Flower symmetry in Saxifraga revisited

Abstract

Monosymmetry in flowers has evolved many times from ancestors with polysymmetrical flowers. Flower symmetry characteristics often lack intraspecific variability. The genus Saxifraga represents an exception with high variations of floral symmetry between and within species and even within individual plants. Lab-based studies could show that this variation is to some extent caused by the flowers’ response to gravity and thus varies regularly between horizontally and vertically oriented flowers. Here, we investigate inter- and intra-individual variation in flower size, flower colouration, and floral symmetry in three species, Saxifraga stellaris, S. rotundifolia, and S. paniculata. In all three species, the stamen that initiates the sequential stamen movement is mostly downward pointing. In S. stellaris and S. paniculata, the terminal flowers are larger than the other flowers. In S. stellaris and S. rotundifolia, the size of floral guides is larger and the colour is stronger on the upward pointing 3 petals of diagonally and vertically oriented flowers, whereas the other two petals have paler or smaller floral guides. The septum between the two carpels is mostly oriented horizontally. We discuss the evolution of flower monosymmetry in regard to the morphological symmetry referring to symmetry defined by the mirror plane between carpels, petals and moving stamens, and visual symmetry defined by the pollinators view on the floral colour pattern and visible morphological features.