A comparison of early floral ontogeny in wild-type and floral homeotic mutant phenotypes of Primula

Primula flowers are heteromorphic with individual plants producing either pin-form or thrum-form flowers. We have used scanning electron microscopy to observe early development of wild-type flowers of primrose (Primula vulgaris), cowslip (P. veris), and the polyanthus hybrid (P. x tommasinii x P. vulgaris). Floral ontogeny in Primula is different from that observed in the well-studied models Antirrhinum majus and Arabidopsis thaliana and our studies reveal morphological landmark events that define the sequence of early floral development in Primula into specific stages. Pinform and thrum-form flowers are indistinguishable during early development with differentiation of the two floral morphs occurring beyond the differentiation of floral organs. Early ontogeny of flowers with homeotic mutant phenotypes was also studied to determine the timing of developmental reprogramming in these mutants. Phenotypes studied included Hose in Hose and Jack in the Green that develop petaloid sepals and leafy sepals, respectively, and Jackanapes plants that carry both these dominant mutations. Recessive double and semi-double flowers that produce additional whorls of petals and/or stamens in place of carpels were also studied. We describe a previously undocumented recessive Primula mutant phenotype, sepaloid, that produces sepals in place of petals and stamens, and a new nonhomeotic, dominant mutant phenotype Split Perianth, in which sepals and petals fail to fuse to form the typical calyx and corolla structures. The molecular basis of these mutant phenotypes in relation to the ABC model is discussed.