Ultrastructural study of microspore and male gametophyte development in Senecio cannabifolius Less

The genus Senecio, which is a basal angiosperm group, holds significant importance for evolutionary and phylogenetic research. It is notable for possessing male meiotic characteristics that are rarely observed in most angiosperms. However, the current understanding about male meiosis in Senecio remains incomplete. Here, we traced the cell morphological characteristics of microsporogenesis, and male gametophyte development of Senecio cannabifolius was observed by transmission electron microscopy. The results showed that the microspore mother cells were surrounded by callose; the cytokinesis was simultaneous; the tetrad was tetrahedral; and the mature pollen was two-celled pollen with three germination pores. During the meiosis of microspore mother cells, there were abundant organelles in the cytoplasm at leptotene stage. Obvious and clear synaptonemal complex was found in pachytene stage. In the prophase I stage, the number of organelles in the cytoplasm decreased; the cristae of mitochondria decreased; and the electron density of plastids was low. It began to recover at the metaphase I and formed an obvious organelle band in the anaphase I, which separated the two daughter nuclei and contained abundant organelles. During this period, the organelles in the cytoplasm changed regularly, which was the phenomenon of cytoplasmic reorganization. The pollen wall was formed at the late uninucleate stage; the pollen wall was mature at the binucleate microspore stage; and the pollen has abortion phenomenon. These results for the first time revealed the ultrastructure of microspores and male gametophytes during the development of S. cannabifolius and enriched the understanding of the formation of pollen grains by microspores in Compositae plants.