Meiotic recombination resulting in crossover recombinants, as proposed for S. cerevisiae. DSBs, which initiate recombination, are introduced by the Spo11 protein, which forms a covalent complex with the DNA ends (7). In addition to Spo11, a number of other proteins are required for DSB formation, including those that have roles in mitotic DSB repair (i.e., Rad50/Mre11/Xrs2) and those that have meiotic-specific roles (proteins in yellow box). Although homologs have not been identified in mammals for this latter group of proteins, Mei1 mutation in the mouse leads to a similar phenotype as Spo11 mutation, implying that Mei1 is required with Spo11 for DSB formation. See text for further details. Note: recombination is between replicated homologous chromosomes, but only one sister chromatid is shown for each homolog.

Chromosomes from Spo11^–/– spermatocytes are nearly devoid of γH2AX staining (green), except in the brightly staining sex body that contains the sex chromosomes. Spermatocytes at different stages of meiotic progression are shown. In leptotene (L), short stretches of axial elements of the synaptonemal complex are formed as evidenced by Scp3 staining (red). In the Spo11 mutant, little γH2AX staining is apparent, such that nuclei are faint. Wild-type (WT) spermatocytes are usually strongly stained for γH2AX at this stage as a result of DSB formation (e.g., see figure 6D in ref. 5). Nuclei that proceed to zygotene (Z) have longer axial elements that are readily visible with Scp3 staining; γH2AX staining is only rarely seen in the Spo11 mutant, as shown. WT spermatocytes experience a decline of γH2AX staining at zygotene, as DSBs begin to be repaired (data not shown). At late zygotene and early pachytene (EP), the sex chromosomes stain heavily for γH2AX in both mutant (as shown) and WT nuclei (data not shown), whereas the autosomes do not stain. WT pachytene nuclei have complete synapsis of homologous chromosomes (data not shown), whereas full synapsis is never achieved in the Spo11 mutant (see refs. 10 and 11). (Magnification: ×400.)