Global distribution and evolutionary transitions of floral symmetry in angiosperms

Floral symmetry plays an important role in plant-pollinator interactions and may have remarkable impacts on angiosperm diversification. However, spatiotemporal patterns in floral symmetry and drivers of these patterns remain unknown. Here, using newly compiled floral symmetry (actinomorphy versus zygomorphy) data of 279,877 angiosperm species and their distributions and phylogenies, we estimated global geographic patterns and macroevolutionary dynamics of floral symmetry. We found that frequency of actinomorphic species increased with latitude, while that of zygomorphic species decreased. Solar radiation, present-day temperature, and Quaternary temperature change correlated with geographic variation in floral symmetry frequency. Evolutionary transitions from actinomorphy to zygomorphy dominated floral symmetry evolution, although the transition rate decreased with decreasing paleotemperature throughout the Cenozoic. Notably, we found that zygomorphy may not favor diversification of angiosperms as previously observed in some clades. Our study demonstrates the influence of (paleo)climate on spatiotemporal patterns in floral symmetry and challenges previous views about role of flower symmetry in angiosperm diversification.

Effects of floral symmetry and orientation on the consistency of pollinator entry angle

Since the publication of Sprengel's (1793) observations, it has been considered that flowers with zygomorphic (or bilaterally symmetrical) corollas evolved to restrict the movement of pollinators into the flower by limiting the pollinator's direction of approach. However, little empirical support has been accumulated so far. Our aim was to build on previous research that showed zygomorphy reduces variance in pollinator entry angle, aiming to observe whether floral symmetry or orientation had an impact on pollinator entry angle in a laboratory experiment using bumble bees, Bombus ignitus. Using nine different combinations of artificial flowers created from three symmetry types (radial, bilateral and disymmetrical) and three orientation types (upward, horizontal, and downward), we tested the effects of these two floral aspects on the consistency of bee entry angle. Our results show that horizontal orientation significantly reduced the variance in entry angle, while symmetry had little effect. We also found either little or no significant interactions between angle and symmetry in their effect on entry angle. Thus, our results suggest that horizontal orientation forces the bees to orient themselves relative to gravity rather than the corolla and stabilizes their flower entry. This stabilizing effect may have been mistaken for the effect of zygomorphic corolla as it is presented horizontally in most species. Consequently, we suggest that the evolution of horizontal orientation preceded that of zygomorphy as indicated by some authors, and that the reason behind the evolution of zygomorphy should be revisited.

Fragmentary Blue: Resolving the Rarity Paradox in Flower Colors

Blue is a favored color of many humans. While blue skies and oceans are a common visual experience, this color is less frequently observed in flowers. We first review how blue has been important in human culture, and thus how our perception of blue has likely influenced the way of scientifically evaluating signals produced in nature, including approaches as disparate as Goethe's Farbenlehre, Linneaus' plant taxonomy, and current studies of plant-pollinator networks. We discuss the fact that most animals, however, have different vision to humans; for example, bee pollinators have trichromatic vision based on UV-, Blue-, and Green-sensitive photoreceptors with innate preferences for predominantly short-wavelength reflecting colors, including what we perceive as blue. The subsequent evolution of blue flowers may be driven by increased competition for pollinators, both because of a harsher environment (as at high altitude) or from high diversity and density of flowering plants (as in nutrient-rich meadows). The adaptive value of blue flowers should also be reinforced by nutrient richness or other factors, abiotic and biotic, that may reduce extra costs of blue-pigments synthesis. We thus provide new perspectives emphasizing that, while humans view blue as a less frequently evolved color in nature, to understand signaling, it is essential to employ models of biologically relevant observers. By doing so, we conclude that short wavelength reflecting blue flowers are indeed frequent in nature when considering the color vision and preferences of bees.

Effects of floral restrictiveness and stigma size on heterospecific pollen receipt in a prairie community

Plant species vary greatly in the degree to which floral morphology restricts access to the flower interior. Restrictiveness of flower corollas may influence heterospecific pollen receipt, but the impact of floral morphology on heterospecific pollen transfer has received little attention. We characterized patterns of pollinator visitation and quantities of conspecific and heterospecific pollen receipt for 29 species with a range of floral morphologies in a prairie community dominated by the introduced plant Euphorbia esula (leafy spurge) which has an unrestrictive morphology. Pollinator overlap was significantly greater between Euphorbia and other unrestrictive flowers than restrictive flowers. Compared to flowers with restrictive morphologies, unrestrictive flowers received significantly more Euphorbia pollen, more heterospecific pollen from other sources, and a greater diversity of pollen species, but not more conspecific pollen. However, stigmatic surface area was significantly larger for flowers with unrestrictive morphologies, and the density of Euphorbia and other heterospecific pollen per stigmatic area did not significantly differ between flower types. These findings suggest that the smaller stigma size in restrictive flowers partly accounts for their decreased heterospecific pollen receipt, but that restrictiveness also allows species to increase the purity of pollen loads they receive. Given that restrictive flowers receive fewer heterospecific pollen grains but at a higher density, the effect of restrictiveness on fecundity depends on whether absolute quantity or density of heterospecific pollen affects fecundity more. Our results also indicate that abundant neighbors are not necessarily important heterospecific pollen sources since Euphorbia pollen was rarely abundant on heterospecifics.

Floral symmetry affects speciation rates in angiosperms

Despite much recent activity in the field of pollination biology, the extent to which animal pollinators drive the formation of new angiosperm species remains unresolved. One problem has been identifying floral adaptations that promote reproductive isolation. The evolution of a bilaterally symmetrical corolla restricts the direction of approach and movement of pollinators on and between flowers. Restricting pollinators to approaching a flower from a single direction facilitates specific placement of pollen on the pollinator. When coupled with pollinator constancy, precise pollen placement can increase the probability that pollen grains reach a compatible stigma. This has the potential to generate reproductive isolation between species, because mutations that cause changes in the placement of pollen on the pollinator may decrease gene flow between incipient species. I predict that animal-pollinated lineages that possess bilaterally symmetrical flowers should have higher speciation rates than lineages possessing radially symmetrical flowers. Using sister-group comparisons I demonstrate that bilaterally symmetric lineages tend to be more species rich than their radially symmetrical sister lineages. This study supports an important role for pollinator-mediated speciation and demonstrates that floral morphology plays a key role in angiosperm speciation.

FLORAL SYMMETRY AND ITS ROLE IN PLANT-POLLINATOR SYSTEMS: Terminology, Distribution, and Hypotheses

▪ Abstract  Floral symmetry has figured prominently in the study of both pollination biology and animal behavior. However, a confusion of terminology and the diffuse nature of the literature has limited our understanding of the role that this basic characteristic of flower form has played in plant-pollinator interactions. Here, we first contribute a classification scheme for floral symmetry that we hope will resolve some of the confusion resulting from the inconsistent application of terms. Next, we present a short review of the distribution of floral forms in angiosperm families. Finally, we provide a list of hypotheses and, when available, supporting evidence for the causes of the evolution of floral symmetry.