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Flowers can be classified into two basic types according to their symmetry: regular flowers have more than one plane of symmetry and irregular flowers have only a single plane of symmetry. The irregular condition is thought to have evolved many times independently from the regular one: most commonly through the appearance of asymmetry along the dorso-ventral axis of the flower. In most cases, the irregular condition is associated with a particular type of inflorescence architecture. To understand the molecular mechanism and evolutionary origin of irregular flowers, we have been investigating genes controlling asymmetry inAntirrhinum. Several mutations have been described inAntirrhinum, a species with irregular flowers, that reduce or eliminate asymmetry along the dorso-ventral axis. We describe the nature of these mutations and how they may be used to analyse the molecular mechanisms underlying floral evolution.

A major change in the development of plants occurs upon floral induction. Meristems in certain positions become organized to form flowers. We are studying this process using a combination of genetic, molecular and physiological approaches in Antirrhinum . In particular, we are exploiting transposon-induced mutations in genes controlling early switches in floral development. These mutations cause homeotic and heterochronic phenotypes and three categories of genes have been identified. The first includes floricaula ( flo ), which is required to switch inflorescence meristems to a floral state. This gene has been isolated and shown to be expressed transiently in bract, sepal, petal and carpel primordia. The second group of genes controls the identity (and sometimes the number) of organs in a whorl. These genes affect overlapping whorls and their mutant phenotypes suggest a co binatorial model for gene action in determining the fate of floral primordia. Some of the regulatory interactions between these genes have been revealed by studying cis -or trans -acting mutations which have resulted in ectopic gene expression. Genes of the third category determine the identity of organs within one whorl and thus affect the symmetry of the flower. We propose that the interactions of these homeotic genes control the basic patterns of inflorescence and flower development not only in Antirrhinum , but also in a diverse range of plant species.