Aspectos morfológicos da flor e do desenvolvimento do Androceu de Oxypetalum Appendiculatum mart. (Apocynaceae, Asclepiadoideae)

Abstract

Apocynaceae is monophyletic, supported by two synapomorphies, with the presence of latex and gynoecium with two carpels separated in the ovary, but distally united in a stylar head. Asclepiadoideae is a subfamily derived from Apocynaceae, with reduced anthers and only two sporangia, each producing a pollinium, consisting of monads and surrounded by a film derived from the cutinization of the inner walls of the tapetum. Pollinia of adjacent stamens are connected together by a translator, forming the pollinarium. Approximately 80% of neotropical Asclepiadoideae species belong to the MOOG clade (Asclepiadeae). This clade is well supported; however, its internal relations have yet to be resolved. Some studies indicate that the androecium, though little explored, is important for the taxonomy of Apocynaceae. Therefore, the objective of this study was to analyze the floral morphology of an Asclepiadoideae species, Oxypetalum appendiculatum, in order to show the development of its androecium, covering important anatomical aspects of anther development. Microsporogenesis and microgametogenesis were also investigated, besides pollinarium morphology and pollen grain. Therefore, this study provides subsidies to the Apocynaceae systematics and contributes to the understanding of the development of its reproductive structures. The corolla of O. appendiculatum is vinaceous, the corona has five free lobes circling the anthers and the gynostegium is pentagonal, ending in a bifid appendage. The stamen presents a subsessile thread and the anthers have sclerified side wings. The anther is bilocular, with six parietal layers: epidermis, endothecium, two strata of middle layers and a biseriate secreting tapetum, whose development is dicotyledonous. The endothecium is not thickened; however, the anther, as a whole, presents a thickened parenchyma. The tapetum is secretory and biserial, but multistratified at the dorsal portion of the anther. Microsporogenesis is successive and tetrads are linear. Pollen grains are tricellular and inaperturate, transferred as monads in the pollinia. The translator has a retinaculum, which is laterally expanded, conferring it the “clip mechanism”, and the pollinium is surrounded by a hyaline film. The wing expansion in the retinaculum is important for the characterization of the species. For genus, the subsessile stamen and the absence of endothecium thickening seem to be constant characters. Bilocular anther, biseriate tapetum, successive microsporogenesis, linear tetrads and inaperturate pollen grains are important characteristics at the subfamily level. The absence of an endothecium thickening indicates the derivative position of O. appendiculatum in the “MOOG clade”. The subsessile threads guarantee the derived position of the species in relation to other Oxypetalum individuals.