Cell fusion of Saccharomyces cerevisiae fragile mutants

Fragile mutants of Saccharomyces cerevisiae are defective in the structure of the cell wall and plasma membrane. The mutant cells lyse in hypotonic solutions but grow exponentially when osmotic stabilizer is included in the medium. These mutants display a general increase in the permeability of the plasma membrane. We show here that fragile yeast cells of the same mating type can fuse without protoplast formation. The frequency of cell x cell fusion is lower than that observed for protoplast x protoplast fusion and can be significantly increased if the cells of one partner are converted to protoplasts. Microscopic observations and genetic analysis demonstrate that the hybrids obtained are fusion products. The fusion between fragile cells is explained in terms of the existence of local defects on their surface where the cell wall is thinner (or even missing), thus allowing a direct contact of cells by means of their plasma membranes.