Cortical Actin Filaments Form Rapidly during Photopolarization and Are Required for the Development of Calcium Gradients in Pelvetia compressa Zygotes

Previous research has shown that cortical gradients of cytosolic Ca2+ are formed during the photopolarization of Pelvetia compressa zygotes, with elevated Ca2+ on the shaded hemisphere that will become the site of rhizoid germination. We report here that the marine sponge toxin, latrunculin B, which blocks photopolarization at nanomolar concentrations, inhibited the formation of the light-driven Ca2+ gradients. Using low concentrations of microinjected fluorescent phalloidin as a tracer for actin filaments, we found that exposure to light induced a striking increase in actin filaments in the cells as indicated by an increase in fluorescence. The increase was quantified in the cortex, where it was most apparent, and the fluorescence there was found to increase by about a factor of 3. This increase in cortical phalloidin fluorescence was inhibited by latrunculin B at the same concentration required to inhibit Ca2+ gradient formation and photopolarization. The distribution of the increasing phalloidin fluorescence was uniform with respect to the developing rhizoid–thallus axis during the formation of the axis, and no intense patches of fluorescence were observed. After germination, fluorescence suggestive of an apical ring of actin filaments was seen near the rhizoid tip. Finally, inhibitor studies indicated that myosin may be involved in the photopolarization process.