Stochastic occurrence of trimery from pentamery in floral phyllotaxis of Anemone (Ranunculaceae)

Abstract

Merosity, indicating the basic number of?oral organs such as sepals and petals, has been constrained to specifc and stable numbers during the evolution of angiosperms. Te ancestral?ower is considered to have a spiral arrangement of perianth organs, as in phyllotaxis, the arrangement of leaves. How has the ancestral spiral evolved into?owers with specifc merosities? To address this question, we studied perianth organ arrangement in the Anemone genus of the basal eudicot family Ranunculaceae, because various merosities are found in this genus. In three species, A.?accida, A. scabiosa, and A. nikoensis that are normally pentamerous, we found positional arrangement of the excessive sixth perianth organ indicating the possibility of a transition from pentamerous to trimerous arrangement. Arrangement was intraspecifcally stochastic, but constrained to three of fve types, where trimerous arrangement was the most frequent in all species except for a form of A. scabiosa. Te rank of frequency of the other two types was species-dependent. We connect these observations with classical theories of spiral phyllotaxis. Te phyllotaxis model for initiation of the sixth organ showed that the three arrangements occur at a divergence angle <144°, indicating the spiral nature of?oral phyllotaxis rather than a perfect penta-radial symmetry of 144°. Te model further showed that selective occurrence of trimerous arrangement is mainly regulated by the organ growth rate. Di?erential organ growth as well as divergence angle may regulate transitions between pentamerous and trimerous?owers in intraspecifc variation as well as in species evolution.