Floral symmetry is a major determinant of flower diversity and is evolutionary labile.
Most flowers are either radially symmetric (i.e., can be divided into equal halves along multiple axes) or bilaterally symmetric (i.e., with a single plane of symmetry along the dorsal-ventral (D-V) axis) in their final forms. Bilateral symmetry has evolved at least 130 times independently from radial symmetry and is considered a key innovation in many species-rich lineages.
Although the final forms can be elaborated in countless ways, the underlying developmental principle is the same for all bilaterally symmetric flowers: organs initiated in the D vs. V domains follow different developmental trajectories. The D-V axis is established in the FM as soon as it is produced by a growing tip called inflorescence meristem (IM), with cells in the FM closer to the IM defining the D domain while the ones further away defining the V domain.
Due to its biological significance, bilateral symmetry has been extensively studied, and it is well established now that homologs of the CYCLOIDEA (CYC) gene are responsible for the elaboration of bilateral symmetry in many taxa. However, almost all studies over the past 30 years focused on simple expression correlations or, at best, functional studies of CYC homologs. A mechanistic understanding of how the D-V is established in the FM and what is responsible for the pre-patterns the expression of CYC is still missing. Currently, Mimulus is the only emerging model system that not only produces bilaterally symmetric flowers, but is also amenable for rapid stable transformation.
