Ionic requirements for establishment of an embryonic axis in Pelvetia zygotes

Previous reports have shown that fertilized Pelvetia fastigiata (J. Ag.) DeToni eggs generate a transcellular ionic current which correlates temporally and spatially with establishment of a rhizoid/thallus axis. In order to understand more fully the ionic controls of development, we have determined the ionic requirements for axis formation induced by unilateral light. Formation of the developmental axis was independent of the presence of individual ions in artificial seawater. This finding indicates that transcellular circulation of a particular ion is not obligatory for polarization, and it confirms earlier work showing that calcium circulation is not fundamental to axis establishment in Fucus zygotes. Polarization was not, however, completely independent of ionic conditions; zygotes were unable to form an axis in pure sucrose solutions. Single salts were added to sucrose to determine which ions were sufficient to permit polarization. Salts of impermeant monovalent cations and salts of divalent cations supported polarization weakly or not at all. By contrast, zygotes photopolarized well in KCl, and other alkali metals substituted for K(+) with varying effectiveness (Rb(+)>Na(+)> Cs(+)≫ Li(+)). The anion was unimportant; a variety of different potassium salts all supported polarization equally well. The mechanism by which KCl promotes polarization is not yet understood, but may involve transport through K(+) channels.