Spatial organization of cortical actin alignments for the ooplasmic segregation of ascidianCionaeggs

ABSTRACT 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 the fertilization, and then to the future posterior side of the zygotes in later phase of the 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 ∼20s after the fertilization induced [Ca 2+ ] increase. Timing of the vegetal contraction was consistent with the emergence of highly aligned actin filaments at the cell cortex of vegetal hemisphere which ran perpendicular to the animal-vegetal axis. We propose that the first ooplasmic segregation is initiated by the local contraction of laterally aligned cortical actomyosin in the vegetal hemisphere, which in turn generates the convectional flow of cytoplasm within whole eggs. SUMMARY STATEMENT Locally distinct, transient emergence of cortical F-actin alignments were observed in live ascidian Ciona eggs during the first ooplasmic segregation by using fluorescence polarization microscopy.