Mechanisms of polar lobe formation in fertilized eggs of molluscs.

Polar lobe formation in the marine mudsnail, Ilyanassa obsoleta, involves formation of a microtubule-, microfilament-dependent furrow that constricts at two rates, then stops and relaxes. Some artificial seawater mixtures allow relatively normal development, facilitate insertion of microelectrode tips, and prevent artifactual bleb formation during such punctures. Membrane events may affect formation of polar lobe constrictions: (1) Brief treatment with digitonin prevents constrictions, but not cytokinesis per se, and the suppression of constrictions is permanent. Tomatine (but not tomatidine) and filipin act similarly, although filipin often also stops cytokinesis as well. (2) Responses to digitonin, tomatine, and filipin occur with little change in membrane potential. (3) Adhesion to substrata in response to brief treatment at low pH prevents both constrictions and cytokinesis. (4) Adhesion to substrata via polylysine allows both constrictions and cytokinesis, but embryos are smaller in volume and develop abnormally. Formation of lobe constrictions may be sensitive to perturbations of the plasma membrane.