Development of polyspermic zygote and possible contribution of polyspermy to polyploid formation in angiosperms
Fertilization is a general feature of eukaryotic uni- and multicellular organisms to restore a diploid genome from female and male gamete haploid genomes. In angiosperms, polyploidization is a common phenomenon, and polyploidy would have played a major role in the long-term diversification and evolutionary success of plants. As for the mechanism of formation of autotetraploid plants, the triploid-bridge pathway, crossing between triploid and diploid plants, is considered as a major pathway. For the emergence of triploid plants, 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 triploid 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 triploid zygotes. Recently, polyspermic rice zygotes were successfully produced by electric fusion of an egg cell with two sperm cells, and their developmental profiles were monitored. Two sperm nuclei and an 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 further developed and regenerated into triploid plants. These 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.
KeywordsKaryogamy Fertilization Polyploid Polyspermy Triploid Zygote
We thank Ms. T. Mochizuki (Tokyo Metropolitan University) for isolating rice gametes. This work was supported in part by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grants-in-Aid no. 26113715 to T.O.) and the Japan Society for the Promotion of Science (Grant-in-Aid no. 16K14742 to T.O.).
- Grant V (1981) Plant Speciation, Ed 2. Columbia University, New YorkGoogle Scholar
- Guignard ML (1899) Sur les antherozoides et la double copulation sexuelle chez les vegetaux angiosperms. Rev Gén de Bot 11:129–135Google Scholar
- Hagerup O (1947) The spontaneous formation of haploid, polyploid, and aneuploid embryos in some orchids. Kongel Dan Vidensk Selsk Biol Medd 20:1–22Google Scholar
- Hu CH, Ho KM (1963) Karyological studies of triploid rice plants. Bot Bull Acad Sin 4:30–36Google Scholar
- Nawaschin S (1898) Revision der Befruchtungsvorgange bei Lilium martagon und Fritillaria tenella. Bull Sci Acad Imp Sci Saint Pétersbourg 9:377–382Google Scholar