The Evolution of Floral Symmetry

Trends in flower symmetry evolution revealed through phylogenetic and developmental genetic advances

“…Using a parsimony approach, and scoring for flower symmetry at the family level (which is likely to underestimate the number of transitions to bilateral flower symmetry), they identified a single transition to bilateral flower symmetry among the basal angiosperms, 23 transitions in monocots, and 46 independent transitions in the eudicots. Therefore, using a well resolved and densely sampled (at the family level) estimate of flowering plant phylogeny, Citerne et al [16] suggest at least 70 transitions to bilateral flower symmetry-twice as many as previously reported.…”

“…The primary comparative question has been whether the developmental programme identified in A. majus contributes to the establishment of bilateral symmetry in other flowering plant lineages. Strikingly, current evidence suggests that a similar developmental programme, first identified in A. majus, has been recruited many times independently during the parallel evolution of bilateral flower symmetry (reviewed in [12][13][14][15][16]). …”

“…Those of the keel form are well known from Fabaceae, but can also be found in Polygalaceae. Less elaborate forms of bilateral flower symmetry also result from organ differentiation primarily in the petal and/or stamen whorls, and may be due to displacement of organ initiation, size or shape variation in organs along the dorsoventral axis of the flower, or sigmoidal curvature of organs (reviewed in [6,16]). …”

“…This is by no means a critical evaluation of floral symmetry evolution, but illustrates the widely dispersed nature of transitions to bilateral flower symmetry. Citerne et al [16] undertook an excellent analysis of floral symmetry evolution on the estimate of flowering plant family relationships presented in Bremer et al [27]. Using a parsimony approach, and scoring for flower symmetry at the family level (which is likely to underestimate the number of transitions to bilateral flower symmetry), they identified a single transition to bilateral flower symmetry among the basal angiosperms, 23 transitions in monocots, and 46 independent transitions in the eudicots.…”

“…Possibilities include independent recruitment of a CYC-dependent programme to specify dorsal or ventral identity ( figure 3a,b), or novel recruitment of a CYC-independent developmental programme to specify either dorsal or ventral flower identity (figure 3c,d). Results from many comparative studies now demonstrate that there is striking parallelism in the independent evolution of bilateral symmetry with a CYC-dependent programme frequently recruited to specify dorsal identity, and in some cases ventral identity (reviewed in [13,15,16]). …”