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 (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. However, loading of MutS 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 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 complex formation, MutS 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 (MLH1/MLH3). Unexpectedly, Msh5GA/GA diakinesis-staged spermatocytes have no chiasmata at all from any CO pathway, indicating that a functional MutS complex in early prophase I is a pre-requisite for all COs.

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