LAUSR

Spiral cleavage is the ancestral developmental mode within the Spiralia. It covers a period of the early development characterized by asymmetric divisions with alternating division angles giving the embryo a spiral looking appearance. The position and fate of the cells are determined during the spiral cleavage thus it’s crucial for body plan formation. However, the mechanisms controlling spiral cleavage are poorly understood. To elucidate the molecular, cellular and physical mechanisms of spiral cleavage, we use the marine annelid Platynereis dumerilii as a model. We wish to unravel the role of the cytoskeleton during spiral cleavage focusing on the cytoplasmic and cortical contributions. We do this through mRNA injections of fluorescently labeled tubulin (EMTB-3XGFP) and histone (H2BmCherry) into the Platynereis zygote to label cytoplasmic elements. We imaged the live embryos with selective plane illumination microscopy (SPIM) in high-resolution, high-speed, and 3Dreconstructable manners. We processed the data with Fiji softwares (Preibisch et al., 2010, 2014, and 2015), and reconstructed this way early cell cleavages in Platynereis. Next, we extracted dynamic cellular events such as inclination of the mitotic spindles, transportation of the nuclei in the cells, and membrane deformation during cell division. Moreover, we monitored the cortical actomyosin dynamics through syn21-lifeact-mKate2 mRNA injection and imaging with spinning-disc confocal microscope. Actin is shown polarized to the micromeres prior to each division. And the cortical actomyosin chiral flow is observed in the macromeres during the spiral cleavage. Membrane deformation and actomyosin dynamics was not interfered with colchicine or nocodazole-mediated microtubule polymerization inhibition. However, actin polymerization inhibition with latrunculin A severely disrupted spindle. These suggest that the actomyosin play critical roles in both establishing cell polarity and controlling division pattern during spiral cleavages. This study provides mechanistic insights into the spiralian development and a base to compare their degree of conservation among spiralians.