The evolution of the MADS-box gene family in angiosperms has been characterized by numerous duplication events and subsequent diversification of function in the resulting lineages. These regulatory genes are involved in many aspects of plant development, notably as master regulators of floral organ identity. In Arabidopsis thaliana, APETALA1 (AP1) is implicated in specifying the identity of the floral meristem as well as that of sepals and petals. FRUITFULL (FUL), a paralog of AP1, also specifies floral meristem identity and has a role in carpel development, but appears to have no effect on perianth identity. Phylogenetic analysis shows that the AP1 and FUL lineages diverged near the base of the monophyletic core eudicots, and can be distinguished by different conserved amino acid motifs in the C-terminal domain of the protein. In angiosperms other than core eudicots only FRUITFULL-like genes are found. We have shown in the AP1/FUL lineages that a simple frame-shift in the nucleotide sequence can produce the observed change in amino acid motifs, thereby providing a mechanism for the origin of the new motifs in the core eudicot AP1 lineage. Since AP1 has been implicated in specifying perianth identity in core eudicots, the absence of AP1-like genes from other species implies that other genes carry out this role. These findings suggest a correlation between MADS-box gene evolution and the fixation of floral structure in the core eudicots. Examination of expression patterns of AP1/FUL family members from a variety of angiosperm species is being used to address the potential functional diversification of this gene family; preliminary results will be discussed.

Key words: APETALA1, floral development, floral evolution, FRUITFULL, MADS-box genes