Evolution of the APETALA3 and PISTILLATA lineages of MADS-box-containing genes in the basal angiosperms

The B class genes, including homologs of the Arabidopsis loci APETALA3 (AP3) and PISTILLATA (PI), appear to play a conserved role in the determination of petal and stamen identity across core eudicot angiosperms. Understanding how and when these functions evolved is a critical component of elucidating the evolution of flowers, particularly the appearance of petaloid perianth organs. Before comparisons of gene expression patterns or functions can be made, however, it is necessary to establish the orthology of AP3 and PI homologs from basal angiosperms. Here, we report the identification and analysis of 29 new representatives of the B gene lineage from basal ANITA and magnoliid dicot angiosperms. These studies indicate that gene duplications have occurred at every phylogenetic level, both before and after the duplication that produced the separate AP3 and PI lineages. Comparison of genomic structure among PI homologs indicates that a 12-nucleotide deletion that had been considered synapomorphic for the whole PI lineage actually arose within the ANITA grade, after the split of the Nymphaeales but before the separation of the Austrobaileyales. Evidence for alternative splicing of the Nymphaea AP3 homolog is also presented. The implications of these findings for angiosperm systematics, the conservation of AP3 and PI gene function, and the evolution of the ABC program are discussed.