Format: ORAL

Marcel Tunggawihardja1*, Julien B. Bachelier1

1. Institute of Biology/Dahlem Centre of Plant Sciences, Freie Universität Berlin, Berlin, Germany

As sisters to all other eudicots, members of the Ranunculales are pivotal to understanding the evolution of their perianth, and the homology of petals that can be of bracteal or staminal origin. However, recent ancestral character reconstructions suggest that the most recent common ancestor (MRCA) of Ranunculales and several families, including Berberidaceae, had not two but three series or whorls of perianth, and that the innermost was petaloid. To assess perianth organ evolution and homology between extant lineages and the MRCA of Ranunculales, we thus re-evaluated floral development in two genera of Berberidaceae with several whorls of perianth becoming increasingly petaloid toward the center of the flower. However, only the two innermost persist at anthesis in Nandina and four in Vancouveria. Using scanning electron microscopy, we confirmed that in both genera, the outermost whorls of sepals are all caducous. In addition, in Nandina, the two whorls of petals are not delayed in development and interpreted as the two innermost whorls of increasingly petaloid sepals, similar to those persisting in Vancouveria. Primordia at the base of some stamens of Nandina that we interpreted as being homologous to those regularly found in two whorls around the stamens in Vancouveria were also documented but they vanish during development whereas in Vancouveria, while they are typically delayed in development and form modified and nectariferous petals. Our study suggest that like in Vancouveria, the MRCA of Berberidaceae and Ranunculales as a whole likely had a perianth differentiated into outermost and innermost more or less petaloid sepals, and more or less modified and nectariferous petals. As such, their flowers thus had petals of both bracteal and staminal origins at anthesis, supporting the hypothesis of a fading-border ABC model determining floral organ identity and petaloidy in the basalmost eudicot lineage.