Dynamic organization of cortical actin filaments during the ooplasmic segregation of ascidian Ciona eggs

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ishii, H., & Tani, T. Dynamic organization of cortical actin filaments during the ooplasmic segregation of ascidian Ciona eggs. Molecular Biology of the Cell, 32(3), (2021): 274-288, https://doi.org/10.1091/mbc.E20-01-0083.Spatial reorganization of cytoplasm in zygotic cells is critically important for establishing the body plans of many animal species. In ascidian zygotes, maternal determinants (mRNAs) are first transported to the vegetal pole a few minutes after fertilization and then to the future posterior side of the zygotes in a later phase of cytoplasmic reorganization, before the first cell division. Here, by using a novel fluorescence polarization microscope that reports the position and the orientation of fluorescently labeled proteins in living cells, we mapped the local alignments and the time-dependent changes of cortical actin networks in Ciona eggs. The initial cytoplasmic reorganization started with the contraction of vegetal hemisphere approximately 20 s after the fertilization-induced [Ca2+] increase. Timing of the vegetal contraction was consistent with the emergence of highly aligned actin filaments at the cell cortex of the vegetal hemisphere, which ran perpendicular to the animal–vegetal axis. We propose that the cytoplasmic reorganization is initiated by the local contraction of laterally aligned cortical actomyosin in the vegetal hemisphere, which in turn generates the directional movement of cytoplasm within the whole egg.We deeply thank Shalin Mehta for use of his MATLAB codes for our fluorescence polarization analyses and Mark Terasaki for reading our manuscript and giving us helpful feedback. We thank William Jeffery for sharing information about local C. intestinalis around the Marine Biological Laboratory. We also thank Takahito Nishikata and Takehiro G. Kusakabe for their generous support and kind suggestions for our experiments. Our research was supported by National Institutes of Health grant R01 GM100160, Japan Society for the Promotion of Science KAKENHI grant JP18K19962 to T.T., institutional funds of the Marine Biological Laboratory to T.T. and H.I., and a TOYOBO Biotechnology Foundation long-term fellowship to H.I

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