No abstract

No abstract

Establishment of a primary developmental axis generally is thought to involve rearrangements in the plasma membrane or cytoplasm of the egg. In this report the additional requirement for cell wall in polarization of Fucus zygotes was investigated. Protoplasts of fertilized eggs were tested for their ability to establish an axis in accordance with an orienting vector of unilateral light. The results demonstrate that cell wall is not required for axis formation. However, the orientation of the axis remains labile until new cell wall is synthesized. The presence of a cell wall is an absolute requirement for axis fixation.

During the establishment of polarity, fucoid algal zygotes adhere to the substratum and select a growth axis according to environmental cues. Since little is known about the early events leading to axis selection, we investigated the chronology of cell adhesion, adhesive deposition, and axis selection induced by light (photopolarization). The requirements for secretion and the cytoskeleton in these processes and in the process of changing the orientation of an axis in response to new environmental cues (axis realignment) were also tested. Adhesive deposition occurred in two distinct stages: it was deposited uniformally on young zygotes (uniform primary adhesive) and later was deposited asymmetrically (polar secondary adhesive). Uniform primary adhesive deposition, cell adhesion, and photopolarization occurred simultaneously, and shortly thereafter, polar secondary adhesive deposition occurred at the future growth site. Uniform primary adhesive deposition and cell adhesion required secretion, but were independent of filamentous-actin (F-actin) and microtubule function. Photopolarization of young zygotes and polar secondary adhesive deposition required secretion but not microtubules. F-actin served to localize secondary adhesive deposition at the rhizoid pole; its function in polarization was more complex. F-actin was required for axis selection; however, its role in realignment of an axis depended on the light regime. The differing requirements for F-actin during development indicates that the axis is not static, but changes with time. These findings indicate that previous and future work on "axis formation" must be interpreted in the context of the developmental stage of the zygote.

No abstract