Embryology and character evolution in the tribe Stewartieae (Theaceae)

The Theaceae family, consisting of nine genera within three tribes, is classified under the order Ericales. While embryological studies of the tribes Theeae and Gordonieae are well-documented, research on the tribe Stewartieae, the earliest diverging lineage within the family, remains limited. To address this gap, we conducted a literature review and examined three species of Stewartia ( S. monadelpha , S. pseudocamellia , and S. serrata ) to elucidate the evolutionary trajectories of embryological characters in Theaceae. Our findings propose novel evolutionary polarities in these characters. The absence of vascular bundles in the integuments is identified as a plesiomorphy, while their presence in the inner integument of Theeae and the outer integument of Gordonieae represents respective apomorphies. Ribbed pseudopollen grains are a plesiomorphy, whereas a clade comprising Franklinia and Schima exhibits porous pseudopollen grains and an anther middle layer with more than three cell layers, which are shared as synapomorphies. Stewartieae possess a bitegmic ovule, a character that Theaceae share as a synplesiomorphy with Pentaphyllaceae and Sladeniaceae. In contrast, the unitegmic ovule is a synapomorphy for a clade that includes the Ericoid, Sarracenioid, and Styracoid families. Both integuments in Stewartieae are of dermal origin, and the transition from bitegmic to unitegmic ovules involves the development of a subdermatogen at the base of the integuments. The presence of endothelium and hypostase are synapomorphies of the family. Additional evolutionary trends in embryological traits within Theaceae are discussed in detail.