Anther wall and pollen development in Neotropical species-rich Miconia (Melastomataceae)

Miconia is one of the largest exclusively Neotropical genera, inserted in the tribe Miconieae. Although the monophyly of the tribe has recently been recognized, the delimitation of its genera is considered quite arbitrarily defined, mainly due to the great diversity combined with little morphological and genetic characterization of its members. Recent findings have associated this diversity with polyploidy, apomixis and hybridization, mechanisms commonly found in the group. The conventional delimitation of sections in Miconia, which has been largely based on stamen morphology, has proved to be undoubtedly artificial; nevertheless, the potential taxonomic significance of anther wall and pollen ontogenetic characters within the genera Miconia has been little explored. Hence, this study intended to fill that gap in our knowledge, by investigating the anther wall and pollen development in six closely related species of Miconia: M. albicans, M. fallax, M. latecrenata, M. paucidens, M. pepericarpa and M. stenostachya. Routine techniques for both light and electronic microscopy were used to examine anthers and pollen grains in several developmental stages. The species studied here share several character states, such as the remarkable monocotyledonous anther wall development, the persistent endothecium in the mature anthers with no cell wall thickenings, the bilocular dehiscent anther and the psilate pollen exine. This study also reports for the first time bisporangiate anthers, crystals and proteinoplasts in tapetal cells for Melastomataceae, and ultrastructural features of anther and pollen wall for Miconia.