Fig. 4. slk1 is required for proper engulfment of the nuclei by the forespore membrane at the end of meiosis. Homothallic h⁹⁰ strains S1478 (wild-type) and S1883 (slk1) carrying the plasmid pREP81-Psy1-GFP were sporulated on MEA plates at 25°C. (A) A percentage of slk1 cells show defects in nucleus engulfment by the forespore membrane, as revealed by Psy1-GFP fluorescence. Three representative cells are shown. Left panel: bright-field images. Central panels: Hoechst and GFP fluorescence. Right panel: merge. (B) Frequency of abnormal forespore membrane formation in the slk1 mutant. Cells are classified according to the number of defective spores. In class I, all nuclei are engulfed by the forespore membranes; class II, one forespore membrane fails to engulf a nucleus; class III, there are two defective forespore membranes; class IV, there are three defective forespore membranes; class V, there are four defective forespore membranes; class VI, there are fewer than four forespore membranes; and class VII, forespore membrane formation is incomplete. Means and standard deviations of three independent experiments are presented. In each experiment, at least 200 cells were counted. (C) Time-lapse experiment showing forespore membrane growth in a slk1 strain (S1883) carrying the plasmid pREP81-Psy1-GFP. Cells were sporulated on MEA plates and DNA was stained with Hoechst. Stacks of five images separated by 1 µm were taken every 5 minutes. GFP (green) and Hoechst (red) merged images were generated with ImageJ. Arrows mark the abnormal engulfment of DNA by the forespore membrane in one spore. Scale bars: 10 µm.