The systematic value of pollen morphology in Homalolepis and other six Neotropical genera of Simaroubaceae

Here we describe the pollen morphology and ultrastructure of 36 species, including representatives of Homalolepis, the largest genus of Simaroubaceae, and other 6 Neotropical genera of the family, using light microscopy and scanning electron microscopy. The objective was to provide novel relevant information on the extent of diversity of pollen morphology among Simaroubaceae taxa and thus contribute to the understanding of the group's taxonomy and recently published phylogeny. Pollen grains from the studied taxa are essentially isopolar, oblate-spheroidal to prolate, tricolporate, small to medium in size, with endoapertures lalongate, except for H. ferruginea, in which it is lolongate, and H. cedron and H. pohliana, in which it is circular. There is great variation in sexine sculpture, resulting in the identification of five pollen types. Most species were of the reticulate type. The pollen key presented herein can be used to identify the 36 taxa at the species level on the basis of morphological characters. The findings demonstrate that most pollen features of species belonging to Picramniaceae and Surianaceae differ from those of Simaroubaceae. For example, pollen grains are tricolpate in Picramnia and tricolporoidate in Alvaradoa arborescens; ornamentation is finely striate in Suriana maritima, striate in Picramnia, and nanogranulate in A. arborescens. Furthermore, two species of Simarouba (not described here) have finely granulate sexine. Species of Castela were described as reticulate in previous studies, in agreement with our findings.