Polar lobe formation and cytokinesis in fertilized eggs of Ilyanassa obsoleta: I. Ultrastructure and effects of cytochalasin B and colchicine

Abstract

Formation of polar lobe constrictions and cleavage furrows in fertilized eggs of the marine mudsnail, Ilyanassa obsoleta, is associated with localized concentrations of microfilaments in the cortical cytoplasm. These microfilaments disappear after treatment with cytochalasin B, with concomitant regression of polar lobe constrictions and cleavage furrows. Microtubules are present in the noncortical cytoplasm of the polar lobe and disappear after treatment with colchicine. Colchicine application early in lobe development inhibits both polar lobe formation and cytokinesis; however, drug treatment later in lobe development, although still inhibiting cytokinesis, fails to inhibit the continued constriction of the polar lobe neck and subsequent lobe resorption. The data therefore suggest that separate colchicine-sensitive steps are required for the complete constriction of the polar lobe neck and for initiation of cytokinesis. Polar lobe necks constrict at two markedly different rates in contrast to the smooth, single-phase progression of cleavage furrows. As with cytokinesis, polar lobe formation may occur by the contraction of a microfilament ring whose polymerization or activity is regulated by microtubules.