Anther and ovule development in Pittosporopsis and its implications for the systematics of Metteniusaceae (Metteniusales)

Abstract

Pittosporopsis Craib, previously considered a member of the Icacinaceae sensu lato (s.l.), was transferred recently to the expanded Metteniusaceae, a family of 11 genera that needs morphological reevaluation to assess possible synapomorphies given its new circumscription. We investigated the anther and ovule developmental characters of Pittosporopsis and compared them with those of other members of Metteniusaceae as well as Icacinaceae s.l. to the extent possible. These characters are important to establish morphological synapomorphies of Metteniusaceae and to provide insights into embryology of the early diverging clades of core asterids. Within the family, Pittosporopsis shares several uncommon embryological characters with both Metteniusa H. Karst. and Emmotum Ham., such as a connective with numerous tanniferous cells and two superposed ovules within one locule. The ovule of Pittosporopsis is bitegmic, the third report of this condition (after Emmotum and Quintinia Baker f.) in the recently recircumscribed campanulids. Characters not shared with other members of Metteniusaceae include an unusual outward protuberance in the anther wall derived from the division and enlargement of endothecial cells, and a hypostase connecting the embryo sac and the ovular vascular bundle. Interestingly, a hypostase is known from Bruniaceae, which is sister to the core campanulids. Although further studies are needed to fully characterize the embryology and floral development of Pittosporopsis and the other genera now placed in Metteniusaceae, our study provides new insights into the embryology of the first diverging campanulid clades.