We determined the distribution of F-actin in fucoid (Pelvetia, Fucus) embryos with nitrobenzoxadiazole-phallacidin, and studied the effect of cytochalasin upon the endogenous currents associated with cell polarization by using the vibrating probe. F-actin is not localized at the presumptive rhizoid immediately after experimental induction of the polar axis with a light gradient; however, a preferential distribution of F-actin develops at the presumptive rhizoid by the time the position of the polar axis is fixed. F-actin continues to be localized at the tip of the rhizoid after germination, except during cytokinesis, when the furrow is the only brightly staining region of the embryo. Incubation with cytochalasin can result in either an enhanced or a diminished pool of F-actin in the embryonic cortex (see Results). Cytochalasin D (100 micrograms/ml) significantly reduces the inward current at the rhizoid pole (n = 11) after a 2.5-h incubation. This drop is concentration dependent and occurs within approximately 30 min at 100 micrograms/ml and approximately 60 min at 10 micrograms/ml. Cytochalasin treatment eliminates the pulsatile component of the current. Preliminary results suggest that 100 micrograms/ml cytochalasin D prevents development of inward current at the presumptive rhizoid but does not completely delocalize this locus if added after photopolarization. We conclude that microfilaments are required for the establishment and maintenance of the pattern of endogenous currents observed during early embryogenesis. This suggests a new model for axis formation and fixation.

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