Fig. 2. Association of M31 with the PAR during male meiosis. (M31, green; SYCP3, red; CREST, which labels the functional centromeres, pseudocoloured white; long arrow, X chromosome; short arrow, Y chromosome; arrowhead, PAR). (A) Schematic representation of the X, Y, XSxra and X^Y*O chromosomes; circles represent centromeres. Sequences in the PAR are represented by the letters A-C. The XSxra chromosome possesses a complete PAR with some additional Y-chromosome short-arm-derived material attached distally. The X^Y* chromosome comprises a complete X and Y chromosome, with a compound PAR that is deleted for distal PAR sequence (in this case, ‘C’). The open circle represents the original Y centromere, which, in this context, is functionally inactive (Burgoyne et al., 1998). (B) High power late pachytene XY bivalent. M31 associates with the synapsed sex chromosome PARs; remnants are also present at the centromeric end. (C) Late pachytene XSxra chromosome, with the X centromeric end marked with CREST. M31 associates with the axial element at a less distal position than in the XY bivalent owing to the presence of most of the Y short arm distal to the PAR (compare with Y short arm in B). (D) Late pachytene X^Y* chromosome, with the X centromeric end again marked with CREST (the inactive Y centromere is negative for CREST). An M31 focus is never seen in the vicinity of the PAR on the X^Y* chromosome although remnants of M31 remain at the X centromere. Bars, 5 µm.