S-CDK is required and sufficient to drive SPB separation and spindle formation during prophase I in <i>ipl1-md</i> cells.

<p>(A)Images for tubulin and Zip1 staining in <i>dmc1</i>Δ <i>ipl1-md</i> strains with normal S-CDK and M-CDK (left image), lacking S-CDK activity (<i>clb5</i>Δ <i>clb6</i>Δ; middle image), or without M-CDK proficient for Clb5 only (<i>clb1</i>Δ, <i>clb3</i>Δ, <i>clb4</i>Δ, <i>clb6</i>Δ <i>CLB5⁺</i>; right panel). Strains: Y4495, Y4435, and Y4496, respectively. Bars, 2 µm. (B) Quantification on the proportion of fixed cells with spindles and separated SPBs at 8 hours and 12 hours. (C, D) <i>ipl1-md ndt80</i>Δ <i>cdc28-as1</i> (Y2577) cells were treated with either 50 µM 1-NM-PP1 (+) or solvent only (DMSO) (−) to inhibit Cdc28/CDK kinase activity at 6 hours, when spindles have formed in at least 20% of <i>ipl1-md ndt80</i>Δ cells. Examples of spread, meiotic nuclei are shown to the left. Note that there was no effect on inhibiting Cdc28-as1 in <i>ndt80</i>Δ alone bars, 2 µm. The graph shows that Quantification of prophase spreads with spindles or aberrant spindle pole structures (<i>e.g.</i> multipolar spindles).