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 f ormed during the photopolarization of Pelvetia compressa zygotes, with elev ated Ca2+ on the shaded hemisphere that will become the site of rhizoid ger mination. We report here that the marine sponge toxin, latrunculin B, which blocks photopolarization at nanomolar concentrations, inhibited the format ion of the light-driven Ca2+ gradients. Using low concentrations of microin jected fluorescent phalloidin as a tracer for actin filaments, we found tha t exposure to light induced a striking increase in actin filaments in the c ells as indicated by an increase in fluorescence, The increase was quantifi ed in the cortex, where it was most apparent, and the fluorescence there wa s found to increase by about a factor of 3, This increase in cortical phall oidin fluorescence was inhibited by latrunculin B at the same concentration required to inhibit Ca2+ gradient formation and photopolarization. The dis tribution of the increasing phalloidin fluorescence was uniform with respec t to the developing rhizoid-thallus axis during the formation of the axis, and no intense patches of fluorescence were observed. After germination, fl uorescence suggestive of an apical ring of actin filaments was seen near th e rhizoid tip. Finally, inhibitor studies indicated that myosin may be invo lved in the photopolarization process. (C) 2000 Academic Press.