Rs. Pu et al., Cortical actin filaments form rapidly during photopolarization and are required for the development of calcium gradients in Pelvetia compressa zygotes, DEVELOP BIO, 222(2), 2000, pp. 440-449
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.