Polar lobe formation and cytokinesis in fertilized eggs of Ilyanassa obsoleta: II. Large bleb formation caused by high concentrations of exogenous calcium ions

Abstract

Fertilized eggs of the mollusk Ilyanassa obsoleta ( Nassarius obsoletus) form large blebs resembling polar lobes within 12 min of exposure to solutions of isotonic CaCl 2, whereas control eggs in sea water remain spherical. Under identical conditions, fertilized eggs of the sea urchin, Strongylocentrotus purpuratus, an organism which normally does not form polar lobes, do not form blebs upon exposure to solutions of isotonic CaCl 2. The calcium-induced blebbing in Ilyanassa still occurs if other cations such as Na +, Mg 2+, or Mn 2+ are present in addition to Ca 2+, but not if comparable concentrations of K + are present. Cytochalasin B prevents the calcium-induced blebbing, whereas colchicine does not. Cytokinesis in both Ilyanassa and Strongylocentrotus and normal polar lobe formation in Ilyanassa appear to require exogenous K + but not exogenous Ca 2+. Preliminary electron microscopy of Ilyanassa eggs exposed to isotonic solutions of CaCl 2 has shown microfilaments in the cortical cytoplasm in the region of the bleb constriction but no microfilaments in spherical control eggs in sea water. These data suggest that high concentrations of exogenous Ca 2+ trigger the polymerization and contraction of a ring of microfilaments in the cortical cytoplasm of the Ilyanassa egg which results in the formation of a lobelike bleb of cytoplasm. The observation that K + antagonizes this Ca 2+-induced blebbing has led to the formulation of a theory which postulates that the ratio of intracellular Ca 2+ to intracellular K + is critical in the control of polar lobe formation and cytokinesis.