Formation of triploid plants via possible polyspermy

Polyploidization is a common phenomenon in angiosperms, and polyploidy has played a major role in the long-term diversification and evolutionary success of plants. Triploid plants are considered as the intermediate stage in the formation of stable autotetraploid plants, and this pathway of tetraploid formation is known as the triploid bridge. As for the mechanism of triploid formation among diploid populations, fusion of an unreduced gamete with a reduced gamete is generally accepted. In addition, the possibility of polyspermy has been proposed for maize, wheat and some orchids, although it has been regarded as an uncommon mechanism of polyploid formation. One of the reasons why polyspermy is regarded as uncommon is because it is difficult to reproduce the polyspermy situation in zygotes and to analyze the developmental profiles of polyspermic zygotes. In the study, we produced polyspermic rice zygotes by electric fusion of an egg cell with two sperm cells and monitored their developmental profiles. The two sperm nuclei and the egg nucleus fused into a zygotic nucleus in the polyspermic zygote, and the triploid zygote divided into a two-celled embryo via mitotic division with a typical bipolar microtubule spindle. The two-celled proembryos developed and regenerated into triploid plants. These results suggest that polyspermic plant zygotes have the potential to form triploid embryos, and that polyspermy in angiosperms might be a pathway for the formation of triploid plants.