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

2,4 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 fl oral 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 fl oral 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 fash- ion, in that CYC2 -like genes are likely expressed briefl y in the fl oral meristem during fl ower 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 pet- als 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.