Floral ontogeny of Aquilegia, Semiaquilegia, and Enemion (Ranunculaceae)

Using scanning electron microscopy, we studied the comparative floral ontogeny of closely related taxa in the tribe Isopyreae of the Ranunculaceae ( Aquilegia olympica, Aquilegia ecalcarata, Semiaquilegia adoxoides, and Enemion occidentale) that vary in a number of floral features. Phylogenetic reconstruction of the Isopyreae indicates that floral nectar spurs are derived in Aquilegia ( but secondarily lost in A. ecalcarata), lack of petals is derived in Enemion, and staminodia are derived in Aquilegia + Semiaquilegia. Order of initiation is shared among all species studied: helical for sepals and simultaneous whorled for all other organs. Species of Aquilegia all have 10 orthostichies ( vertical rows of organs), with alternate orthostichies either sepal based or petal based. Scalelike staminodes terminate the orthostichies of stamens in Aquilegia; these staminodes are few or absent in Semiaquilegia and other Ranunculaceae. Carpels terminate the sepal- based orthostichies. Organogenesis in Enemion and A. ecalcarata are similar except that the alternisepalous orthostichies contain only stamens in Enemion and lack petals. Spurs in A. olympica originate as an abaxial bulge late in petal development that projects downward. Despite the inference that the lack of spurs is ancestral in S. adoxoides but derived in A. ecalcarata, their early petal development is remarkably similar, with the formation of an abaxial knob that fails to develop further. This indicates that A. ecalcarata has lost a key early developmental feature for spur development. Lack of petals in Enemion has clearly resulted from a homeotic shift as stamens develop in the same position as petals in the other species. In Aquilegia and Semiaquilegia, flowers with more stamens have more staminodia. This pattern indicates that staminodia may represent the addition of a novel floral whorl rather than simply a suppression of the development of some stamens. Variation in floral features in other members of the Isopyreae suggests that this tribe will be especially fruitful for future morphological and molecular studies of flower development.