[
International Worm Meeting,
2021]
Establishment of the major body axes is essential for embryonic development. The C. elegans dorsoventral axis is established during the two-cell stage, and its orientation is defined by the division of the anterior AB blastomere. Earlier work in various holoblastic animals has shown that embryo geometry strongly affects early embryonic division patterns. However, whether and how embryo geometry affects the orientation of the dividing AB cell, and thereby the dorsoventral axis is unknown. To study this, we performed quantitative 3D live imaging of the cytoskeletal machinery in dividing AB blastomeres of embryos that are compressed perpendicular to the anteroposterior axis. In agreement with previous studies, we find that the AB cell division, and therefore the future dorsoventral axis, aligns parallel to the long cellular axis (which is perpendicular to the compression direction). Moreover, we show that, when viewed down the anteroposterior axis, the mitotic spindle of the AB blastomere is initially oriented randomly with respect to the long axis. During spindle elongation, it undergoes a rapid, large scale rotation to finally align parallel with the long axis. This mitotic spindle rotation is accompanied by a rotation of the cytokinetic furrow, ultimately resulting in the cell division axis to be aligned with the long cellular axis. Although the molecular mechanisms are still topic of debate, the mitotic spindle in many contexts is known to find the long axis of the cell. However, by performing conditional genetic perturbations, we find that force generation in the actomyosin layer is required and governs the kinetics of alignment of the mitotic spindle along the future DV axis. We speculate that the cooperative action of spindle and cortex promotes timely alignment of the cytokinetic machinery in fast-dividing cells.