Disentangling effects of flower symmetry and orientation on pollination – Saxifraga as a study case

Perspective matters: Pollinator guidance and pollen transfer in Saxifraga stellaris is shaped by within-plant differences in symmetry and orientation

by Manuel Wiegel & Klaus Lunau

Perspective matters: Floral symmetry and orientation are key traits shaping pollinator behaviour and pollen placement, yet botanists usually classify flowers based on symmetry seen from above, but flower-visiting insects experience flowers very differently. In the genus Saxifraga floral displays exhibit high intraindividual variation: Flowers positioned at the top of inflorescences are radially symmetrical, whereas laterally positioned flowers are bilaterally symmetrical. These differences are induced by gravity, resulting in variation of petal length, interpetal angles, presence and/or size of floral guides on petals coordinated with sequential stamen movement. Radially symmetrical flowers provide similar visual information to flower visitors from all approaching directions, while bilaterally symmetrical flowers present directional cues that define landing sites and guide pollinator movement in line with the sequential anther dehiscence. Using field observations, video analyses, and experimental measurements of pollen surrogate deposition, we studied how flower-visiting flies respond to variation in flower symmetry and orientation of Saxifraga stellaris, S. cuneifolia and S. fortunei. Our results show that landing behaviour and subsequent movement patterns were strongly influenced by flower orientation and, to lesser extent, by flower symmetry. The development of vertically oriented bilaterally symmetrical flowers likely improves pollen placement and reduces self-pollination. Our findings highlight the combined role of gravity, flower symmetry, and pollinator behaviour in shaping bilateral flower evolution in Saxifraga.

Read the scientific publication in JPE