No abstract

Genomic studies indicate that basal angiosperms, and by inference the earliest angiosperms, had a rich tool kit of floral genes. Homologues of the ABCE floral organ identity genes are also present in basal angiosperm lineages; however, C-, E- and particularly B-function genes are more broadly expressed in basal lineages. There is no single model of floral organ identity that applies to all angiosperms; there are multiple models that apply depending on the phylogenetic position and floral structure of the group in question. The classic ABC (or ABCE) model may work well for most eudicots. However, modifications are needed for basal eudicots and, the focus of this paper, basal angiosperms. We offer 'fading borders' as a testable hypothesis for the basal-most angiosperms and, by inference, perhaps some of the earliest (now extinct) angiosperms.

The perianth of Persea americana, like that of most Lauraceae, consists of two whorls of morphologically similar petaloid organs, termed tepals. In closely related Persea borbonia however, a sepaloid outer whorl of tepals contributes to a dimorphic perianth. To determine whether Persea homologues of the A-, B-, C-, and E-class MADS-box genes that determine organ identity in eudicot flowers have played a role in shaping this dimorphic perianth, their expression levels were assessed across the floral whorls of P. americana and P. borbonia. We find that homologues of A-and B-function genes from Persea are expressed at comparable levels in the perianth whorls of the two species, but homologues of AG (C-function) and SEP3 (E-function) have shifted away from sepaloid tepals of Persea borbonia. Genetic studies suggest a conserved role for AG homologues in specifying stamen and carpel identity in Persea, but the coincidence of phenotypic and gene expression shifts argues for a hitherto unprecedented role in perianth development. Phylogenetic conservation of SEP3-AG interaction in floral organ identity transcription factor quartets may underlie the coincident shift of Persea AG and SEP3 gene expression. Based on our results, we hypothesize: 1) A role for Persea AG and SEP3 homologues in shaping perianth morphology is indicated by the coincident shift to a dimorphic perianth with loss of their expression in the outer tepals of Persea borbonia. 2) Reproductive organ identity in Persea is specified through the action of three AG homologues. 3) The expression of floral organ identity homologues in the tepals of Persea, and perhaps other Lauraceae, is suggestive of staminal origin, i.e., andropetals.