The effects of cytochalasin B on the events of fertilization in the surf clam, Spisula solidissima. I. Polar body formation.

Abstract

AbstractInvestigations, employing the drug cytochalasin B (CB), have been carried out in an effort to determine the mechanisms underlying the processes involved in polar body formation and their possible relation to cytokinesis of mitosis. Fertilized eggs of the surf clam, Spisula were treated with 1 or 10 μg/ml CB and prepared for light and electron microscopy. Germinal vesicle breakdown and formation of the first meiotic spindle are similar in CB treated zygotes and in controls. At anaphase I a cytoplasmic protrusion develops at the site of the peripheral aster and contains dyad chromosomes. Although the inner aspect of the plasma membrane of this projection in CB treated zygotes is lined with a discontinuous layer of fine textured material, a cleavage furrow fails to form and the cytoplasmic bleb gradually regresses in size. The failure of the presumptive first polar body to cleave results in the presence of two groups of dyads within the zygote. These become aligned on separate or shared meiotic spindles in preparation for the second meiotic division. At anaphase II a protrusion, structurally similar to the first, is formed which also fails to separate from the zygote via a cleavage furrow. Due to the absence of polar body formation, four groups of maternally derived chromatin may be observed at the animal hemisphere of the zygote. These groups are organized into female pronuclei (usually 4) which may not be of equal ploidy. Failure of the cleavage furrow to develop during polar body formation in CB treated zygotes suggests that the filamentous material associated with the leading edge of the furrow of control specimens may function in a manner proposed for the contractile ring of mitotic cells.