Evolution of Symmetry in Plants

Abstract

Symmetry provides organisms with an efficient means to cope with physical constraints and explore three-dimensional space. We describe the diversity and evolution of symmetry types in the aerial parts of the major group of land plants, the angiosperms. Two main types of symmetry occur: bilateral symmetry, where structures can be divided into two mirror halves, and radial symmetry, with multiple planes of symmetry. Different organ arrangements or phyllotactic patterns produce different types of symmetry, which may vary within a plant’s life span. Leaves are usually flat bilaterally symmetrical organs with a bifacial organization resulting from an abaxial-adaxial differentiation associated with photosynthetic activity. Alterations in the genetic pathway underlying this asymmetry are thought to play a role in the repeated evolution of unifacial leaves. Flowers are composed of a series of organs that are considered to be highly modified leaves on a short compact axis. The symmetry of flowers as a whole is one of the most studied traits in plant evolutionary developmental genetics. Bilateral symmetry is derived from radial symmetry, probably from coevolution with specialized pollinators. Nearly 200 transitions in floral symmetry types have been recorded over the course of angiosperm evolution. Symmetry can change during flower development, and the timing of this change can vary between species. CYCLOIDEA-like transcription factors have been recruited repeatedly for the control of floral bilateral symmetry in angiosperms. The establishment of bilateral symmetry in leaves and flowers thus relies on different growth processes and gene networks.