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.
Highlights
Apiales is an order of campanulid asterids comprising seven families [1], which considerably differ from each other in flower groundplan and the range of its variation
Our results show that the variation of floral symmetry in Araliaceae largely represents a consequence of certain developmental constraints
We argue here that the most important factor leading to the disymmetry of the entire flower is the occurrence of a constraint that maintains the arrangement of a large number of elements in a single whorl
Summary
Apiales is an order of campanulid asterids comprising seven families [1], which considerably differ from each other in flower groundplan and the range of its variation. Araliaceae, in contrast, show a considerable variation in certain features of flower structure, which are commonly constant or almost so in many other euasterid families These features include the number of whorls and their merism, and the presence of calyx and corolla tubes [2,3]. The terms “actinomorphy” and “zygomorphy” are aimed to divide the diversity of floral shape into two contrasting and recognizable groups, they are structurally imprecise, as they lack any strict criteria These two terms do not cover the cases of presence of two different (i.e., dividing in different ways) symmetry planes in a flower or the absence of any planes (disymmetry and asymmetry: [9,10,11]). In the literature on the genetics of floral development, e.g., [17,18,19], zygomorphic (monosymmetric) flowers are often confused with asymmetric ones, which is geometrically incorrect [11]
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