Evolution of floral structures in basal angiosperms

The evolution of selected characters of the perianth, androecium, and gynoecium was reconstructed on a modification of the recent three- gene topology for angiosperms to address patterns of evolution in the flowers of basal angiosperms. The reconstructions indicate that the patterns of perianth and stamen phyllotaxis are usually, but not always, closely associated. The attainment of a stable perianth and stamen phyllotaxis preceded stability in the gynoecium. There are several surprising reversals from whorled to spiral phyllotaxis in different clades. The developmental mechanisms responsible for changes in stamen number have rarely been evaluated in basal angiosperms, in contrast to the eudicots. Our reconstructions reveal a close relationship between a whorled phyllotaxis, a reduction of the number of stamen whorls, and stamens in double ( paired) positions. Staminodes appear frequently in different lineages. A strict distinction between a sepal- derived and a staminodial, stamen- derived perianth is not always clear and may reflect shifting boundaries between organ-determining genes. The frequent coexistence of spiral phyllotaxis, trimery, and dimery in the same clades, with the occasional presence of an intermediate pentamerous condition, emphasizes the instability of merosity ( merism) in basal angiosperms; transitions between seemingly highly different flower configurations occur often. A stable merosity is strictly linked with few synorganized carpels, except for the presence of a single carpel. Symmetry of flowers is either structurally correlated with the basic merosity, the result of extreme reduction, or developmentally correlated with shifts in organ initiation. Despite uncertainty about character evolution in certain lineages or their equivocal relationships, floral characters are generally consistent with phylogenetic relationships inferred from molecular data. The distribution of floral characters is discussed for the major clades. Several clades ( e. g., Magnoliales- Laurales, monocots) are strongly supported by synapomorphic morphological characters.