Induction of Polarity in Fucoid Zygotes.

Early events 'in the vegetative development of higher land plants are difficult to investigate because the young embryo is encased in the ovarian tissue of the previous gametophytic and sporophytic generations. Recently, genetic approaches have succeeded in identifying mutations that alter early development (Jürgens, 1995; see also Laux and Jürgens, 1997, in this issue), but cellular and physiological analyses of higher plant embryogenesis remain tedious. Brown algae of the family Fucales (comprising the genera Fucus and Pelvetia) are, however, very tractable for investigating early plant embryogenesis, especially the establishment of developmental polarity. These marine organisms grow attached to rocks in the intertidal zone and are typically 0.25 to 0.5 m in height at maturity. Zygotes, eggs, and sperm are shed from fronds of mature plants in the laboratory and can easily be harvested in gram quantities, free of other tissues. Young zygotes attach firmly to the substratum, and populations of adhering zygotes transit synchronously through early development. In the Fucales, mature eggs are radially symmetric, and cell polarization does not occur until after fertilization. The primary developmental axis is organized early in the first cell cycle and defines the growth axis of the young embryo. Unlike higher plants, in which positional cues are often conveyed by neighboring cells, fucoid zygotes develop autonomously and therefore rely on environmental signals for spatial information to orient their nascent axes. In the laboratory, an entire population can be induced to establish axes in parallel by the application of externa1 vectors such as unidirectional light (Jaffe, 1968). A few hours after polarization, localized growth occurs at one pole of the nascent axis, as shown in Figure 1. The first zygotic division is an invariant, asymmetric division, with the cell plate oriented transverse to the growth axis. The smaller rhizoid cell contains the growing apex and is the precursor to the holdfast of the alga; the larger thallus cell gives rise to the stipe and fronds of the mature plant (Figure 1). After the initial division of the zygote, the rhizoid cell elongates and repeatedly divides transverse to the growth axis, whereas the thallus cell undergoes proliferative divisions, each of which is transverse to the previous division.