The morphological diversity of pollen in the genus Gossypium

Background Plant pollen has diverse morphological characteristics that can be consistently passed down from generation to generation. Information on pollen morphology is thus immensely important for plant classification and identification. In the genus Gossypium , however, in-depth research on pollen morphology is lacking, with only few reports on limited cotton species. To evaluate the diversity of pollen in Gossypium , we therefore conducted a comprehensive analysis of the pollen morphology of 33 cotton species and varieties using scanning electron microscopy. Results The 33 analyzed cotton samples exhibited common pollen morphological features, including spherical shapes, radial symmetry, echination, panporation, and operculation, while the pollen size, spine shape, spine density and length showed distinctive features. Pollen size varied significantly among species, with diameters ranging from 62.43 μm in G. harknessii to 103.41 μm in G. barbadense . The exine had an echinate sculptural texture, and spines were mostly conical or sharply conical but occasionally rod-like. Spine density varied from 173 in G. incanum to 54 in G. gossypioides , while spine length ranged from 3.53 μm in G. herbaceum to 9.47 μm in G. barbadense . In addition, the 33 cotton species and varieties were grouped at a genetic distance of 3.83 into three clusters. Cluster I comprised five allotetraploid AD-genome cotton species, four D-genome species, and one K-genome species. Cluster II included 13 diploid species from A, B, D, E, and G genomes, whereas Cluster III only consisted one E-genome species G. incanum . Conclusions Although pollen characteristics alone are not enough to resolve taxonomic and systematic relationships within the genus Gossypium , our results add to knowledge on palynomorphology and contribute to phenological information on these taxa. Our findings should aid future systematic and phylogenetic studies of the Gossypium genus.