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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.
It has been suggested that 60–70% of flowering plants have polyploid ancestry1-3 and that polyploidy has played a major role in the long-term diversification and evolutionary success of plants.4,5 There are two distinct types of polyploids: autopolyploids, which arise within a population of an individual species; and allopolyploids, which are the product of interspecific hybridization.6,7 In the formation of autopolyploid plants, triploid plants are considered as the intermediate stage in the formation of stable tetraploid plants, and this pathway of tetraploid formation is known as the triploid bridge.5,7 Triploid plants spontaneously emerge in natural populations and in cultivated plants,8 and the frequency of triploids was approximately 0.1% in approximately 55,000 field-grown tomato plants9 and 0.01–0.29% in 22 varieties of barley.10
As for the mechanism of triploid formation among diploid populations, it is generally accepted that a reduced gamete with one set of chromosomes fuses with an unreduced gamete with the somatic number of chromosomes, producing a triploid zygote.11 In addition to fusion of a reduced and an unreduced gamete, the possibility of polyspermy has been proposed for maize, wheat12,13 and some orchids14. However, polyspermy has been regarded as an uncommon mechanism for polyploid formation,7,15 probably because it is difficult to provide evidence for this phenomenon at the cellular level. That is, it is difficult to reproduce the polyspermy situation in zygotes and to analyze the developmental profiles of polyspermic zygotes.
Fertilization is a general feature of eukaryotic uni- and multicellular organisms for restoring a diploid genome from female and male gamete haploid genomes. However, in many animals and fucoid algae, polyspermy occasionally occurs and multiple microtubule-organizing centers (MTOCs) are generated by polyspermy-derived extra centrioles, because their eggs are generally infiltrated by more than one sperm. The multiple MTOCs in such zygotes cause aberrant nuclear and cell division, resulting in zygotic or early embryonic lethality.16-19 To ensure production of diploid zygotes, animals have developed two kinds of responses. One is the blocking of fusion of additional sperms, which operates in monospermic zygotes at the step of membrane fusion, and the other is selective use of one sperm nucleus from several sperm nuclei in polyspermic zygotes, which is known as event of physiological polyspermy.20 As for polyspermy block of monospermic zygotes, a positive shift in the egg membrane induced by sperm fusion functions as a fast polyspermy block, and Ca2+-dependent formation of the fertilization envelope in fertilized eggs, a process mediated by exocytosis of cortical granules, is considered to be slow polyspermy block.21,22
In angiosperms, upon double fertilization, one sperm cell from the pollen grain fuses with the egg cell, and the other sperm cell fuses with the central cell. The egg and central cells fused with each sperm cell develop into an embryo, which transmits genetic material from the parents to the next generation, and into an endosperm, which nourishes the developing embryo/seedling, respectively.23-26 It has been indicated that polyspermy block functions in the egg cell27-30 and the central cell30 to promote faithful double fertilization31. In addition, Ca2+-dependent egg activation occurs in plants as well as animals,32-34 and it has been postulated that the increased Ca2+ level triggers the polyspermy block of fertilized egg cells via Ca2+-dependent exocytosis of cell wall materials.28,33,35 In the case of maize and rice zygotes that were produced by electro-fusion of isolated gametes, deposition of cell wall materials on the whole surface of the zygote is generally observed 20 min after egg–sperm fusion.28,36 The possible Ca2+-dependent polyspermy block in plant zygotes, such as cell wall deposition, might be equivalent to slow or permanent polyspermy blocks in fertilized animal eggs. In angiosperms, further studies will be required to clarify the details of the fast polyspermic block such as a positive shift in the egg membrane.
The egg:sperm ratio in angiosperms is generally considered as 1:1, because one pollen tube containing two sperm cells reaches the embryo sac harboring two female gametes, an egg cell and a central cell.25 However, it has been reported that additional pollen tubes can invade the embryo sac37-39 when the sperm cell–egg cell fusion and/or the sperm cell–central cell fusion does not progress successfully40,41. This suggests that the egg:sperm ratio can vary, and that penetration of extra pollen tubes into the embryo sac provides the opportunity for multiple sperm cells to fuse with an egg cell. In addition, the possibility that polyspermic zygotes arise via multiple fusions has been reported.7,12,13 Interestingly, regarding the developmental fate of the polyspermic zygote, it has been considered that polyspermic plant zygotes formed in angiosperms do not die because plant cells do not use centrosomes as the spindle pore for chromosome separation during nuclear division.42 However, experimental evidence for the developmental fate of polyspermic zygotes in angiosperms has not been collected, because it is extremely difficult to identify and prepare polyspermic zygotes for analysis.
