Anther wall and pollen development in two species of Rauvolfia L. (Apocynaceae)

Although dioecy and gynodioecy are widely reported in Gentianales, they are rare in Apocynaceae, with reports for only three species. In these species, the pistillate flowers have empty anthers. To understand how this process occurs, it is important to gather information about the formation of anthers and pollen grains, but studies on the embryology of Apocynaceae are scarce and restricted. Here, we investigate anther wall and pollen grain development and the processes leading to pollen grain absence in R. vomitoria, a gynodioecious species. We compare it with R. weddelliana, a species with hermaphrodite flowers. To do so, the flowers of both species were collected at several developmental stages and processed using usual anatomical techniques. Mature pollen grains were analyzed via scanning electron microscopy. In both species, the anthers are tetrasporangiate, and the anther wall development is of the dicotyledoneous type. The tapetum is secretory, and the endothecium has fibrous thickenings. The tetrad of microspores is tetrahedral as a result of simultaneous cytokinesis. Each pollen grain is tricolpate, two celled, oblate, and smooth. Small perforations occur in the equatorial axis of the pollen only in R. weddelliana. Thick margins occur in the colpi of both species. Surprisingly, when the pistillate flowers of R. vomitoria, which came from the same population in which gynodioecy was reported, were analyzed, fertile anthers were found during this flower season. The only evidence of the sterility was the presence of some sterile microsporangia. Our results indicate that R. vomitoria may exhibit sexual lability.