Embryological studies on Abelia tyaihyoni Nakai (Caprifoliaceae)

Abelia tyaihyoni is a Korean endemic species and also designated as an endangered taxon in the IUCN Red List. We present a comprehensive embryology of A. tyaihyoni, comparing it with previously available information on the Caprifoliaceae and related families, and identifying possible evolutionary trends. Overall, comparisons showed that Abelia is similar to the other genera of Caprifoliaceae in many embryological features, including the tetrasporangiate anther, the anther wall, dicotyledonous wall formation, fibrous endothecium, simultaneous cytokinesis, tetrahedral tetrads, and three-celled pollen grains. In addition, Abelia share anatropous, unitegmic, and tenuinucellate ovules, ephemeral antipodals, and a cellular-type endosperm with many caprifoliaceous genera. Some embryological features, such as the amoeboid tapetum with several nuclei forming a polyploid mass, bisporic Allium-type embryo sac, obturator, bilayered endocarp, and compressed exotesta, are here described for the first time in this genus. The results confirm that A. tyaihyoni shares some significant embryological features with other Caprifoliaceae. Although previous embryological studies on the Caprifoliaceae lack convincing evolutionary explanations, our comparative approach demonstrates some possible apomorphies in Abelia such as the presence of two sterile carpels, bisporic Allium-type embryo sac, single-seeded fruit, bilayered and sclerified endocarp, and compressed exotesta.