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Hydrostatic pressure treatment during the first mitosis does not suppress the first cleavage but the second one |
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| Authors: | Xuelian Zhang and Hiroshi Onozato |
| Institution: | Department of Biology, Faculty of Science, Shinshu University, Matsumoto 390-8621, Japan |
| Abstract: | Suppression of cell division causes chromosome set doubling. Some chemical agents or physical shocks such as temperature or hydrostatic pressure are effective tools for suppression of cell division. As spindles are obviously inactivated or disorganized by these treatments, it has been supposed that inactivation or disassembly of spindles blocks the anaphase movement of chromosomes and a duplicated nucleus is formed without cell division. The present study demonstrated that hydrostatic pressure treatment (650 kg/cm2 for 6 min) around the time of metaphase of the first cell cycle of the rainbow trout embryos did not suppress the first cleavage but the second one. Spindles disassembled by the hydrostatic pressure or heat shock regenerated soon after treatment, resulting in the occurrence of the first mitosis. Interestingly, a monopolar spindle was assembled in each blastomere in the second cell cycle, and disjunction of duplicated chromosomes and the cleavage was prevented, leading to the chromosome set doubling. From the third cell cycle, normal cell division resumed. No significant difference was found between the area of the nucleus plate of the treated embryos and twice the area of the nucleus plate of control embryos in the third cell cycle, meaning that the chromosome sets had been doubled at the end of the second cell cycle. The process of chromosome set doubling caused by heat shock seemed to be fundamentally the same as that caused by hydrostatic pressure. To the best of our knowledge, this is the first time that the mechanism of chromosome set doubling in animal eggs treated with hydrostatic-pressure or heat shock has been clarified. Haploid–diploid or diploid–tetraploid mosaics sometimes occur among individuals treated for cleavage inhibition. The mechanism of such occurrence of mosaicism is, however, not clear. In this study, we found interesting two-cell stage embryos, which had a monopolar spindle in one blastomere and a bipolar spindle in the other during the second mitosis in a batch subjected to tetraploidization treatment. These embryos have a high potential of developing diploid–tetraploid mosaics. This paper also discusses the mechanism of occurrence of these aberrant embryos and discusses their relationship to diploid–tetraploid mosaicism. |
| Keywords: | Cleavage suppression Hydrostatic pressure Heat shock Monopolar spindle Chromosome set doubling Diploid–tetraploid mosaicism |
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