PMC

A. Timecourse of pairing in wildtype and cmt-1 mutant germlines. B. Timecourse of synapsis in wildtype and cmt-1 mutant germlines. C. Images of meiotic nuclei stained with antibodies against HTP-3, SYP-1 and HIM-8 in cmt-1 and cmt-1;pch-2Δ mutants. The circled nucleus has undergone non-homologous synapsis. D. Quantification of non-homologous synapsis wildtype animals, cmt-1, cmt-1;pch-2^E253Q and cmt-1;pch-2Δ mutants. All error bars indicate 95% confidence intervals. Significance was assessed by performing two-tailed Fisher exact tests. A *** indicates a p value < 0.0001.

(A) Pairing of HIM-8 and ZIM-2 foci were scored after shifting to 25° C for 6 hours. ZIM-2 pairing was reduced in both zyg-12(or577) and zyg-12(ct350) hermaphrodites, while HIM-8 pairing was reduced only in zyg-12(ct350). Asterisks indicate a difference from controls at p<0.03. (B) Nuclei from the pachytene region stained for SYP-1(green), HTP-3 (red), and HIM-8 (blue). Animals were fixed after 6 hours at restrictive temperature (25° C). A mixture of unsynapsed, homologously synapsed and non-homologously synapsed chromosomes were observed, with more extensive asynapsis in the zyg-12(ct350) mutant. The arrowhead in the top right panel indicates a nucleus with non-homologous synapsis. The images of zyg-12(ct350) include nuclei with paired HIM-8 foci and SYP-1 polycomplex (arrows, middle row), and non-homologous synapsis (arrowhead, bottom row).

A nucleus containing a single pair of chromosomes is shown for simplicity. In early meiosis, the NE proteins SUN-1 (orange) and ZYG-12 (green) concentrate by association with individual PCs. Motion of the patches and associated chromosomes is facilitated by microtubules, and promotes encounters between chromosomes. In addition to mediating chromosome encounters, the NE bridge components SUN-1 and ZYG-12 also collaborate to inhibit initiation of synapsis between transiently associated chromosomes. Homology between associated chromosomes is assessed by dynein, acting tangentially at the NE surface. Non-homologous chromosomes normally separate rapidly. However, when dynein exerts force opposing the association between homologous PCs, the resulting tension overcomes a thermodynamic barrier to license synapsis initiation; once initiated, the SC polymerizes processively along the chromosomes, completing their alignment.

Transmission electron microscopy of silver-stained spreads of synaptonemal complexes (SC) from wild-type A344 inbred and as1 and mtm-99-25 mutants. Bar=1 μm. (A) Fragments of two SCs and associated chromatin at the pachytene stage of wild-type showing complete synapsis of homologous chromosomes. Note that LEs of each SC are not connected at the telomere ends (arrows). (B, C) Short and long foldbacks, and hairpin-like structures in the middle of chromosomes in as1 showing non-homologous synapsis. Compare the ends of foldbacks (arrowheads) and the normal telomere ends (arrows in A). Note that the central element (CE) has assembled between the LEs of foldbacks (large arrowheads). (D) An early diplotene in mtm99-25. Both pairing partner switches and foldbacks are present in the same chromosome and a CE has formed between non-homologous chromosomes. The SC is beginning to disassemble; unsynapsed AEs disappear while synapsed LEs are more stable.