The Immense Diversity of Floral Monosymmetry and Asymmetry Across Angiosperms

Abstract

Floral monosymmetry and asymmetry are traced through the angiosperm orders and families. Both are diverse and widespread in angiosperms. The systematic distribution of the different forms of monosymmetry and asymmetry indicates that both evolved numerous times. Elaborate forms occur in highly synorganized flowers. Less elaborate forms occur by curvature of organs and by simplicity with minimal organ numbers. Elaborate forms of asymmetry evolved from elaborate monosymmetry. Less elaborate form come about by curvature or torsion of organs, by imbricate aestivation of perianth organs, or also by simplicity. Floral monosymmetry appears to be a key innovation in some groups (e.g., Orchidaceae, Fabaceae, Lamiales), but not in others. Floral asymmetry appears as a key innovation in Phaseoleae (Fabaceae). Simple patterns of monosymmetry appear easily "reverted" to polysymmetry, whereas elaborate monosymmetry is difficult to lose without disruption of floral function (e.g., Orchidaceae). Monosymmetry and asymmetry can be expressed at different stages of floral (and fruit) development and may be transient in some taxa. The two symmetries are most common in bee-pollinated flowers, and appear to be especially prone to develop in some specialized biological situations: monosymmetry, e.g., with buzz-pollinated flowers or with oil flowers, and asymmetry also with buzz-pollinated flowers, both based on the particular collection mechanisms by the pollinating bees. Floral monosymmetry has developed into a model trait in evo-devo studies, whereas floral asymmetry to date has not been tackled in molecular genetic studies.

Trends in flower symmetry evolution revealed through phylogenetic and developmental genetic advances

“…Radially symmetrical flowers (figure 1a) display several planes of symmetry that bisect the flower into mirror images, and bilaterally symmetrical flowers ( figure 1d) [38][39][40][41]. Bilateral flower symmetry itself can range from elaborate to subtle patterns of low complexity (reviewed in [6]). Most familiar forms of complex bilateral flower symmetry are the bilabiate (lipped and keeled) flower forms.…”

“…Those of the keel form are well known from Fabaceae, but can also be found in Polygalaceae. Less elaborate forms of bilateral flower symmetry also result from organ differentiation primarily in the petal and/or stamen whorls, and may be due to displacement of organ initiation, size or shape variation in organs along the dorsoventral axis of the flower, or sigmoidal curvature of organs (reviewed in [6,16]). …”

“…Bilateral symmetry in these lineages is not only common, but also highly elaborate. However, a survey of flowering plant lineages demonstrates that both elaborate and subtle forms of bilateral flower symmetry have evolved from radially symmetrical ancestors many times, and that reversals from bilateral to radial, or approximately radial symmetry are not uncommon (reviewed in [6]). …”

The Maize PI/GLO Ortholog Zmm16/sterile tassel silky ear1 Interacts with the Zygomorphy and Sex Determination Pathways in Flower Development

“…Floral zygomorphy is a derived condition, characteristic of a few large and important families (e.g., Orchidaceae, Poaceae, Asteraceae, and Fabaceae), although dispersed throughout the angiosperms (Endress, 2012). In the eudicots, there is evidence that the CYCLOIDEA (CYC)-like TCP transcription factors have been repeatedly recruited to control the development of zygomorphy (Luo et al, 1996(Luo et al, , 1999Feng et al, 2006;Yang et al, 2012).…”

“…Within the order that contains the grasses, the Poales, zygomorphy is rare, although it is pervasive in the grass family (Rudall and Bateman, 2004;Endress, 2012). Although most often discussed in terms of the second and third floral whorls (petal and stamen homologs), zygomorphy can affect all four whorls of a flower, and symmetry patterns can differ within a flower (Endress, 2012). Several aspects of the maize flower demonstrate asymmetry.…”