An Evaluation of A‐Function: Evidence from theAPETALA1andAPETALA2Gene Lineages

The ABC model for the specification of floral organ identity, published 15 years ago, has proven to be a useful framework for interpreting floral development data from a wide variety of species. In Arabidopsis, A‐function (specification of sepal and petal identity) is attributed to two unrelated genes, APETALA1 (AP1) and APETALA2 (AP2). An examination of the available information regarding orthologues and paralogues of these genes in other species shows that although some are required for sepal identity, none is required for both sepal and petal identity. Combined with phylogenetic analyses that show gene duplication and loss specific to Brassicaceae, this suggests that the two‐whorl phenotype attributed to loss of A‐function in Arabidopsis may be unique to Brassicaceae. Furthermore, all genes that are required for proper sepal identity, including AP1 and AP2, are also implicated in floral meristem identity, suggesting that these two functions may not be separable. Available data are all consistent with a previous Antirrhinum‐based model for floral organ identity that required only two gene functions. The loss of sepal identity seen in some AP1‐ and AP2‐lineage mutants can be explained as loss of floral meristem identity; the available evidence suggests that a discrete perianth identity gene function is not required.