Anther wall and pollen development in Aspidosperma Mart & Zucc. (Apocynaceae)

Apocynaceae is one of the richest flowering plant families, and the complexity of their reproductive structures and morphological diversity of the flowers has been considered central to understanding its evolutionary success and diversification. However, the diversity of its embryological characters is poorly known, especially in taxa that are early diverging in the family, such as the Aspidospermateae tribe. Moreover, the knowledge gap on early divergent taxa forbids the reconstruction of evolutionary trends in Apocynaceae. Here, we investigated the anther wall and pollen development of five species of Aspidosperma using conventional plant anatomy techniques. We found that the primary conditions for the Aspidosperma species are fibrous endothecium, middle layer, and secretory tapetum having one-layered and uninucleate cells, besides simultaneous cytokinesis. We also found pollen grains 4–6 colporate, alternately with pseudocolporate in all the species, with ridges formed by the thickened inequal deposition of the infratectum layer in the exine that delimits aperture regions. The differentiated deposition of exine confers a distinctive appearance to pollen grains and appears to be associated with the hydration/dehydration process during pollination. Similar deposition patterns have been identified in other genera within the tribe Aspidospermateae, suggesting that it may constitute a synapomorphy for the tribe.