Fig. 1. Double immunolabelling on spread T. elegans primary spermatocytes with anti-SCP3 (green) and anti-centromere CREST (red) sera. (A) Zygotene. The anti-SCP3 antibody labels axial structures that are polarised in a `bouquet' configuration. Paired regions (arrows) appear thicker than the unsynapsed autosomal AEs (arrowheads). The sex chromosomal AEs (X,Y) are recognisable in the bouquet area, and they are separated. At higher magnification (A') the irregular outline of the X AE can be seen. The AE of the Y chromosome (inset) is regular and the centromere is subterminal. (B-F) Pachytene. The bouquet arrangement is lost and the six autosomal pairs are fully synapsed. (B,B'). Early pachytene. The sex AEs are thickened and separated. The X AE loses its irregular outline. (C,C') Early-mid pachytene. Sex chromosomes start to make contact by means of fine threads labelled in green with anti-SCP3 (arrowheads in C'). (D,D') Mid pachytene. Sex chromosomal AEs (XY) become thinner. An anti-SCP3-labelled structure (arrow) with a horseshoe plate appearance is associated with the ends of the sex AEs. The AE of the Y chromosome is immersed in this plate. (E-E') Mid pachytene. The AE of the X chromosome loses its stiffness and folds. The AE of the Y chromosome is immersed in the anti-SCP3 labelled plate (arrow). (F) Late pachytene. The autosomal SCs are fragmented (arrowheads). (F') Enlargement of the XY body. The AE of the X chromosome is seen as a fine and fragmented thread tracing several loops. X and Y AEs are still associated with the anti-SCP3-labelled plate (arrow). Centromeres are still detectable but are fainter than in previous stages (arrowheads). Bars, 10 µm in (A-F); 2 µm in (A'-F').