Pollen morphology and its implication in the taxonomy of some selected tribes of the Asteraceae of Hainan Island South China

Pollen morphological traits are mostly used in resolving taxonomic problems at various taxonomic ranks to define species boundaries for long in the systematics and evolution. This study aims to provide new and useful information regarding the extent of pollen morphological diversity among the Asteraceae taxa and thus to contribute a better understanding of the taxonomy of studied species. These species belonged to three subfamilies Asteroideae having eight tribes (Senecioneae, Heliantheae, Milleriae, Eupatorieae, Coreopsideae, Inulaceae, Plucheeae, Tageteae, followed by Cichorioideae (Cichorieae) and the Vernonioideae (Vernonieae). Both light and scanning electron microscopy were used to investigate the ultrastructure of the exine. Principal component analysis (PCA) was performed to show the relationship among the Asteraceae taxa. The results showed two main types the echinolophate and echinate pollen. The echinate and echinolophate features were found to have a significant taxonomic value at the family level that delimits the Asteraceae from rest of the families. Besides, the ehinolophate feature was found to be the diagnostic trait of the tribe Vernonieae (Elephantopus tomentosus), Cichorieae (Youngia japonica) and echinate mixed with other morphological features in the inter-spinal area i.e. perforate, verrucate, scabrate, micro-reticulate and psilate were found in rest of the tribes. Aperture traits were found useful to differentiate between tribes. The maximum spines length was found in Bidens pilosa (5.43 μm) while maximum pollen size was observed in Elephantopus tomentosus (28.43 × 21.93 μm) followed by Tagetes erecta (28.36 × 19.51 μm). Furthermore, the size, spines length, density, arrangement and number of the spines at the apocolpium region were found useful taxonomically important to discriminate the species. The overlapped features showed interspecies relationships because to be in the same families while the interspecies variation in pollen traits validate their existence as distinct species. The PCA analysis showed that the spine length, polar axis, equatorial diameter, colpus length and width, P/E ratio and inter-spinal area were found useful pollen morphological traits. In conclusion, our results showed that pollen morphology has a significant taxonomic potential to identify and define species boundaries at the tribe and generic levels. Hence, the pollen features can be used as an additional tool for regrouping taxa within the family Asteraceae by using both light and scanning electron microscopic techniques.