Model for the evolution of <i>Prdm9</i> alleles and hotspot erosion.

<p>(A) Predicted timeline for the origin of <i>Prdm9</i> alleles based on SNP frequency found at hotspots comparing B6 and CAST to SPRET. PRDM9<sup>Cst</sup> hotspots show an increase in SNPs in the B6 background, suggesting this allele was active in a shared lineage between <i>M.m. castaneus</i> and <i>M.m. domesticus</i>. PRDM9<sup>Dom2</sup> hotspots do not have increased SNPs in the CAST background suggesting this allele was never active in <i>M.m. castaneus</i>. (B) The PRDM9/hotspot lifecycle. Evolutionary erosion driven by biased PRDM9 initiation of recombination decreases hotspot activity over time at many hotspots in parallel. Mutation of <i>Prdm9</i> creates a new binding domain subsequently shifting the genome-wide position of hotspots.</p>