Title: Axis Convergence in C. elegans embryos

Speaker: Dr. Archit Bhatnagar, Institute of Molecular Genetics of the Czech Academy of Sciences, Prague

Abstract: Body axes established during embryonic development are often observed to align with specific geometric features of the embryo or its environment. For instance, in fruit flies and worms, the anteroposterior (AP) body axis consistently aligns with the geometric long axis of the encompassing eggshell. However, the mechanisms ensuring the convergence of the AP axis with the eggshell's long axis remain elusive. We investigate this issue in the context of early C. elegans development, where the nascent AP axis actively realigns to converge with the embryo's long axis when misaligned. Our investigation reveals that active mechanical flows in the actomyosin cortex are required for this axis convergence. We identify two physical mechanisms through which cortical flows could drive axis convergence: First, bulk cytoplasmic flows, driven by cortical flows, could directly reposition the AP axis. Second, active forces originating within the pseudocleavage furrow—a transient structure within the actomyosin cortex resembling a contractile ring—can induce a mechanical reorientation of the furrow itself, aligning the furrow perpendicular to the egg's long axis; which in turn ensures AP axis convergence. Combining numerical simulations with experimental interventions—such as removal of the pseudocleavage furrow and changing embryo shape—we demonstrate that the pseudocleavage furrow-dependent mechanism is the major driver of axis convergence with cytoplasmic flows-dependent mechanism playing a minor role. In essence, our findings underscore that active force generation within the actomyosin cortical layer drives axis convergence during the early stages of nematode development.