Pollen morphology of Praxelis (Asteraceae, Eupatorieae, Praxelinae) in Brazil

Abstract

Studies on the pollen morphology of 11 species of Praxelis Cass. was conducted (Praxelinae, Eupatorieae-Asteraceae). Asteraceae is the second biggest family of the angiosperms the equivalent 25,000 species distributed around the world. Praxelis, genus predominantly South American, is the second biggest in number of the species of the subtribe Praxelinae. In Brazil, the species analyzed here is represented well in “cerrado” biome, less often in the other biomes. The botanic material was utilized by exsicatas deposited in Brazilian herbarium. The pollen was acetolyzed, measured, described and illustrated under light microscope. To observe details of aperture and surface, non-acetolyzed pollen grains were analyzed and photographed under scanning electron microscope, and transmission electron microscope. Characteristics like shape, size, ornamentation and aperture constitution were studied. The species were described as: small to medium, isopolar, oblate spheroidal to subprolate, 3(4)-colporate, ornamented margin, endoaperture with or without median constriction, presence of cavea, the sexine is spinulous and always greater than nexine. Hierarchical cluster analysis (HCA) was used to divide the main pollen taxa into three groups based on pollen morphology. Statistically distinctive taxa were identified using principal components analysis (PCA). Pollen analysis revealed that the main characteristics that defined the clusters were: the shape for cluster 1, the dimensions of the aperture for cluster 2 and the size of the pollen grains, the spines and the cavea to cluster 3. It is hoped that this study contributes to a better understanding of the taxonomy of the genus and provide data for a phylogeny future.