For centuries, bilaterally symmetrical flowers have been thought to stabilize the entry of animal visitors and improve pollination accuracy. However, this long-held belief may not be true. Laboratory experiments using bumble bees, Bombus ignitus, and artificial flowers exhibited that the observed stabilization of flower entry by animals in bilaterally symmetrical flowers is largely due to the horizontal orientation shared by most of these flowers.

Tsukuba, Japan—In nature, flowers may share morphological features, even across distantly related species. Bilateral symmetry is one such feature found in flowers of various taxa, such as orchids and legumes. Animals enter these bilaterally symmetrical flowers from the front with their ventral side down. This observation has led to the belief that bilateral floral symmetry represents an adaptation to allow the stamens and pistils to touch specific areas of an animal's body, thereby increasing pollination accuracy. However, this widely held notion since the 18th-century has been recently refuted.

This study noted that over 90% of bilaterally symmetrical flowers, such as orchids, are presented horizontally. Accordingly, animals naturally approach these flowers from the front with their ventral side down. This indicates that the observed stabilization of animal flower entry may not be attributed to bilateral symmetry but to the accompanied horizontal orientation. Thus, in the present study, indoor experiments were conducted using nine different artificial flowers with exhaustive combinations of floral symmetry (bilateral symmetry, disymmetry, and radial symmetry) and orientation (upward, horizontal, and downward) to investigate the effects of floral symmetry and orientation on the stability of flower entry by bumble bees, Bombus ignitus.

As expected, the results of this study showed that the entry angle of bees was significantly stabilized when flowers were presented horizontally, with a 60% reduction in angular variation compared to the upward and downward orientations. Meanwhile, none of the three types of floral symmetry were found to contribute to the stabilization of flower entry. This remarkable result prompts a reconsideration of the adaptive significance of bilaterally symmetrical flowers that have evolved repeatedly in angiosperms.

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This work was supported by a JSPS Grants-in-Aid for Scientific Research (KAKENHI no.19K06834) to K.O.

Original Paper

Title of original paper:

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

Journal:

The Science of Nature - Naturwissenschaften

DOI:

10.1007/s00114-023-01845-w

Correspondence

Assistant Professor OHASHI Kazuharu
Institute of Life and Environmental Sciences, University of Tsukuba

Related Link

Institute of Life and Environmental Sciences