Disentangling effects of flower symmetry and orientation on pollination

Some Saxifraga species possess an intraindividual variation of flower symmetry and orientation: flowers with horizontally oriented corolla planes are radially symmetrical, whereas flowers with vertically oriented corolla planes are bilaterally symmetrical. Petal size, petal colouration, sequence of stamen movement, and interpetal angles are induced by gravity and thus correlate with symmetry and orientation. The flower visitors' responses to flower symmetry and orientation were tested in the field with fly-pollinated Saxifraga stellaris and S. cuneifolia. Video analysis of approach and landing behaviour showed that the landing site on the flowers was independent of flower symmetry and orientation. The flower visitors’ body axis, however, was aligned with the flowers’ vertical axis in most cases, since most flies land with their heads facing upwards. The flies' movement on the flower is more constrained in vertically oriented flowers: instead of walking around the protruding carpels, they walk over them. In a lab-based experiment, the deposition of pollen surrogate on stigmas of S. fortunei was tested using Episyrphus balteatus hoverflies that were dusted with pigment particles before a single flower visit. The hoverflies deposited more pigment particles on vertically oriented flowers, irrespective of flower symmetry. It is discussed whether vertically oriented bilaterally symmetrical Saxifraga flowers benefit from the avoidance of self-pollination and reduced pollen clogging. The role of differences in bilateral symmetry among Saxifraga species with regard to the display of floral guides in the upper petals, initiation of stamen movement in the lower stamens, petal length and interpetal angles is discussed in the context of pollination efficiency.