Flower morphology and anatomy of Sabia (Sabiaceae): structural basis of an advanced pollination system among basal eudicots

Abstract

Flowers of four species of Sabia are investigated using scanning electron and light microscopy to understand the complex floral system of Sabiaceae and to contribute to the understanding of the systematic position of the family among early diverging eudicots. The structure of the mature flower and the floral anatomy are here described and compared with that of the sister genus Meliosma. The floral structure is relatively uniform with the greatest variability in the shape of the nectary and the differentiation of the style. Flowers share a similar pollen release mechanism, as pollen is extruded from the monosporangiate thecae through an inward-out dehiscence process leading to seemingly extrorse anthers occasionally accompanied by the curving of the upper part of the filaments. The bicarpellate ovary is divided in a synascidiate zone and a symplicate zone of similar size with two superposed unitegmic ovules per carpel. The floral Bauplan of Sabia can be interpreted as precursory to a further evolution of the monosymmetric flower of Meliosma, as both genera share numerous characters. The isolated position of Sabiaceae in the early diverging eudicots is highlighted by their unique floral morphology, although several features point to a link with Ranunculales, such as Menispermaceae. These probably reflect the existence of apomorphic tendencies shared by members of the early diverging eudicots.