Floral symmetry affects bumblebee approach consistency in artificial flowers

Abstract

Bilateral symmetry has evolved from radial symmetry in several floral lineages, and multiple hypotheses have been proposed to account for the success of this floral plan. One of these hypotheses posits that bilateral symmetry (or, more generally, a reduced number of planes of floral symmetry) allows for more precise pollen placement on pollinators. Greater precision would maximize the efficacy of pollen transfer to conspecifics, while minimizing reproductive interference amongst plant species. Despite the intuitiveness of this hypothesis, it has little experimental support. Here, we tested whether a reduction in the number of floral planes of symmetry (as in the transition from radial to bilateral symmetry) increases the potential precision of pollen placement. We analyzed video recordings of bumblebees (Bombus impatiens) visiting artificial flowers to determine whether consistency in flower entry angle differed between radial (round) and disymmetric (rectangular) “flowers”. We observed more consistent entry angles for disymmetric flowers than for radial flowers, with entry angles to radial flowers 43% more variable on average (standard deviations of 30° vs. 21°). Bees trained on flowers with an intermediate (square) morphology exhibited a slight, non-significant preference for radial symmetry over disymmetry. Our results show that disymmetry—an evolutionarily intermediate form of floral symmetry—has the potential to increase pollen transfer to conspecific stigmas, relative to radial symmetry. Thus, evolutionary reduction in the number of planes of floral symmetry likely provides benefits in terms of pollen delivery, as suggested by the pollen-placement-accuracy hypothesis. These findings offer insight into the evolution of floral symmetry.

Morphological Complexity as a Floral Signal: From Perception by Insect Pollinators to Co-Evolutionary Implications

“…This could increase the contact of floral sex organs with the insect’s body and improve the plant’s reproductive prospects. The angle of entry by bumblebees into dissymmetric artificial flowers was in fact more consistent than when they entered radially symmetric flowers [ 58 ].…”

Gynodioecy in the common spindle tree (Euonymus europaeusL.) involves differences in the asymmetry of corolla shapes between sexually differentiated flowers

“…The completely symmetric subspace involves variation among the flowers, wherein the shapes of all four petals are changed in a coordinated fashion and remain mutually identical. This kind of variation corresponds to an ideal actinomorphy or disymmetry of corolla morphology (Culbert & Forrest, 2016). The positions of four reflected and re-labelled copies of a single corolla on these PCs are exactly identical.…”

Male sterility significantly elevates shape variation and fluctuating asymmetry of zygomorphic corolla in gynodioecious Glechoma hederacea (Lamiaceae)

“…The prominent lower lip of corollas in G. hederacea forms a dominant visual attractor for pollinators approaching the flowers head-on and from above. Several studies have illustrated the innate preference of different insect pollinators, such as honeybees and bumblebees, for bilateral flower symmetry ( Moller and Sorci 1998 ; Rodríguez et al 2004 ; Wignall et al 2006 ), which also correlates with higher pollen placement during individual insect visits ( Culbert and Forrest 2016 ). In addition, in Erysimum mediohispanicum , a species with a continuous range of corolla morphology ranging from radial to bilateral symmetry, the flowers with zygomorphic shapes were comparatively more attractive for pollinators than actinomorphic or strongly asymmetric flowers ( Gómez et al 2006 ).…”