Evolution of the arils of Euonymus in comparison with some other genera of Celastraceae

The family Celastraceae R. Br. is characterized by the difference between the genera according to the types of fruits and the presence of arils in fruits of representatives of some genera. Attempts to reconstruct the evolution of arils in the Celastraceae were made earlier, while trying to trace the relationship of arils with the mucilagenous pulp and wings on the seed. In particular, earlier attempts were made to reconstruct the evolution of Euonymus arils. But, at present, significant changes in the phylogeny of the genus Euonymus and the family Celastraceae as a whole, as well as new accumulated material require another revision of the evolution of arils of Euonymus. We have studied the morphology-anatomical structure of mature arils of 28 species of Euonymus, as well as (additionally) the development of arils in 8 species of the genus. It is assumed that the aril of ancestral taxa of Euonymus had a funicular-exostomal origin, partially covered the seed, was colored orange, and composed of a single-layered epidermis and undifferentiated multi-layered parenchyma. In the cells of aril there were rounded chromoplasts, and large oil drops were absent. In the process of evolution of the aril of Euonymus, funicular arils appeared, while the funicular-exostomal ones were preserved. The degree of seed covering by aril in some modern species has increased and decreased again, arils have appeared with a small “window”, partially (by 1/2–2/3) covering the seed and small fleshy structures in the basal part of the seed near funicule. The color of arils has been preserved in orange hues, but arils of some species have become red in color. The anatomical structure of the arils has changed. At present, along with multi-layer arils with a multi-layer undifferentiated or weakly differentiated parenchyma, there are arils with a strongly differentiated parenchyma and, conversely, with a parenchyma that is reduced in the process of development. In the process of evolution, fibrous and crystalline chromoplasts and large oil drops of different sizes appear in the cells of the arils.