Pollen morphology of the genus Gossypium and its systematic implications

Background: Plants develop a variety of pollen morphological features during long-term evolution, which are controlled by genes and are not easily affected by the external natural environment. Therefore, pollen morphology has great significance in plant taxonomy, evolution and identification. However, there is no detailed study on the pollen morphology of the Gossypium genus although some cotton species have been investigated in the scattered reports. In this study, the pollen morphology of 23 cotton species was comprehensively examined using scanning electron microscopy to evaluate the pollen diversity of the genus and its taxonomic significance. Results The common characteristics of Gossypium pollen are spherical, radially symmetrical, echinate, panporate and operculate. The pollen diameter ranges from 62.43µm in G. harknessii to 103.41µm in G. barbadense , revealing that there are great variations in pollen size among cotton species. Moreover, the exine sculpture is echinate. The exine echini shape is mostly conical or sharply conical and occasionally rodlike. Echini density is found maximum in G. incanum (173) and minimum in G.gossypioides (54), meanwhile, echini length varies from 3.53 µm in G. herbaceum to 9.47 µm in G. barbadense . In addition, all cotton species are divided into three clusters based on cluster analysis, which provides new understanding of the evolution and phylogeny of the Gossypium genus. Conclusion Although the pollen characteristics alone are insufficient to reconstruct taxonomic and systematic relationships within the genus Gossypium , our findings can enrich our knowledge of sporopollen morphology and fill the phenological gap of these taxa and will contribute to future systematic and phylogenetic studies of the Gossypium genus.