Mutation of the ATPase domain of MutS homolog-5 (MSH5) reveals a requirement for a functional MutSγ complex for all crossovers in mammalian meiosis

During meiosis, induction of DNA double strand breaks (DSB) leads to recombination between homologous chromosomes, resulting in crossovers (CO) and non-crossovers (NCO). Only 10% DSBs resolve as COs, mostly through a class I pathway dependent on MutS{gamma} (MSH4/ MSH5). Class II CO events represent a minor proportion of the total CO count and also arise from DSBs, but are not thought to involve MutS{gamma}. However, loading of MutS{gamma} occurs very early in prophase I at a frequency that far exceeds the final number of class I COs found in late prophase I. Moreover, loss of MutS{gamma} in mouse results in apoptosis before CO formation, preventing analysis of its CO function. We generated a mutation in the ATP binding domain of Msh5 (Msh5GA). While this mutation was not expected to affect MutS{gamma} complex formation, MutS{gamma} foci do not accumulate during prophase I. Nevertheless, while some spermatocytes from Msh5-/- animals progress into pachynema, most spermatocytes from Msh5GA/GA mice progress to late pachynema and beyond. Some spermatocytes from Msh5GA/GA mice complete prophase I entirely, allowing for the first time an assessment of MSH5 function in CO formation. At pachynema, Msh5GA/GA spermatocytes show persistent DSBs, incomplete homolog pairing, and fail to accumulate MutL{gamma} (MLH1/MLH3). Unexpectedly, Msh5GA/GA diakinesis-staged spermatocytes have no chiasmata at all from any CO pathway, indicating that a functional MutS{gamma} complex in early prophase I is a pre-requisite for all COs. ARTICLE SUMMARYMSH4/MSH5 are critical components of the class I crossover (CO) machinery, which is responsible for >90% of the COs that arise in mammalian meiosis. We generated a point mutation in the ATP binding motif of Msh5, and found that mutant spermatocytes lose all COs, not just those arising from the class I pathway.