Floral Organogenesis and Morphogenesis of Staphisagria (Ranunculaceae): Implications for the Evolution of Synorganized Floral Structures in Delphinieae

Premise of research. Floral synorganization is a structural feature of many speciose angiosperm taxa and is considered a morphological innovation paving the way for evolutionary diversification. Staphisagria is sister to the remaining Delphinieae, the only lineage of Ranunculaceae characterized by zygomorphic flowers. We aim at providing a description of floral organogenesis and morphogenesis in both Staphisagria species, presenting the disparity of Delphinieae hyperorgans in a phylogenetic framework, and proposing a scenario of likely developmental pathways underlying the different types of hyperorgans in Delphinieae. Methodology. We carried out morphological, anatomical, and developmental studies on flowers of S. macrosperma and S. picta. Pivotal results. Synorganization is complex in Staphisagria and Delphinieae as a whole and involves flower dorsoventralization, the nesting of spurs, the postgenital fusion of petals, and the formation of a shared cavity. From a choripetalous ancestor, late and partial postgenital fusion among dorsal petals evolved once or twice in the tribe. Conclusions. The Delphinieae flower includes nested spurs and nested floral parlors. These key innovations, unique in angiosperms, probably led to the diversification of this species-rich tribe in the Northern Hemisphere. The lengths of the inner (nectariferous) spurs and the nested floral parlors determine the range of pollinators able to collect nectar. These traits could be used to revise the circumscription of taxonomic groups within the tribe and should be taken into account when examining the possible coevolution between Delphinieae flowers and their pollinators. Integrating this new knowledge about the hyperorgan will be essential for future research in taxonomy, evo-devo, and pollination ecology in Delphinieae.