How homologous chromosomes (homologs) find their partner, pair, and recombine during meiosis constitutes the central phenomenon in eukaryotic genetics. It is widely believed that, in most organisms, SPO11-mediated DNA double-strand breaks (DSBs) introduced during prophase I precede and are required for efficient homolog pairing. We now show that, in the mouse, a significant level of homolog pairing precedes programmed DNA cleavage. Strikingly, this early chromosome pairing still requires SPO11 but is not dependent on its ability to make DSBs or homologous recombination proteins. Intriguingly, SUN1, a protein required for telomere attachment to the nuclear envelope and for post-DSB synapsis, is also required for early pre-DSB homolog pairing. Furthermore, pre-DSB pairing at telomeres persists upon entry into prophase I and is most likely important for initiation of synapsis. Our findings suggest that the DSB-triggered homology search may mainly serve to proofread and stabilize the pre-DSB pairing of homologous chromosomes.
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