Figure 2.

Chromosome remodeling establishes a LAB-1-specific domain on bivalents. (A) Schematic representation of one arm of the C. elegans gonad. Germline nuclei undergoing mitotic proliferation in the premeiotic region (distal tip is marked by a single asterisk) enter meiosis as they move proximally toward the spermatheca (sm) and the uterus (double asterisk). A chromosome remodeling process initiated in late pachytene culminates in the formation of six discrete bivalents at diakinesis, where cellularization individualizes single oocytes. A sperm-associated signal then promotes the maturation of the oocyte closest to the spermatheca (−1), which in turn signals its own ovulation. Fertilization than ensues with sperm (sp) entry in the oocyte triggering completion of the meiotic program with subsequent segregation of homologs (anaphase I) and sister chromatids (anaphase II). The presence of two polar bodies (pb1 and pb2) at the surface of the fertilized oocyte marks the end of meiosis. Finally, the female and male pronuclei migrate toward each other and fuse to initiate embryo development. MI, metaphase I; MII, metaphase II; PNF, pronuclei fusion. (B) Immunolocalization of LAB-1 and SYP-1 in mid to late prophase I nuclei of wild-type and lab-1(tm1791) gonads. Bar, 2 μm. (C) Punctate and weak expression of LAB-1 in pachytene nuclei of lab-1(tm1791) mutants. Bar, 2 μm. (D) LAB-1 and SYP-1 immunolocalization on a single bivalent from a −3 oocyte at diakinesis in wild-type and lab-1(tm1791) gonads. Bar, 1 μm. (E) LAB-1 and SYP-1 immunolocalization on a single bivalent from a −1 oocyte at diakinesis in wild-type and lab-1(tm1791) gonads. Bar, 1 μm.