Evolution of genetic mechanisms controlling petal development

Molecular genetic studies in Arabinopsis thaliana and other higher-eudicot flowering plants have led to the development of the 'ABC' model of the dete rmination of organ identity in flowers, in which three classes of gene, A, B and C, are thought to work together to determine organ identity(1,2). Acc ording to this model, the B-class genes APETALA3 (AP3) and PISTILLATA (PI) act to specify petal and stamen identity, Here we test whether the roles of these genes are conserved throughout the angiosperms by analysing the expr ession of AP3 and PI orthologues in the lower eudicot subclass Ranunculidae , We show that, although expression of these orthologues in the stamens is conserved, the expression patterns in the petals differ from those found in the higher eudicots, The differences between these expression patterns sug gest that the function of AP3 and PI homologues as R-class organ-identity g enes is not rigidly conserved among all angiosperms. These observations hav e important implications for understanding the evolution of both angiosperm petals and the genetic mechanisms that control the identities of floral or gans.