of flower types. Flowers are often viewed as organs that attract pollinators and pollinator-mediated selection has been considered the major mode of selection on floral traits. However, a review of phenotypic selection on floral traits highlighted many low estimates of phenotypic selection with more than half of the studies having estimates that did not differ statistically from zero. In addition, alternative hypotheses emphasizing the role of abiotic factors and non-pollinator biotic agents of selection on floral traits have been proposed. Finally, a recent meta-analysis supported the presence of pollinator-mediated selection on floral traits but detected similar strength of selection by abiotic factors. The paradigm of pollinator-driven floral trait evolution must therefore be reexamined. The collection of articles in this special issue broadens the scope of floral evolution research beyond plant-pollinator interactions and expands the perception of flowers beyond mere organs of pollinator attraction. The studies in this special issue convincingly show that non-pollinator biotic and abiotic selection agents can shape the evolution of flowers. Some of the studies indicate how more controlled factorial experiments that estimate the relative contribution of pollinator-mediated selection relative to other factors on floral traits are warranted. Other studies demonstrate how phenotypic plasticity and sexual selection should be considered in studies of floral evolution. By moving a floral trait closer or further from its new optimum, phenotypic plasticity can, respectively, slow down or accelerate selection on standing genetic variation. Sexual selection could accelerate or oppose the effect of natural selection on floral traits. One study examines the impact of plant physiology on selection on floral traits and determines that reducing the constraints on the water and carbon costs of flowers can increase the lability of flowers and enable stronger pollinator-mediated selection. Another study presents some evidence for the role of abiotic factors in the maintenance of a flower color polymorphism. Finally, one study reexamines a well-accepted although weakly supported hypothesis and determines that competition for pollinators does not select for staggered flowering. By presenting and combining different perspectives, this special issue on floral evolution broadens our understanding of the mechanisms responsible for the floral diversity prevalent in the world today.