Petal evolution and morphogenesis in Ranunculaceae

Ranunculaceae comprise ca. 2500 species that display a broad range of floral diversity, particularly at the level of the perianth. Petals, when present, are often referred to as ‘elaborate’ because they have a complex morphology. In addition, the petals usually produce and store nectar, which gives them a crucial functional role in the interaction with pollinators. Its morphological diversity and species richness make this family a particularly suitable model group for studying the evolution of complex morphologies. For this purpose, we scored and analysed traits that characterize in detail the complexity of mature petal morphology in 32 genera (out of ca. 55), and used an already published robust and dated phylogeny of the family to conduct ancestral state reconstruction. Furthermore, we described petal development using high resolution X-Ray computed tomography in six species with contrasting petal forms. Our results suggest a flat and shortly clawed ancestral petal for the entire family, and for the ancestors of all tribes except Adonideae. Elaborations would have therefore evolved independently in different lineages, with some convergences in shape. To further explore the morphogenesis of elaborations, we focused on the spurred petals of taphisagria picta. We described the cellular characteristics (volume, sphericity, orientation in space) of the developing spur using a newly designed automated method to analyse in a three-dimensional space images generated using confocal microscopy. Our results show that spur development is marked by an early phase of dominant cell proliferation, followed by a phase of dominant anisotropic (directional) cell expansion. The acquisition of the spur’s curvature seems to be achieved by a complex interplay of anisotropic cell elongation and cell division, both mechanisms being unevenly distributed among the different parts of the spur.