Spurs in a Spur: Perianth Evolution in the Delphinieae (Ranunculaceae)

Delphinieae (Ranunculaceae) comprise ∼650 species of temperate herbs in Eurasia, North America, and Africa. Their zygomorphic flowers have been the object of numerous studies in morphology, ecology, and developmental genetics, and new phylogenetic insights make it timely to synthesize knowledge about their evolution. Key features of Delphinieae flowers are unusual nectaries consisting of paired organs of the inner perianth whorl that are completely enclosed by a single dorsal organ of the outer whorl. We investigated the floral development of five annual, unicarpellate species of Delphinium, focusing on perianth organization. The results show that the nectar-storing organ in these species results from the postgenital fusion of two primordia of the internal perianth whorl. Eleven floral traits traced on a phylogeny of Delphinieae reveal only two homoplasies in the perianth, namely, the nightcap shape of the dorsal organ of the external perianth whorl and the reduction of the internal perianth whorl to two organs, traits that each evolved once in Aconitum and once in Delphinium. The length of the inner spur(s), the type of pollinator (bees, hummingbirds, hawkmoths), and species altitudinal ranges are unrelated, but most species are exclusively bumblebee adapted, and bee tongue lengths may match the internal nectar spur lengths. The paired inner spurs present in most Delphinieae require a back-and-forth movement of the tongue while the pollinator is inside the flower or hovering close to it. A new evolutionary scenario reconciles the diversity of perianth organization in Delphinieae with the tribe’s conserved pollination mechanism.