Embryology of Styracaceae and Implications for the Evolution of the Integument Number in Ericales

Phylogenetic relationships in Styracaceae are well understood, but embryological characters and the ontogeny of integument(s) are still uncertain in many species. The goals of this study are to evaluate the systematic implications of embryological characters in Styracaceae, clarify the character evolution of the number of integuments and suggest a mechanism for the transition between unitegmic and bitegmic ovules. We examined the embryological characters of four genera and five species of Styracaceae, most of which were shared across taxa. However, Styrax has specific embryological features including bitegmic ovules, a multiplicative and sclerotic outer mesotesta and vascular bundles in the testa, all possible autapomorphies. The other three genera of Styracaceae share a unitegmic ovule, a parenchymatous mesotesta and a seed coat without vascular bundles, possible plesiomorphies with Diapensiaceae and Symplocaceae. The transition from a unitegmic to a bitegmic condition can be interpreted to be caused by a downwards shift of the boundary between the inner and outer integument, due to reduced activity in the subdermal initials and increased activity in the dermal initials of the outer integument at its base.