Genetic Footprints of Stamen Ancestors Guide Perianth Evolution in Persea (Lauraceae)

The perianth of Persea americana (Lauraceae) consists of two whorls of morphologically similar laminar organs, termed tepals. Closely related Persea borbonia, however, produces a dimorphic perianth with smaller outer tepals. To assess whether homologues of floral organ identity genes in Persea may play a role in shaping this dimorphic perianth, we compared their expression patterns in the two species. A homologue of AP1 (A-function) is expressed at low levels in both perianth types but was not tepal specific. Homologues of AGL6, however, show the tepal-specific expression pattern expected of A-function genes. Homologues of AP3 and PI (B-function) are expressed in tepals of both perianth types, indicating that perianth dimorphism in Persea is not regulated by these genes. Differential expression across the dimorphic perianth as absence late in outer tepal development was evident for homologues of AG (C-function) and SEP3 (E-function). Genetic studies in model systems indicate a conserved role for AG homologues in specifying stamen and carpel identity, but the expression pattern in Persea indicates a novel role in perianth development. On the basis of gene expression and the occasional presence of tepaloid organs in stamen whorls, we hypothesize that the tepals of Persea and perhaps other Lauraceae are derived from stamens.