Approximately 80% of vegetative cells of Saccharomyces cerevisiae form asci, the majority of which are four-spored, when incubated in a suitable sporulation medium. Ascospore delimitation is initiated by a pair of unit membranes, the spore-delimiting membranes, between which the spore wall is eventually laid down. Numerous lipid vesicles are observed to be in close association with the spore-delimiting membranes during delimitation and spore wall formation. Their role during these processes is discussed. The dry weight of cells increases by about 75% during sporulation while the lipid content increases by a factor of four. The increase in lipid content is attributable to increased synthesis of sterol esters, triacylglycerols and phospholipids. Uniformly labelled [14C] acetate is incorporated mainly into sterol esters, triacylglycerols and phospholipid. Pulse-labelling by adding [U-14C] acetate to sporulating cultures and harvesting after a further six hours incubation reveals two main periods of lipid synthesis, namely between To and T18' and between T24 and T30. The composition of sterols and phospholipids does not change appreciably, but there is a marked increase in the proportion of unsaturated fatty acids. The significance of these changes in lipid content and composition are discussed in relation to the appearance of the spore-delimiting membranes, endoplasmic reticulum membranes and lipid vesicles. Electrophoretic measurements on individual ascospores indicate the presence of a surface protein layer. Inhibition of ascosporogenesis by ammonium ions was investigated. Asci from cultures in the period T20 to T30 were found to be particularily susceptible to inhibition by ammonium ions.
|Date of Award||1973|