Sex chromosome recombination failure, apoptosis, and fertility in male mice.

Lack of crossing-over in meiosis can trigger an apoptotic response at metaphase I by the spindle assembly checkpoint (SAC). In contrast to females, segregation of sex chromosomes in males poses a particular challenge as recombination and chiasma formation is restricted to the pseudoautosomal region, the small region of homology between X and Y chromosomes. Existing data indicate that low levels of crossover failure in male meiosis can be tolerated without compromising fertility, while high levels of X-Y dissociation (in ≥70 % of cells) result in widespread apoptosis and subsequent infertility, demonstrated earlier, e.g., in Spo11β-only mice. Here, we explore the threshold of X-Y recombination failure frequency that is compatible with fertility. We show that in Spo11β-only(mb) mice with a mixed genetic background, in contrast to Spo11β-only mice with a C57BL/6 background, X-Y pairing fails in ~50 % of cells but this still allows for sperm production without any overt impact on fertility. We also review data on apoptosis and fertility from other achiasmate mouse models and propose that the incidence of homolog dissociation that can be tolerated in vivo without compromising male fertility lies between 50 and 70 %.