Homology and Evolution of Petals in the Core Eudicots

Seventeen morphological characters are described and plotted on a phylogeny of the eudicots. The distribution of perianth characters demonstrates that the currently held view that petals originated from stamens in the core eudicots is not consistent with the predominance of bract-derived petals (bracteopetals). Petals in the core eudicots have the same morphological affinities and structure as sepals or bracts, with a few notable exceptions. Depending on the most likely topology, Berberidopsidales may represent the plesiomorphic perianth condition in the core eudicots, giving rise to pentamerous flowers with sepals and petals of a similar tepaline origin. Within the core eudicots several divergent trends have led to a wide variety of perianth morphologies involving to different degrees the presence of a calyx and corolla. Different factors contribute to the development of a biseriate perianth, including the upward shift of bracts and their absorption in the flower and a progressive differentiation of tepals through shifts in petaloidy. A number of correlations in perianth evolution are highlighted, such as petaloid sepals linked with loss of petals and presence of a hypanthium, a reversal to tepal-like petals with secondary polymery in the androecium and gynoecium, or the loss of sepals linked with the development of stamen-petal tubes. The relationship between petal morphology and the molecular basis for petaloidy is discussed.