Phylogeny and the evolution of flower symmetry in the Asteridae

Phylogenetic trees imply that flowers with a single plane of symmetry (zygomorphic flowers) have evolved several times independently from radially symmetrical (actinomorphic) ancestors within the Asteridae. However, there also appear to have been reversals to actinomorphy. A few evolutionarily derived actinomorphic flowers resemble mutants caused by loss-of-function mutations in genes such as CYCLOIDEA. However, a majority of the shifts from zygomorphy to actinomorphy appear to have entailed a reduction in petal number and flower size, implying a mechanism other than loss of CYCLOIDEA function. Within the Asteridae there appear to be three common forms of zygomorphy. An explanation for the virtual absence of other forms rests on the near universality of the basic orientation of the flower in the Asteridae.