The ABC model of floral development postulates that three classes of genes are responsible for determining the identity of the four types of floral organs. The genes overlap in function in adjacent organ whorls, thus A-gene function alone determines sepal identity, A+B determine petal, B+C determine stamen, and C alone determines carpel identity. APETALA1 (AP1) of Arabidopsis thaliana has been identified as an A-function gene, but it is also involved in the determination of floral meristem identity. The phenotype of the mutant has features that can be interpreted as a failure of either function, thus there is no clear evidence that AP1 is involved in specifying floral organ identity as a separate function. AP1 homologs from other species have not been shown to affect floral organ identity, thus there is little documentation of the A component of the ABC model. AP1 belongs to a large and complex subfamily of the MADS-box gene family, and it is not clear that homologs have been correctly identified in other species. This is a requirement for comparative studies, therefore we are studying the phylogeny of AP1-like genes to be able to identify homologs for further expression and function studies and to determine where significant evolutionary events have occurred. Preliminary results suggest that during angiosperm evolution there was a duplication that produced the paralagous AP1 and FRUITFULL lineages present in higher eudicots. There is some evidence that this duplication may have occurred within the basal eudicots, and may coincide with a similar duplication in the AP3 (B-function gene) lineage. These duplication events may be related to the fixation of basic floral structure that is seen in the higher eudicots.

Key words: ABC model, APETALA1, floral evolution, floral structure, MADS-box genes