Prior to cell cleavage, cytokinetic proteins are recruited into the nascent actomyosin contractile ring. Interactions between spindle microtubules and the cell cortex play a critical role in this recruitment. However, direct evidence for physical interaction between microtubules and the cortex has been lacking. Here we reveal the physical connection between astral microtubules and cortical actin filaments, by micromanipulating the fluorescently tagged cytoskeleton in living grasshopper spermatocytes.When microtubules were tugged with a microneedle, they in turn pulled on cortical actin filaments, interrupting the filaments’ journey toward the equator. Further displacement of the actin dragged the cell membrane inward, demonstrating that the cortical actin network physically linked spindle microtubules to the cell membrane. Regional disruption of the connection by breaking spindle microtubules prevented actin accumulation in a segment of the ring, which locally inhibited furrowing. Dynamic astral microtubules were shown to be able to move actin aggregates with their tips in Cytochalasin D-treated cells. These data support our model in which dynamic astral microtubules physically redistribute cortical actin into the incipient contractile ring.
During meiosis, homologous chromosomes pair, synapse, and commonly exchange genetic material, creating genetic variation among offspring. These meiotic specific events take place in prophase I nucleus to ensure the formation of paired homologous chromosomes. However, the source of the mechanical forces that drive chromosome pairing and crossing over remains unclear. Here we show the dynamic distribution of phosphorylated myosin II between the homologous chromosomes during meiosis in grasshopper spermatocytes. The phosphorylated myosin II was localized specifically to paired homologues that undergo crossing over. The myosin II remained phosphorylated during prophase and metaphase, then became abruptly dephosphorylated at anaphase onset, which coincided with separation of the homologues. Pharmacological dephosphorylation of myosin II in prophase dissembled SMC3 axis in pachytene cells and induced partial separation of homologues in diakinesis cells. These results suggest that the activated myosin II motors could provide forces for pairing and/or other meiotic processes of the homologues. Furthermore, inactivation of the motors could be required for dissolution of the chiasmata at the onset of anaphase I, allowing separation of the bivalents. I anticipate that our results will initiate a reassessment of how homologous chromosomes recombine and separate during meiosis, expanding upon the conventional perception of myosin II motors in cell contraction and motility. This basic research could provide a foundation for understanding and eventually treating a subset of medical conditions that arise from meiotic errors.