ROLES OF SECRETION AND THE CYTOSKELETON IN CELL-ADHESION AND POLARITYESTABLISHMENT IN PELVETIA-COMPRESSA ZYGOTES

During the establishment of polarity, fucoid algal zygotes adhere to t he substratum and select a growth axis according to environmental cues . Since little is known about the early events leading to axis selecti on, we investigated the chronology of cell adhesion, adhesive depositi on, and axis selection induced by light (photopolarization). The requi rements for secretion and the cytoskeleton in these processes and in t he process of changing the orientation of an axis in response to new e nvironmental cues (axis realignment) were also tested. Adhesive deposi tion occurred in two distinct stages: it was deposited uniformly on yo ung zygotes (uniform primary adhesive) and later was deposited asymmet rically (polar secondary adhesive). Uniform primary adhesive depositio n, cell adhesion, and photopolarization occurred simultaneously, and s hortly thereafter, polar secondary adhesive deposition occurred at the future growth site. Uniform primary adhesive deposition and cell adhe sion required secretion, but were independent of filamentous-actin (F- actin) and microtubule function. Photopolarization of young zygotes an d polar secondary adhesive deposition required secretion but not micro tubules. F-actin served to localize secondary adhesive deposition at t he rhizoid pole; its function in polarization was more complex. F-acti n 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 wo rk on ''axis formation'' must be interpreted in the context of the dev elopmental stage of the zygote. (C) 1998 Academic Press.