Pollen Diversity in the Tribe Cardueae (Asteraceae) and Its Taxonomic Significance

The Cardueae with about 74 genera and 2500 species is one of the largest tribes of the family Asteraceae. The taxonomy of the Cardueae is complicated and unresolved, as it contains the largest and most diverse genera. The main distribution centers of the Cardueae are in the eastern and western Mediterranean, the western Irano-Turanian region, central Asia, and North Africa. Using pollen morphology can be a useful tool for the identification and delimitation of Cardueae taxa. Therefore, the current study aims to describe the pollen types of tribe Cardueae based on pollen features. Furthermore, the study addresses the relationships among the genera and species based on palynological features as well as species delimitation based on pollen morphology. Pollen morphology of 39 species of Cardueae distributed in 28 genera was examined by scanning electron microscopy. In the present study, statistical analyses and various clustering and ordination methods were provided for palynological data. Based on these results, the members of nine subtribes of Cardueae showed overlap in palynological data. This extreme heterogeneity of relationships among the taxa of tribe Cardueae could be attributed to the large number of taxa and their high morphological diversity. However, suitable palynological features were distinguished for taxonomic differentiation among the species of Cardueae, including pollen shape, size, polar axis to equatorial diameter ratio [P/E], exine sculpture, and spine size. Consequently, six pollen types were identified in the studied taxa of Cardueae.