Actomyosin active torques determine body plan handedness in C. elegans

Left-right (LR) asymmetries are a defining feature of bilaterian body plans, but the mechanisms that convert molecular chirality into a conserved body-axis handedness remain unclear. In C. elegans, LR axis specification proceeds via a chiral spindle skew that establishes a handed cell-cell contact pattern (CCP) at the 6-cell stage; an arrangement that, when reversed, leads to situs inversus nematodes [1]. Chiral acto-myosin flows that arise from active chiral torques have been previously identified as the driver of this LR asymmetric spindle skew [2]. Here, we show that high amounts of Lifeact::mKate2, widely used to visualize F-actin, reverse the handedness of active torques in the cortex. We find that this results in reversed chiral acto-myosin flows during LR symmetry breaking, leading to mirrored 6-cell embryos and situs inversus nematodes. Our findings demonstrate that reversing the handedness of active torques changes the LR handedness at the organismal level and establish active chiral torques as instructive for LR axis specification in worms.