Fig. 1. Expression of M31 during XY male mouse meiosis (M31, green; SYCP3, red; DAPI, blue; long arrow, X chromosome; short arrow, Y chromosome; arrowhead, PAR). (A-D) Leptotene nucleus. (A) Axial element formation has just begun. (B) Same nucleus stained for M31, which is concentrated at the centromeric heterochromatin. (C) Superimposition of (A) and (B). (D) Same nucleus stained with DAPI, which highlights areas of centromeric heterochromatin. (E-H) Zygotene nucleus. (E) Synaptonemal complex (SC) formation has just begun. (F) Same nucleus stained for M31, which is concentrated at the centromeric heterochromatin. (G) Superimposition of (E) and (F). (H) Same nucleus stained with DAPI, which highlights areas of centromeric heterochromatin. (I-L) Mid-pachytene nucleus. (I) In contrast to the autosomes, which synapse over their entire length, the X and Y chromosomes show only limited synapsis that includes the PARs. (J) Same nucleus stained with M31. (K) Superimposition of (I) and (J). At this stage, the centromeric heterochromatin of the X chromosome (thin arrow in J) is very compact in comparison to that of the autosomes, and is correspondingly brightly stained with M31. M31 also assembles in the region of X-Y synapsis (open arrowhead in J,K). (L) Same nucleus stained with DAPI. Notice the compact heterochromatin of the X centromere. (M-P) Early diplotene nucleus; the sex chromosomes have partially desynapsed. (N) Same nucleus stained for M31. (O) Superimposition of (M) and (N). At this stage, autosomal centromeric heterochromatin is more compact and brightly stained with M31, whereas that of the X chromosome is barely stained. The focus in the region of X-Y synapsis (open arrowhead in N,O) decorates the very tip of the SC of the sex chromosomes. (P) Same nucleus stained with DAPI. (Q-T) Mid-diplotene nucleus, showing extensive autosomal desynapsis and marked contraction of the XY bivalent. (R) Same nucleus stained with M31. (S) Superimposition of (Q) and (R). At this stage, M31 associates with the sex body (open arrow in R). (T) Same nucleus stained with DAPI. (U-X) Metaphase I nucleus. At this stage, SYCP3 associates exclusively with the centromeric regions of each bivalent. (V) Same nucleus stained with M31. (W) Superimposition of (U) and (V). As well as centromeric heterochromatin, M31 continues to associate with the X and Y chromatin. (X) Same nucleus with X (green) and Y (red) paints. Bars, 10 µm.
