Recombinational DNA double-strand breaks in mice precede synapsis

In Saccharomyces cerevisiae, meiotic recombination is initiated by Spo11-de pendent double-strand breaks (DSBs), a process that precedes homologous syn apsis. Here we use an antibody specific for a phosphorylated histone (gamma -H2AX, which marks the sites of DSBs) to investigate the timing, distribut ion and Spo11-dependence of meiotic DSBs in the mouse. We show that, as in yeast, recombination in the mouse is initiated by Spo11-dependent DSBs that form during leptotene. Loss of gamma -H2AX staining (which in irradiated s omatic cells is temporally linked with DSB repair) is temporally and spatia lly correlated with synapsis, even when this synapsis is 'non-homologous'.