Ultrastructural changes in germinating ascospores of Saccharomyces cerevisiae

Abstract

Ascus walls of a two-spored strain of Saccharomyces cerevisiae were shown by the freeze-etching technique to possess an outer fibrillar zone which was not observed in walls of vegetative cells and ascospores. Invaginations were less abundant in the plasmalemma of the spore protoplast than in that of the vegetative cell. Those in the latter were, however, less elongate. The walls of buds produced by germinating spores were continuous with the spore wall and the mechanics of bud formation appeared similar in spores and vegetative cells. Buds produced by spores resembled the spore in possessing thick walls and in containing abundant lipid granules. The transition to typical vegetative cells with thin walls and sparse lipid deposits required several cell division cycles. Bud scars on asci could be observed clearly with the scanning electron microscope.