Introduction

The study of plant gametogenesis, fertilisation and embryonic development has been hindered by the inaccessibility of the gametes and embryo in higher plants. It has therefore been necessary to turn to lower plants as model systems for studying these early phases of development. Large brown marine algae of the genus Fucus have been used extensively for this purpose (Quatrano, 1990). This and another recent study (Masters, Shirras & Hetherington, 1992) provides evidence that the eggs of Fucus serratus contain maternal mRNA and that this mRNA may be sequestered in mRNP particles in a manner similar to that found in several animal systems. Further, as previous work has demonstrated that phosphorylation–dephosphorylation of messenger ribonucleoproteins plays a role in the control of the translational availability of Xenopus stored mRNA, we have sought evidence that a similar strategy is employed in Fucus eggs.

Maternal mRNA

The unfertilised Fucus egg shows no apparent asymmetry, an axis of symmetry forming after fertilisation in response to external stimuli such as light. When unidirectional light is shone on Fucus zygotes the shaded side forms a protuberance. The first cell division is asymmetric, dividing the protuberance from the larger cell mass of the original zygote. The polarisation of the embryo can be divided into two phases: a reversible axis formation and an irreversible axis fixation. Axis fixation depends on a reorganisation of actin filaments and an interaction between cell surface molecules and the forming cell wall (Kropf, Kloareg & Quatrano, 1988; Kropf, Berge & Quatrano, 1989a).