Abstract
This study aimed to clarify the morphological properties and the role of homeotic genes in the development of paracorolla, a petaloid organ fused with stamens at the base in Antirrhinum majus. Two primordia of paracorolla initiated at the lateral and slightly outer (i.e., basal) position of a stamen primordium, suggesting that the paracorolla is a stipule of the stamen. In the developed paracorolla, expression of the A-class gene SQUAMOSA (SQUA) and the B-class genes DEFICIENS (DEF) and GLOBOSA (GLO) were high, while those of the C-class genes PLENA (PLE) and FARINELLI (FAR) were very low. In contrast, in the paracorolla primordia, expressions of SQUA and PLE were not detected. Thus, the expression patterns of homeotic genes in the paracorolla were basically the same as those of the petal, and this pattern was established during paracorolla development. In the stamen, which fused with the paracorolla at the base, DEF, GLO, PLE, and FAR were expressed in the anther. However, in the filament, SQUA was also expressed at a higher level in addition to these genes, indicating that the expression pattern shifted from the stamen type in the anther, to the petal type in the paracorolla, through the intermediate type in the filament. This gradient of expression pattern among these organs, which apparently belong to the same whorl of the flower, may be essential for the development of the paracorolla into a petaloid organ.
