Myosin and actin are necessary for polar lobe formation and resorption in Ilyanassa obsoleta embryos.

During the first mitotic divisions many spiralian embryos form a cytoplasmic protrusion at the vegetal pole called the polar lobe. In the gastropod Ilyanassa obsoleta the polar lobe is constricted by a contractile ring composed of filamentous actin, myosin, and associated proteins, similar to the contractile ring of the cleavage furrow. To resolve the role of myosin and actin in polar lobe formation and resorption, we have applied 2,3-butanedione monoxime and Latrunculin B at different stages of the first cleavage to inhibit myosin and F-actin, respectively. Our results show that myosin is important for both cytokinesis and polar lobe formation. Additionally, we have found that the resorption of the polar lobe is a two-step process: the first step is passive, driven by the tension of the actin-cortex and the second step is active, in which the ATP-hydrolysis of myosin/actin interaction supplies the force to complete the resorption of the polar lobe. We have summarized our results in a scheme of the first cleavage of Ilyanassa obsoleta.