Patterns of Diversity of Floral Symmetry in Angiosperms: A Case Study of the Order Apiales

Abstract

Floral symmetry is widely known as one of the most important structural traits of reproductive organs in angiosperms. It is tightly related to the shape and arrangement of floral parts, and at the same time, it plays a key role in general appearance (visual gestalt) of a flower, which is especially important for the interactions of zoophilous flowers with their pollinators. The traditional classification of floral symmetry divides nearly all the diversity of angiosperm flowers into actinomorphic and zygomorphic ones. Within this system, which is useful for ecological studies, many variations of symmetry appear to be disregarded. At the same time, the diversity of floral symmetry is underpinned not only by ecological factors, but also by morphogenetic mechanisms and constraints. Sometimes it is not an easy task to uncover the adaptive or developmental significance of a change of the floral symmetry in a particular lineage. Using the asterid order Apiales as a model group, we demonstrate that such changes can correlate with the merism of the entire flower or of its particular whorl, with the relative orientation of gynoecium to the rest of the flower, with the presence of sterile floral elements and other morphological characters. Besides, in some taxa, the shape and symmetry of the flower change in the course of its development, which should be taken in consideration in morphological comparisons and evaluations of synapomorphies in a particular clade. Finally, we show that different results can be obtained due to employment of different approaches: for instance, many flowers that are traditionally described as actinomorphic turn out to be disymmetric, monosymmetric, or asymmetric from a more detailed look. The traditional method of division into actinomorphy and zygomorphy deals with the general appearance of a flower, and mainly considers the shape of the corolla, while the geometrical approach handles the entire three-dimensional structure of the flower, and provides an exact number of its symmetry planes.