Nuclear migration during karyogamy in rice zygotes is mediated by continuous convergence of actin meshwork toward the egg nucleus
Fertilization is comprised of two sequential fusion processes; plasmogamy and karyogamy. Karyogamy completes with migration and fusion of the male and female nuclei in the fused cell. In animals, microtubules organized by the centrosome control female/male pronuclei migration. In contrast, the nuclear migration in fused gametes of angiosperms is controlled by actin filaments, but the mechanism that regulates actin filament-dependent nuclear migration is not clear. In this study, we prepared fused rice (Oryza sativa L.) gametes/zygotes using in vitro fertilization and observed the spatial and temporal movements of actin filaments and sperm nuclei. Our results show that actin filaments in egg cells form a meshwork structure surrounding the nuclei. Quantitative analysis of the actin meshwork dynamics suggests that actin meshwork converges toward the egg nucleus. In egg cells fused with sperm cells, actin filaments appeared to interact with a portion of the sperm nuclear membrane. The velocity of the actin filaments was positively correlated with the velocity of the sperm nucleus during karyogamy. These results suggest that sperm nuclear membrane and actin filaments physically interact with each other during karyogamy, and that the sperm nucleus migrates toward the egg nucleus through the convergence of the actin meshwork. Interestingly, actin filament velocity increased promptly after gamete fusion and was further elevated during nuclear fusion. In addition to the migration of gamete nuclei, convergence of actin meshwork may also be critical during early zygotic developments.
KeywordsFertilization Karyogamy Nuclear migration Actin meshwork Oryza sativa
We thank Ms. H. Maeda for help in preparing transformed plants, and Ms. N. Koiso, Ms. A. Matsuura and Ms. M. Shinada (Tokyo Metropolitan University) for help with data entry for quantitative analysis. Thank you to Dr. T. Igawa (Chiba University) for the DD45 promoter::H2B-TagRFP in pENTRTM3C and the DD45 promoter::Lifeact-TagRFP in pGWB1, Dr. K. Tamura (Kyoto University) for the GFP–SUN2 in pGWB405, and Dr. T. Nakagawa (Shimane University) for the pGWB1 and pGWB405 vectors. We also thank Dr. HS. Weiner and Mr. MH. Rahman for the critical reading of this manuscript. This work was supported by a research fellowship from the Japan Society for the Promotion of Science (15J05885 to Y.O.). This work was also partly supported by the Ministry of Education Culture Sports Science and Technology of Japan (26113715 to T.O.) and the Japan Society for the Promotion of Science (25650083 to T.O.).
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