Embryological studies in Thinouia mucronata and Serjania meridionalis (Paullinieae, Sapindaceae): development of gametophytes in both floral morphs and its phylogenetic implications

Abstract

The tribe Paullinieae is distinguished by a monoecious reproductive system; it exhibits two floral morphs, namely staminate flowers, with gynoecium reduced to a pistillode, and morphologically hermaphrodite but functionally pistillate flowers. The aim of this study was to analyze the development of sporogenesis and gametogenesis in flowers of Thinouia mucronata Radlk. (subtribe Thinouiinae) and Serjania meridionalis Cambess. (subtribe Paulliniinae) and to elucidate the moment when pollen grains of pistillate flowers and the ovule of staminate ones stopped their development. Light and scanning electron microscopy was applied using standard techniques. In T. mucronata flowers are actinomorphic; the pistillate ones have anatropous, pseudocrassinucellate ovules, without hypostase and with placental obturator, and stamens with indehiscent anthers and well-developed pollen grains that remain inside the pollen sac at the end of anthesis. Serjania meridionalis has zygomorphic flowers, the pistillate ones have campylotropous, crassinucellate ovules, with presence of hypostase, and funicular obturator, and stamens with indehiscent anthers collapsed at the end of anthesis, showing remnants of aborted sporogenous tissue in each pollen sac. Both species share the anatomy of the anther wall; microsporocytes form tetrahedral or decussate tetrads; monads are bicellular; the pistillode has abortive megaspore; gynoecium with bithegmic ovules and Polygonum-type megagametophyte. One difference in antheral wall ontogeny between species was that T. mucronata displayed the dicotyledoneus type development, whereas S. meridionalis exhibited the basic type. These embryological characters clearly support the basal position of Thinouia in the tribe recently validated from molecular phylogeny studies and supported by the present morphological data.