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. 2008 Jun 6;4(6):e1000088. doi: 10.1371/journal.pgen.1000088

Figure 6. A Low Frequency of Synapsis in cra-1 Mutants.

Figure 6

(A–E) TEM images of late pachytene nuclei in wild type and cra-1 germlines. (A) Equatorial section of a wild type late pachytene nucleus. The large dark body slightly off-center is the nucleolus surrounded by SC stretches. The SC stretches are visible as faint zipper-like tracks flanked by electron-dense patches of chromatin (indicated by a yellow arrow). (B) Equatorial section of a cra-1 late pachytene nucleus, in which chromatin patches are observed surrounding the nucleolus and SC stretches are absent. (C) High magnification of a stretch of SC from a different wild type late pachytene nucleus. The distance between electron-dense chromatin patches arranged in parallel was approximately 100nm; the yellow arrow indicates a region where transverse filaments are more apparent. (D) High magnification of chromatin patches observed in a different cra-1 late pachytene nucleus. Distances between chromatin patches occasionally observed in a parallel arrangement in cra-1 mutants were measured as indicated by the yellow dotted arrow. (E) High magnification of an occasional SC stretch observed in a different cra-1 late pachytene nucleus. The yellow arrow indicates a region where transverse filaments are more apparent and measurements confirmed the approximately 100nm distance in these SC stretches. Bars, 500 nm (A–B), 100 nm (C and E), and 200 nm (D). (F–M) High magnification of a single pachytene nucleus simultaneously assessing pairing at the PC end of chromosome I (IR; green) by FISH and observing the chromosomal association of SYP-1 (red) by immunostaining. Panels H–I and L–M are higher magnification images of regions indicated by arrows respectively in F and J. In wild type (F–I), thick DAPI signals flank the SYP-1 signal at the paired loci, indicating homologous synapsis. In contrast, in cra-1 mutants (J–M), SYP-1 is mainly observed localizing to very thin DAPI-stained chromatin tracks at unpaired loci, suggesting that SYP-1 is associated along unsynapsed chromosomes and that synapsis is infrequent. Bars, 2 µm.