The embryological insight into the relationship between Forsythia and Abeliophyllum (Forsythieae, Oleaceae)

Forsythia saxatilis, a Korean native Forsythia, is recognized as an endangered species in the Korean Red List. We carried out a comprehensive embryological investigation, including a study of the pericarp development, of F. saxatilis and compared it with previously acquired information on the family Oleaceae and also with Abeliophyllum distichum, a close relative of Forsythia. Our results revealed that several embryological features of Forsythia are exclusively comparable with the family Oleaceae, particularly in relation to A. distichum. Despite the differences in fruit types and pericarp ontogeny, Forsythia shares some unique embryological features with Abeliophyllum, for instance the basic type of anther wall development, formation of the nucellar cap, a long micropyle, formation of the obturator and hypostase, and the crushed endotesta. Furthermore, the loculicidal capsule of Forsythia seems to be primitive within the tribe Forsythieae and samara of Abeliophyllum might be evolved from it. In conclusion, a considerable number of synapomorphies of embryological characters supports recent molecular reports and provide additional embryological evidence for the sister-group relationship of Forsythia and Abeliophyllum.