Floral bilateral symmetry: one axis, many details

Symmetry is a major component of floral diversity. Most flowers are radially or bilaterally symmetric, with bilateral symmetry (monosymmetry) having evolved repeatedly from radial ancestors. Since the identification of the dorsal identity gene CYCLOIDEA (CYC) in snapdragon, floral monosymmetry has become a classic example of convergent evolution through repeated recruitment of the same developmental program: across nearly all taxa examined, monosymmetry is associated with asymmetric expression of CYC along the dorsal-ventral axis. Despite extensive comparative evidence, the mechanisms by which CYC contributes to the development and diversification of floral symmetry remain fragmentary. Here, I examine three key questions concerning CYC function and regulation, summarize current mechanistic insights, and highlight directions for future research.