Chromosome length is not the sole determinant of sexually dimorphic crossover rates during mammalian meiosis: Insights from genetically diverse mouse strains

Meiotic recombination generates crossovers (COs), reciprocal exchanges between homologous chromosomes critical for accurate chromosome segregation. Inappropriate CO frequency and distribution drive aneuploidy in human oocytes, with error rates up to 10-fold higher than in sperm despite females exhibiting higher CO frequencies. COs form in the context of the proteinaceous synaptonemal complex (SC) that tethers homologs during prophase I. SC length strongly correlates with CO number, and sexual dimorphism in recombination has long been attributed to longer SCs in females. However, this model is challenged by wild-derived PWD mice in which males consistently generate more COs despite having shorter SCs. Here, we exploit natural genetic variation among inbred mouse strains to dissect the structural and regulatory basis of sexually dimorphic CO regulation. Using cytological markers of SC assembly (SYCP3), recombination progression (RAD51, MSH4), class I CO designation (HEI10, MLH1/MLH3), and chiasmata, we show that SC length is not the sole predictor of CO number. PWD males exhibit stronger CO interference and higher CO number than females, despite reduced SC length. Notably, females show reduced efficiency in designating recombination intermediate to become COs, whereas PWD males display exceptional proficiency. Unexpectedly, although class II COs are rare, they play a disproportionate role in ensuring that every chromosome pair receives at least one CO, thereby safeguarding against aneuploidy. Together, these findings challenge the prevailing view that SC length is the primary determinant of sexually dimorphic CO rates and instead highlight sex-specific regulation of CO designation and pathway usage as key drivers of recombination outcomes.