Actin-driven chromosomal motility leads to symmetry breaking in mammalian meiotic oocytes.

Movement of meiosis I (MI) chromosomes from the oocyte centre to a subcortical location is the first step in the establishment of cortical polarity. This is required for two consecutive rounds of asymmetric meiotic cell divisions, which generate a mature egg and two polar bodies. Here we use live-cell imaging and genetic and pharmacological manipulations to determine the force-generating mechanism underlying this chromosome movement. Chromosomes were observed to move toward the cortex in a pulsatile manner along a meandering path. This movement is not propelled by myosin-II-driven cortical flow but is associated with a cloud of dynamic actin filaments trailing behind the chromosomes/spindle. Formation of these filaments depends on the actin nucleation activity of Fmn2, a formin-family protein that concentrates around chromosomes through its amino-terminal region. Symmetry breaking of the actin cloud relative to chromosomes, and net chromosome translocation toward the cortex require actin turnover.