Floral Symmetry–What It Is, How It Forms, and Why It Varies

Abstract

Flowers are compressed shoots in which floral organs are arranged in whorls, generally in a symmetrical manner that may be an intrinsic feature of the flower. Floral symmetry varies across angiosperms; the states range from symmetrical (polysymmetrical, disymmetrical and monosymmetrical) to asymmetrical, with the polysymmetrical form perhaps being the most common state. The development of different states of symmetry has been widely studied at the morphological level, while the range of underlying molecular processes is not fully understood. Causes for this variation may be found in the ecological interactions of flowers, especially with pollinators, which may exert selective pressures that favour one or other symmetry state. However, the complexity of interactions among ecological, developmental and other intrinsic factors means that it is difficult to generalize. For instance, because pollinators such as bumblebees appear to prefer symmetry, it is possible that they and similar insect visitors exert selection pressure that favours symmetry over asymmetry; however, asymmetry has evolved several times. Phylogenetic analyses show that disymmetry, monosymmetry and asymmetry evolved multiple times across angiosperms, and that in most es that have been studied, the same genetic pathway (involving the regulatory gene CYC2) has been involved in several independent origins of monosymmetry. It has also been shown that several lineages never alter their polysymmetrical state, which must have some underlying reasons. Finally, the question of how different symmetry states arise, and why they arise, must include the integration of a wide range of developmental, ecological and evolutionary explanations.