Symmetry in flowers: Diversity and evolution

This article traces research on floral symmetry back to its beginnings. It brings together recent advances from different fields that converge in floral symmetry and new unpublished material on diversity and development of floral symmetry. During floral development, symmetry may change: monosymmetric flowers may have a polysymmetric early phase; polysymmetric flowers may have a monosymmetric or even asymmetric early phase; more than one symmetry change is also possible. In Lamiales s.l. (comprising the model plant Antirrhinum, where the cycloidea gene produces monosymmetric flowers with the adaxial side of the androecium reduced), taxa also occur in which the androecium is reduced on both sides, adaxial and abaxial. As a trend in asymmetric flowers, enantiomorphy (with two mirror-image morphs) at the level of individuals seems to occur only in groups in which the flowers are predominantly of a relatively simple construction. In contrast, one morph is fixed at the level of species or higher taxa in groups with more complicated flowers. This is indicated by the apparent lack of enantiomorphy in corolla contortion in asterids but its predominance in rosids with contort flowers, or by the apparent lack of enantiomorphy in the pollination organs of asymmetric flowers in Faboideae but its presence in asymmetric flowers in Caesalpinioideae. To study the evolution of the diverse symmetry patterns, a concerted approach from different fields including molecular developmental genetics, pollination biology, and comparative diversity research is necessary.