Pollen Wall Ultrastructure of Araceae and Lemnaceae in Relation to Molecular Classifications

A comparative ultrastructural study of Araceae pollen walls was conducted for the first time. The results are based mainly on investigations by transmission electron microscopy of 101 species in 70 genera (out of a total of 105 genera), while pollen of 83 genera with 219 species were studied by scanning electron microscopy, generally without acetolysis. Special attention was given to Lemnaceae (including Limnobiophyllum) considered to be closely related to Araceae (especially to Pistia). The ultrastructural pollen wall characters are mostly in accordance with and strongly support the morphological classifications and the arrangement of genera within recent molecular trees. For example, pollen ultrastructural characters favor the placement of Lemnaceae (as Lemnoideae) between the two subfamilies Orontioideae and Pothoideae, and this novel view is strongly supported by molecular data. Within the large subfamily Aroideae, especially, the tribal arrangement based on molecular and morphological data is supported by palynological evidence with respect to the ektexine, which is mostly an unusual outer exine layer devoid of sporopollenin. In the morphologically basal tribes and in the corresponding "lower" clades of the molecular trees, a highly reduced or specifically modified smooth outer exine layer is typical. In contrast, a spiny outer exine layer is representative of the morphologically derived tribes, corresponding to the "higher" clades in the molecular trees.