Fucus Embryogenesis : A Model to Study the Establishment of Polarity Brad Goodner

It has been known for more than 100 years that when spherical zygotes of the brown alga Fucus are subjected to a gradient of light, the plane of the first cell division is always perpendicular to the light axis. As illustrated in Figure 1, the resulting cell plate divides the zygote into two unequal cells; the smaller rhizoid cell emerges from the shaded portion of the gradient, whereas the larger thallus cell is on the lighted side (Figures 1A to 1C). Each cell has a different organellar composition. Golgi and mitochondria are enriched in the rhizoid cell, whereas the thallus cell contains most of the chloroplasts (Brawley et al., 1977). Each cell of the two-celled embryo also has a different developmental fate. For the most part, cells derived from the initial rhizoid cell form the holdfast portion of the mature plant, whereas descendents of the initial thallus cell form the embryo proper and fronds (Figures 1D and 1E). Although the developmental fate of an isolated thallus or rhizoid cell has not been determined directly, and the exact cell lineage from each cell of the two-celled embryo has not been mapped precisely, it is clear that the polar axis imposed on the zygote by an external gradient establishes the polarity and division plane of the two-celled embryo, results in the differentiation of the first two cells of the embryo, and sets the developmental axis for the whole organism (for reviews, see Quatrano, 1978,1990; Harold, 1990; Jaffe, 1990; Kropf, 1992). The basic mechanisms involved in the generation of cell asymmetry in the zygote and the directed transport of specific cytoplasmic components to the resulting daughter cells are developmental phenomena of critical importance. These polar processes are common not only to algae but also to many different organisms, from flowering plants, yeasts, and nematodes to insects, amphibians, and mammals. In flowering plants, for example, polar asymmetric divisions play critical roles in the first zygote division (Mayer et al., 1993) as well as in the formation of pollen grains, guard cells, and epidermal hairs (Wardlaw, 1968). However, in most systems, how a cell transports cytoplasmic material to a specific site defined by an extracellular gradient and orients the division plane