Rethinking floral reference models in Rosales: an ontogenic analysis of flower structure in Elaeagnaceae

One of the most emblematic angiosperm flowers is the rose (Rosa, Rosaceae, Rosales). However, roses represent only a small fraction of the floral diversity across the nine families of Rosales. To explore this diversity, we examined the floral structure and development of the less-studied family Elaeagnaceae, which comprises apetalous flowers that are diclinous or monoclinous and possess a gynoecium considered pseudomonomerous, focusing on two of its three genera, Elaeagnus and Hippophae. Floral buds and flowers of Elaeagnus angustifolia, Hippophae rhamnoides, and H. salicifolia were processed for surface and anatomical analyses, and vascularization was studied using high-resolution X-ray computed tomography. Our data show that organ absence from inception is the main developmental mechanism shaping floral architecture, thereby promoting dicliny and apetaly. The gynoecium is monomerous and entirely plicate, vascularized by two ventral and one dorsal trace. Elaeagnaceae exhibits specializations within Rosales, including a silvery indument of early-developing peltate trichomes, a hypanthial nectary in E. angustifolia, hypanthium outgrowths with papillose epidermis in pistillate Hippophae flowers, and species-specific hypanthium tube lengths. These highly specialized and reduced floral forms diverge from the rosalean floral reference models, which is showy, dichlamydeous, actinomorphic, and multi-ovulate, as exemplified by most Rosaceae members.