Phosphorylation of SYP-1 is required for the asymmetric localization of factors involved in chromosome segregation. (A) Top: Diagram of chromosome axis remodeling during meiotic prophase from pachytene through meiosis I. Cruciform axis of HTP-3/cohesin with short arm (S) and long arm (L) is shown in diakinesis. Bottom: Diagram of meiotic prophase substages in the C. elegans gonad with meiotic nuclei indicated by blue circles. Ooc, oocyte nucleus; Sp, sperm pronucleus. Diakinesis nuclei are referred to by their position relative to the spermatheca, with the nearest diakinesis nucleus being the −1 oocyte. After entering the spermatheca and becoming fertilized, the −1 oocyte nucleus goes through the meiosis I and II divisions to generate the oocyte pronucleus. (B and C) Partial Z-projection images of a representative chromosome pair in −1 or −2 oocytes with immunostaining for HTP-1/2 (green), SYP-1, and HTP-3 (magenta; B) or LAB-1 (green), SYP-1, and HTP-3 (magenta; C) in the syp-1(wt); syp-1(me17) (−1 oocyte), syp-1(12A); syp-1(me17) (−1 oocyte), and syp-1(T452A); syp-1(me17) (−2 oocyte) gonads. The 12A and T452A mutant gonad shows aberrant persistence of SYP-1 on both arms (see also Fig. S3 D), and HTP-1/2 and LAB-1 on short arms, in −1 or −2 oocytes. In B and C, representative chromosomes are presented with the color of image label indicating the color of immunostaining in the merged image. Full-projection images of each nucleus are shown in the rightmost column, with the chromosome pair shown on the left encircled. The cruciform axis of HTP-3 is visible only for chromosomes lying perpendicular to the optical axis. Bars: (magnified insets) 1 µm; (full projection images at right) 5 µm.