Stepwise evolution of corolla symmetry in CYCLOIDEA2 ‐like and RADIALIS ‐like gene expression patterns in Lamiales

• Premise of the study: CYCLOIDEA2 ( CYC2 )‐like and RADIALIS ( RAD )‐like genes are needed for the normal development of corolla bilateral symmetry in Antirrhinum majus L. (snapdragon, Plantaginaceae, Lamiales). However, if and how changes in expression of CYC2 ‐like and RAD ‐like genes correlate with the origin of corolla bilateral symmetry early in Lamiales remains largely unknown. The asymmetrical expression of CYC2 ‐like and/or RAD ‐like genes during floral meristem development could be ancestral or derived in Plantaginaceae. • Methods: We used in situ RNA localization to examine the expression of CYC2 ‐like and RAD ‐like genes in two early‐diverging Lamiales. • Key results: CYC2 ‐like and RAD ‐like genes are expressed broadly in the floral meristems in early‐diverging Lamiales with radially symmetrical corollas, in contrast to their restricted expression in adaxial/lateral regions in core Lamiales. The expression pattern of CYC2 ‐like genes has evolved in stepwise fashion, in that CYC2 ‐like genes are likely expressed briefly in the floral meristem during flower development in sampled Oleaceae; prolonged expression of CYC2 ‐like genes in petals originated in the common ancestor of Tetrachondraceae and core Lamiales, and asymmetrical expression in adaxial/lateral petals appeared later, in the common ancestor of the core Lamiales. Likewise, expression of RAD ‐like genes in petals appeared in early‐diverging Lamiales or earlier; asymmetrical expression in adaxial/lateral petals then appeared in core Lamiales. • Conclusions: These data plus published reports of CYC2 ‐like and RAD ‐like genes show that asymmetrical expression of these two genes is likely derived and correlates with the origins of corolla bilateral symmetry.