Evolutionary developmental genetics of floral symmetry: the revealing power of Linnaeus' monstrous flower.

Actinomorphic flowers have several planes of reflectional symmetry while zygomorphic flowers have just one. In a number of independent cases, actinomorphic flowers have arisen from zygomorphic ones during evolution. A famous example, studied by Linnaeus, is an actinomorphic variety of the common toadflax Linaria vulgaris. It has been shown now that this mutant carries a defect in LCYC, a homolog of the CYC gene, which controls zygomorphy in Antirrhinum majus.((1)) Interestingly, the mutant phenotype is not due to changes in the LCYC nucleotide sequence but rather to an extensive, heritable methylation of the gene.((1)) A second gene controlling zygomorphy in snapdragon, DICH, has recently also been shown to be a CYC homolog and both genes share significant sequence similarity with TB1, one of the key genes of maize domestication. The respective family of genes, probably encoding transcription factors, might thus become both a useful instrument and a target of future plant evolutionary developmental genetics. Copyright 2000 John Wiley & Sons, Inc.