Actomyosin-driven mechanics of starfish oocytes*

Actomyosin networks underlie most force generation by eukaryotic cells. These networks are driven out of equilibrium in part by myosin which crosslinks and exerts forces on actin filaments. While myosin’s role in force generation is well studied, how actin structure and dynamics influence the active mechanics of the networks during force generation is not understood. Here, we address this issue using oocytes from the starfish Patiria miniate. During maturation, the oocytes undergo surface contraction waves driven by the actomyosin cortex. Using pharmacological inhibitions, which target actin polymerization dynamics, we find that cellular deformation during the contraction wave is not a monotonic function of cortical actin density and is peaked near the wild type. This is reminiscent of in vitro actomyosin networks, which have been shown to have maximal contractility at intermediate levels of network connectivity. To test if this is also the case in starfish oocytes, we probe the oocyte’s mechanical properties and how these change under targeted molecular perturbations.