Deep into the Aristolochia Flower: Expression of C, D, and E-Class Genes in Aristolochia fimbriata (Aristolochiaceae)

Aristolochia fimbriata (Aristolochiaceae) is a member of an early diverging lineage of flowering plants and a promising candidate for evo-devo studies. Aristolochia flowers exhibit a unique floral synorganization that consists of a monosymmetric and petaloid calyx formed by three congenitally fused sepals, and a gynostemium formed by the congenital fusion between stamens and the stigmatic region of the carpels. This floral ground plan atypical in the magnoliids can be used to evaluate the role of floral organ identity MADS-box genes during early flower evolution. In this study, we present in situ hybridization experiments for the homologs of the canonical C-, D-, and E-class genes. Spatiotemporal expression of the C-class gene AfimAG is restricted to stamens, ovary, and ovules, suggesting a conserved stamen and carpel identity function, consistent with that reported in core-eudicots and monocots. The D-class gene AfimSTK is detected in the anthers, the stigmas, the ovary, the ovules, the fruit, and the seeds, suggesting conserved roles in ovule and seed identity and unique roles in stamens, ovary, and fruit development. In addition, AfimSTK expression patterns in areas of organ abscission and dehiscence zones suggest putative roles linked to senescence processes. We found that both E-class genes are expressed in the anthers and the ovary; however, AfimSEP2 exhibits higher expression compared to AfimSEP1. These findings provide a comprehensive picture of the ancestral expression patterns of the canonical MADS-box floral organ identity genes and the foundations for further comparative analyses in other magnoliids.