Basal lineages of angiosperms: Relationships and implications for floral evolution

Patterns and trends in angiosperm evolution can only be inferred using a well-resolved and well-supported phylogenetic tree, and many inferences of character evolution across the angiosperms rely on reconstructions at or near the base of the tree. For this reason, inferences of phylogeny and evolution in basal angiosperms have implications for understanding evolution in the angiosperms as a whole. Recent studies have concluded that the first three branches of extant angiosperms are Amborella, Nymphaeales, and a clade of Austrobaileya, Trimenia, and Illiciales; relationships among basal angiosperms following these early diverging branches are less clear. In this article, we (1) review the evidence for these basal relationships inferred from sequence data for three genes, (2) address the relationships among the clades of noneudicot angiosperms, including the monocots, based on both this large analysis of the angiosperms and new, more focused analyses of basal angiosperms alone, and (3) consider the implications of these phylogenetic relationships for the evolution of floral traits, particularly of the perianth. Using the best tree currently available and data on floral characters taken from the literature, we hypothesize that the early angiosperm flower had spiral phyllotaxis, with a small but indeterminate number of undifferentiated perianth organs. From this simple flower, a whorled floral arrangement, with a fixed number of sepals and petals, evolved quite early in angiosperm evolution. Reversions to spiral phyllotaxis, changes in merosity, and losses of floral parts occurred several times in the early evolutionary history of the angiosperms.