Step by step: Floral structure and developmental changes to the formation of the gynostegium in Apocynaceae s.l.

The gynostegium is a key floral innovation in Apocynaceae, formed by the fusion between anthers and the style head and is essential for pollination. We investigated how this complex structure develops by comparing early- and late-divergent species to elucidate the developmental processes underlying its formation and diversification. Floral buds and flowers were analysed using light microscopy and scanning electron microscopy, allowing detailed examination of morphological and anatomical traits throughout floral ontogeny. We identified the timing and sequence of conation, adnation and fusion events during development. All species exhibited petal union during floral ontogeny, generally consistent with postgenital fusion, although in Asclepias curassavica, this process occurs at an earlier developmental stage and shows a distinct ontogenetic pattern. All species also showed adnation of staminal filaments to the corolla tube. We documented progressive stamen specializations, including the formation of a staminal tube, sclerenchymatous differentiation in the anthers, secretory tissues presentation and guide rails, particularly in the APSA clade. Our findings demonstrate developmental steps that extend beyond anther-style head fusion. We show that reproductive structures are reshaped, both functionally and anatomically, towards gynostegium formation. The reduction of the corolla tube and emergence of a staminal tube suggest a shift in pollinator-guiding roles from the corolla to the androecium, accompanied by a partial transfer of secretory functions from the gynoecium to the stamens. These results provide new insights into the developmental mechanisms driving floral complexity in Apocynaceae.