Contradicting the widespread assumption that bilaterally symmetrical flowers enhance pollination precision, a recent study suggests that the orientation of the flower, specifically when it is horizontally aligned, significantly affects the stability of bee entry angles, not the symmetry. This novel finding encourages a reassessment of the evolutionary importance of symmetry in angiosperms’ flowers. This research was conducted by the University of Tsukuba.
It is frequently observed that flowers of unrelated species share similar physical traits. Bilateral symmetry is one such attribute, commonly seen in a variety of flowers, including those of orchids and legumes.
Animals, when approaching these symmetrically-shaped flowers, usually do so headfirst, their undersides facing the ground. Such observations have fueled the belief, dating back to the 18th century, that the evolution of flower symmetry aimed at ensuring contact between the flower’s stamens and pistils and specific body parts of the animals, thereby optimizing pollination. However, this widely-held theory has recently come under scrutiny.
The study observed that a majority (over 90%) of bilaterally symmetrical flowers, like orchids, present themselves horizontally. Animals then approach these flowers from the front, their ventral side down. These findings suggest that it is the horizontal positioning, rather than the bilateral symmetry, that causes the stabilization of animal entry into the flower.
The researchers carried out indoor experiments involving nine different artificial flowers with a variety of symmetry (bilateral, dissymmetric, and radial) and orientation (upward, horizontal, and downward) combinations. The objective was to explore how these factors affected the stability of flower entry by bumble bees, specifically Bombus ignitus.
The results confirmed the hypothesis that horizontal flower presentation led to significant stabilization of the bee entry angle, reducing angular variation by 60% in comparison to upward and downward orientations. Surprisingly, the three types of floral symmetry didn’t contribute to stabilizing flower entry. These unexpected results urge a rethinking of the adaptive value of bilaterally symmetrical flowers, a trait that has evolved repeatedly in angiosperms.
Reference: “Effects of floral symmetry and orientation on the consistency of pollinator entry angle” by Nina Jirgal and Kazuharu Ohashi, 16 May 2023, The Science of Nature.
This research was funded by a JSPS Grants-in-Aid for Scientific Research.