Embryology of Zippelia begoniaefolia (Piperaceae) and its systematic relationships

Abstract

Microsporogenesis and embryology of the monotypic Zippelia (Z. begoniaefolia) Blume (Piperaceae) is described for the first time to assess its systematic relationships. The formation of the anther wall is of Basic Type such that the anther wall, consisting of an endothecium with fibrous thickenings, two middle layers, and a glandular septum with 2-nucleate cells, is derived from a primary parietal layer. Simultaneous cytokinesis follows meiosis of the microspore mother cell thence forming a tetrahedral tetrad of microspores. The single basal ovule is orthotropous, crassinucellate and bitegmic but only the inner integument forms the micropyle. The sporogenous cell of the nucellus functions directly as a megaspore mother cell. A coenocyte with four nuclei forms after meiosis of the megaspore mother cell. The formation of the embryo sac is tetrasporic ab initio and is of, or similar to, the Drusa Type of embryo sac in which the nuclei of the coenocyte undergo two successive mitoses and forms a 16-celled or 16-nucleate embryo sac that is ovoid in shape. The embryo sac has an egg apparatus consisting of an egg cell and two synergids (but one of the latter is less discernable). Two polar cells occur just beneath the egg apparatus and 11 antipodal cells or nuclei are arranged along the lower part of the inner wall of the embryo sac. They are linked by threads of cytoplasm. The two polar cells are separated or fused before fertilization. A large primary endosperm nucleus with many nucleoli, which resulted from the fertilized polar cells and with the participation of antipodal cells, divides into a free nuclei stage. The free nuclei are arranged along the lower part of the inner wall of the embryo sac or rarely assemble at the central part. The development of endosperm is thus of the Nuclear Type. The zygote remains undivided and fails to develop even when the seed is nearly mature. Frequently, the zygote and the endosperm abort later and leave an empty chamber in the top part of the seed. Most of the seed content is starchy perisperm. Only the inner integument forms the seed coat and the pericarp develops glochidiate hairs (anchor-like hairs) when the endosperm begins to develop. By comparison with the other piperaceous taxa using embryological and botanical features, Zippelia is referred to as a basal taxon and a more isolated evolutionary line or a blind branch in the Piperaceae.