Embryology of Biebersteinia (Biebersteiniaceae, Sapindales): characteristics and comparisons with related families

Biebersteinia, a perennial herb of five species distributed from Greece to Central Asia, was long considered to be placed in, or near Geraniaceae. Recent molecular analyses, however, have shown that the genus is the sole member of the family Biebersteiniaceae in Sapindales (not including Geraniaceae). Here, we report the embryological features of Biebersteinia and provide embryological corroboration for the molecular sapindalean affinities of the genus. We compared its embryology with those of eight other families of Sapindales, as well as with those of the related orders Huerteales, Malvales, and Brassicales. Overall comparisons showed that Biebersteinia fits in Sapindales because of the presence of anther tapetal cells with polyploid nuclear mass and non-fibrous exotegmen. Further, the genus is characterized by three-celled pollen grains, tetrasporic 16-nucleate Penaea-type female gametophyte, unitegmic ovules, pseudoporogamy, and the chalaza shifting its position near the concave side in the post-fertilization stage. A considerable number of autapomorphies, combined with the lack of synapomorphies with other sapindalean families, supports placing Biebersteinia in its own family. Biebersteiniaceae appear to represent an early divergent lineage of Sapindales. Previous descriptions of seed coats, which were considered to have developed from bitegmic ovules, were revised.