After isolation of gametes from flowers of rice plants (Oryza sativa L. cv. Nipponbare), egg cell was fused with a sperm cell, and the resulting fused egg cell (zygote) was further electro-fused with a second sperm cell within 10 min after the first fusion (Fig. 1A and B). Sperm cells expressing histone H2B-GFP under control of the ubiquitin promoter were used, because the chromatin in the sperm cells and subsequent zygotes/embryos was fluorescently labeled to observe precisely the karyogamy progression and developmental fate of the polyspermic zygote after electric fusion.36,43,44
When the progression of karyogamy in polyspermic zygotes was observed in detail, two karyogamy pathways were detected. First, one of the two sperm nuclei first fused with the egg nucleus, resulting in decondensation of sperm chromatin in the fused nucleus (Fig. 1C). Thereafter, the other sperm nucleus fused with the nucleus to form a triploid zygotic nucleus. Second, the two sperm nuclei in the polyspermic zygote came into contact and fused together (Fig. 1D), and then the united sperm nuclei further fused with the egg nucleus, resulting in a triploid nucleus. In both cases, karyogamy was completed within 4 h,36 and the time-course for karyogamy in these polyspermic zygotes was equivalent to that in diploid zygotes.44
In addition to fast and slow polyspermy blocks, which operate at the plasmogamy step, the polyspermy barrier at the karyogamy step has been demonstrated in polyspermic zygotes of some animal taxa, in which only one sperm nucleus fuses with the egg nucleus, and the remaining extra sperm nuclei degenerate or disappear.22,45,46 Interestingly, in some gymnosperms such as Pinus nigra and Picea glauca, two sperms enter the egg, but only one sperm nucleus migrates toward the egg nucleus and fuses with it.47-49 These indicate that selective karyogamy producing a diploid zygote operates as a polyspermy barrier in some animals and plants. In angiosperm zygotes, however, such selective karyogamy to promote diploid progeny would not occur, because two sperm nuclei fuse with an egg nucleus to form a triploid zygote without degradation or rejection of excess sperm nuclei (Fig. 1C and D).
After karyogamy in polyspermic zygotes, chromosomes were formed and then arranged at the equator with a microtubule spindle at metaphase (Fig. 2A).36 In anaphase zygotes, the chromosomes were evenly separated toward each pole via possible action of the microtubule spindle (Fig. 2B), and phragmoplasts were formed at telophase in dividing polyspermic zygotes (Fig. 2C).36 These observations suggested that the chromosomes in triploid zygotes were evenly divided into two daughter nuclei via the bi-polar microtubule spindle, and that polyspermy did not affect the mitotic profile in angiosperm zygotes. This is consistent with the expectation that polyspermic plant zygotes lacking centrosomes would divide normally, in contrast to those in animals and fucoid algae.31,42
The divided polyspermic zygotes further developed into a globular embryo-like structure, cell mass and white callus (Fig. 3A–C).36 From the white callus, multiple shoots regenerated (Fig. 3D) and plantlets were obtained (Fig. 3E). The speed of growth from the polyspermic zygote to plantlet was almost the same as that of a diploid zygote produced by the fusion of female and male gametes.36,50 The plantlets grew into mature plants (Fig. 3F). Their flowers were larger than those of diploid plants and formed well-developed awns (Fig. 3G). Although the plants flowered, mature seeds hardly formed on these plants. Being consistent with their sterile phenotype and floral characteristics of triploid rice plants (Fig. 3G),51 the ploidy level of the mature plants was determined to be 3C by flow-cytometry analysis (Fig. 3H).36 These suggest that the ploidy level of triploid in polyspermic zygotes, which is derived from an egg nucleus and two sperm nuclei, was conserved during the development and regeneration of the polyspermic triploid zygote.
The results of this study clearly showed that a zygote prepared from one egg cell and two sperm cells has the potential to divide and regenerate into a mature triploid plant. This suggests that polyspermic zygotes in the embryo sac can develop into mature triploid embryos if the balance of male to female genomes in the endosperm is within a permissible range.52 Polyspermy in angiosperms might be a pathway for the formation of triploid plants, which can contribute significantly to the formation of autopolyploids.
No potential conflicts of interest were disclosed.
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.).