Okamoto Takashi, Ohnishi Yukinosuke, Toda Erika
Department of Biological Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo, 192-0397, Japan.
Plant Breeding Innovation Laboratory, RIKEN Innovation Center, 1-7-22 Suehiro-cho, Tsurumi, Yokohama, 230-0045, Japan.
J Plant Res. 2017 May;130(3):485-490. doi: 10.1007/s10265-017-0913-9. Epub 2017 Mar 8.
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.
受精是真核单细胞和多细胞生物的一个普遍特征，其目的是从雌、雄配子的单倍体基因组恢复二倍体基因组。在被子植物中，多倍体化是一种常见现象，多倍体在植物的长期多样化和进化成功中发挥了重要作用。关于同源四倍体植物的形成机制，三倍体桥途径，即三倍体与二倍体植物杂交，被认为是主要途径。对于三倍体植物的出现，未减数配子与减数配子的融合是普遍接受的。此外，虽然多精受精被认为是三倍体形成的一种不常见机制，但已有人提出玉米、小麦和一些兰花存在多精受精的可能性。多精受精被视为不常见的原因之一是难以在合子中重现多精受精情况并分析多精三倍体合子的发育过程。最近，通过将卵细胞与两个精子细胞进行电融合成功产生了多精水稻合子，并监测了它们的发育过程。在多精合子中，两个精核和一个卵核融合形成合子核，三倍体合子通过典型的双极微管纺锤体进行有丝分裂分裂成二细胞胚。二细胞原胚进一步发育并再生为三倍体植物。这些结果表明多精植物合子有形成三倍体胚的潜力，被子植物中的多精受精可能是三倍体植物形成的一条途径。
