Surface Control of Cell Polarity, the Plane of Cell Division, and Cell Fate in Fucus Embryos

Abstract

When spherical zygotes of the brown alga Fucus are subjected to a gradient of light during the first several hours after fertilization, a bud or rhizoid is formed on the shaded side of the zygote at 14 hours. This polar growth is followed by an asymmetric cell division at 24 hours, with the cell plate always oriented perpendicular to the light axis. The resulting embryo is comprised of two unequal cells; the smaller, more elongated rhizoid cell emerges from the shaded region of the embryo, while the larger, more rounded thallus cell is on the lighted side. These cells differ not only in shape but in organellar content and molecular composition, both in the cytoplasm and cell wall. It is clear that within the first 24 hours after fertilization, external gradients imposed on the zygote can establish a polarity which results in the differentiation of the first two cells of the embryo and sets the developmental axis for the whole organism (Fowler 0026 Quatrano 1995; Quatrano & Shaw 1997).