The ABC model and the diversification of floral organ identity.

Broad studies of the ABC program across angiosperms have found that interactions between gene duplication, biochemical evolution, shifts in gene expression and modification of existing identity programs have been critical to the evolution of floral morphology. Several themes can be recognized in this context. First, the original concept of "A" function applies only very narrowly to Arabidopsis and its close relatives. Second, while many types of petaloid organs are associated with the expression of AP3/PI homologs, there is growing evidence that there are other genetic mechanisms for producing petaloidy, especially in first whorl organs. Third, pre-existing organ identity programs can be modified to yield novel organ types, often in association with gene duplications. Lastly, there are many aspects of ABC gene function outside the major model systems that remain a mystery, perhaps none more so than the C-terminal amino acid motifs that distinguish specific ABC gene lineages.