Fig. 4.

X-Y bivalent configurations at pachytene in XSxr ^bY and XSxr ^bY* males. Each panel has a diagram of the predicted crossover event, a low magnification view showing the X-Y bivalent in a γH2AX-stained (red) sex chromatin cloud together with near-by autosomes, a higher power view of the X-Y bivalent with the γH2AX staining removed, and a drawing of the deduced X-Y bivalent configuration highlighting the centromere positions. Note that the red CREST staining of active centromeres often appears as yellow where it overlaps the green chromosome axis. (a) XSxr ^bY with Yp-Sxr ^b synapsis. (b) XSxr ^bY with PAR-PAR synapsis. (c) XSxr ^bY* with centromere-centromere association—this is likely to be due to synapsis driven by homology of Sstx sequences adjacent to the X and Y*(X-derived) centromeres. (d) XSxr ^bY* with Yp-Sxr ^b synapsis. (e) XSxr ^bY* with centromere-centromere and Yp-Sxr ^b synapsis. (f) XSxr ^bY* with PAR-PAR synapsis—parasynapsis and staggered synapsis cannot be differentiated at this resolution. The staggered configuration is indicated in the black square