No abstract

Within the first hour of zygote maturation, Chlamydomonas reinhardtii cells stop synthesizing certain polypeptides that characterize the vegetative and gametic stages of the life cycle and initiate the synthesis of novel, zygote-specific polypeptides. At least six of these polypeptides are secreted into the medium, and fine-structural studies indicate that they represent components of the cell wall that is synthesized and secreted early in zygote development. We conclude that a new program of protein synthesis, and possibly also gene transcription, is initiated shortly after gametic cells fuse, a program that appears highly suited to celldifferentiation studies. KEY WORDS Chlamydomonaszygotes 9 secretion 9 cell differentiation During its simple life cycle, the unicellular eukaryotic protist Chlamydomonas reinhardtii passes through three states of differentiation: the haploid vegetative state, the haploid gametic state, and the diploid zygotic state. Gametogenesis is triggered by nitrogen starvation of vegetative cells (27) and requires about 12 h (17, 22). Zygotes form when gametes of opposite mating type (mr + and mr-) fuse to form quadriflagellated cells during the mating reaction, and early zygote development proceeds for about 24 h, during which time a thick zygote wall is elaborated (7).We wished to learn whether either of these lifecycle transitions was suitable for studies of eukaryotic cell differentiation, our criterion for suitability being that the transition should involve the production of readily detected, novel polypeptides whose synthesis is specific for a particular state of differentiation. We therefore began by subjecting cells at various stages of differentiation to 2-3-h pulses with [~4C]acetate, and analyzed the labeled polypeptides by slab-gel electrophoresis followed by autoradiography.When such experiments were performed with cells undergoing gametogenesis (24), all of the major polypeptides synthesized by gametes were found to have apparent counterparts in the vegetative-cell samples. The synthesis of many vegetative-cell polypeptides was observed to abate or cease entirely during gametogenesis so that shifts occurred in the relative intensities of various bands on the gels, but no major polypeptides specific to the gametic state could be distinguished by a variety of approaches.In contrast, we discovered that zygotic cell fusion in C. reinhardtii elicits the synthesis of a number of major zygotic polypeptides that are not evident in vegetative or gametic cells. Because the trigger for the onset of this "zygotic program" is gametic and probably nuclear fusion, the phenomenon is at least formally analogous to the trigger-J. CELL BIOLOGY 9 The Rockefeller University Press 9 0021-9525/78