The Expression Profile of the Major Mouse SPO11 Isoforms Indicates that SPO11β Introduces Double Strand Breaks and Suggests that SPO11α Has an Additional Role in Prophase in both Spermatocytes and Oocytes

Both in mice and humans, two major SPO11 isoforms are generated by alternative splicing: SPO11 alpha (exon 2 skipped) and SPO11 beta. Thus, the alternative splicing event must have emerged before the mouse and human lineages diverged and was maintained during 90 million years of evolution, arguing for an essential role for both isoforms. Here we demonstrate that developmental regulation of alternative splicing at the Spo11 locus governs the sequential expression of SPO11 isoforms in male meiotic prophase. Protein quantification in juvenile mice and in prophase mutants indicates that early spermatocytes synthesize primarily SPO11 beta. Estimation of the number of SPO11 dimers (beta beta/alpha beta/alpha alpha) in mutants in which spermatocytes undergo a normal number of double strand breaks but arrest in midprophase due to inefficient repair argues for a role for SPO11 beta-containing dimers in introducing the breaks in leptonema. Expression kinetics in males suggested a role for SPO11 alpha in pachytene/diplotene spermatocytes. Nevertheless, we found that both alternative transcripts can be detected in oocytes throughout prophase I, arguing against a male-specific function for this isoform. Altogether, our data support a role for SPO11 alpha in mid-to late prophase, presumably acting as a topoisomerase, that would be conserved in male and female meiocytes.