Anther development in gourd tree (Crescentia cujete, Bignoniaceae): implications for the evolutionary transition between basic and dicotyledonous types of microsporangium wall formation

In this study, we compared anther development in C. cujete with those of other Bignoniaceae and some related taxa. Our goal was to investigate the ontogeny of microsporangium wall layers, microsporogenesis, and pollen development, in order to provide data useful for studies of the evolution of morphoanatomical features of the anther and to contribute for the embryology of Bignoniaceae. As in other Bignoniaceae, the horseshoe microsporangium of C. cujete exhibits a secretory dimorphic tapetum with dual origin and a parenchymatous placentoid. These features have been also observed in several other asterid families. However, the parenchymatous placentoid in C. cujete and other species of Bignoniaceae is a connective outgrowth, which differs from the septum-derived condition of this structure in certain families. Microsporogenesis and pollen development also proceed as in other species of Bignoniaceae, and the pollen grains are shed at the bicelled stage. The multilayered endothecium with bands of wall thickenings is restricted to four specific sites of the anther. Stomium rupture involves the action of epidermal cells only, but the anther wall opening appears to arise from a combination of the mechanical action of the multilayered endothecium and cell collapse in specific parts of the connective tissue. Formation of the anther wall layers in C. cujete conforms to both basic and dicotyledonous modes (i.e., centripetal and centrifugal derivation of the outer tapetum, respectively), which is congruent with plasticity in the evolutionary transition between these conditions.