Spermatocytes in Msh5- and Dmc1-null males reach midpachytene but do not mount an MSUC response. (a and b) Msh5^−/− spermatogenic cell squash preparation stained for H1t and γH2AX shows that the majority of H1t-positive cells have fragmented γH2AX staining. (c and d) A spread Msh5^−/− spermatocyte judged to be in pachytene because there is some nonhomologous synapsis (arrows). ATR staining is focal and axially restricted, and the associated γH2AX staining is fragmented. (e and f) Dmc1^−/− spermatogenic cell squash preparation stained for H1t and γH2AX shows many relatively weakly H1t-positive cells with fragmented γH2AX staining. (g and h) A spread Dmc1^−/− spermatocyte judged to be in pachytene because there is some nonhomologous synapsis (arrows). ATR staining is again focal and axially restricted, and the associated γH2AX staining is fragmented. (i–l) Dmc1^−/− surface-spread spermatocytes stained with a second H1t antibody reveals that cells with fragmented γH2AX staining can be either H1t negative (i and j) or positive (k and l). Bars: (a, b, e, and f) 15 μm; (c, d, and g–l) 10 μm.