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Fig. 3. The rescue of the prophase arrest of Atm^-/- spermatocytes by Spo11 heterozygosity reveals that ATM plays no essential role in the sex body. (A) A single copy of Spo11 rescues the prophase-arrest characteristic of the ATM KO. Whereas inactivation of both copies of Spo11 in an Atm^-/- background results in a complete prophase arrest (and a meiotic phenotype indistinguishable to that of the Spo11^-/- single mutant), Spo11 heterozygosity rescues the prophase arrest of an ATM KO, allowing spermatocytes to complete prophase I. (B) ATM is dispensable for phosphorylation of H2AX in the sex body. Combined FISH (whole-chromosome paint probes against the X and Y chromosomes, in green) and immunostaining (antibodies against SCP3 in red and those against ${gamma}$ H2AX in blue) performed on structurally preserved spermatocytes from Atm^-/- Spo11^-/- (a-c) and Atm^-/- Spo11^+/- (d-f) mice (two animals for each genetic background). As is the case for the Spo11^-/- single mutant, none of the 50 nuclei scored for the Atm^-/- Spo11^-/- double-mutant strain contained a ${gamma}$ H2AX signal overlapping with either the X or the Y chromatin. By stark contrast, in the rescued Atm^-/-Spo11^+/- background, the X-Y chromatin consistently contains ${gamma}$ H2AX. The ${gamma}$ H2AX signal coincided with the X-Y chromatin in all of the 30 nuclei scored.

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