Arabidopsis thaliana (left) and Antirrhinum majus (right) flowers. A. thaliana courtesy of Pilar Cubas, INIA/CNB-CSIC, Spain. A. majus courtesy of the John Innes Centre, Norwich, UK. Reprinted from P. Cubas et al. Current Biology 11, 1050–1052 © (2001), with permission from Elsevier Science.

Unlike humans, who are said to be attracted by symmetrical (facial) features, pollinating animals are partial to the asymmetrical shape of the flowers that they help fertilize. Asymmetrical flowers have arisen many times in the evolution of flowering plants, but it is unknown whether the same molecular mechanisms have been used over and over again, or whether new strategies have been deployed each time. Pilar Cubas and colleagues show that the asymmetrical expression of a gene that controls floral asymmetry is found even in the meristems (undifferentiated cells) that develop into symmetrical flowers. This indicates that an ancient, radially symmetrical ancestor might have had an incipient asymmetry, from which asymmetrical flowers have arisen repeatedly by the same molecular route.

The flowers of the mustard weed Arabidopsis thaliana are symmetrical, unlike those of its distant relation, the snapdragon Antirrhinum majus , which have dorsoventral asymmetry (see photo). In the snapdragon, floral asymmetry is conferred by the asymmetric expression of a TCP-transcription factor encoded by the gene CYCLOIDEA (CYC). The gene is dorsally expressed from the early stages of development in the floral meristems, where it affects the growth rate of the meristem and so, later on, the asymmetry, size and shape of the reproductive organs. Cubas et al. found that the expression of the A. thaliana orthologue of CYC, TCP1, was also expressed in the dorsal part of flower meristems, but only very early in development. This limited expression might explain why A. thaliana has symmetrical flowers. Molecular and morphological phylogenies agree that the common ancestor of the two species lacked dorsoventral asymmetry; the authors propose that this ancestor had an asymmetrical 'pre-pattern' — evident from early CYC expression — which could be readily elaborated to evolve asymmetry.

There are other examples of morphological traits that have arisen many times in evolution, such as the emergence of visual organs, although in most cases their mechanisms are not known. This study provides a possible explanation for how asymmetrical flower shape evolved repeatedly using essentially the same mechanism; however, a more definitive picture will only emerge from analysing a wider range of plant species.