Mutation of the ATPase domain of mutS homolog-5 (MSH5) reveals a requirement for a functional MutSG 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). In the mouse, only 10% of DSBs resolve as COs, mostly through a class I pathway dependent on MutSg (MSH4/ MSH5) and MutLg (MLH1/MLH3), the latter representing the ultimate marker of these CO events. A second Class II CO pathway accounts for only a few COs, but is not thought to involve MutSg/ MutLg, and is instead dependent on MUS81-EME1. For class I events, loading of MutLg is thought to be dependent on MutSg, however MutSg loads very early in prophase I at a frequency that far exceeds the final number of class I COs. Moreover, loss of MutSg in mouse results in apoptosis before CO formation, preventing the analysis of its CO function. We generated a mutation in the ATP binding domain of Msh5 (Msh5^GA). While this mutation was not expected to affect MutSg complex formation, MutSg foci do not accumulate during prophase I. However, most spermatocytes from Msh5^GA/GA mice progress to late pachynema and beyond, considerably further than meiosis in Msh52/2 animals. At pachynema, Msh5^GA/GA spermatocytes show persistent DSBs, incomplete homolog pairing, and fail to accumulate MutLg. Unexpectedly, Msh5^GA/GA diakinesis-staged spermatocytes have no chiasmata at all from any CO pathway, indicating that a functional MutSg complex is critical for all CO events regardless of their mechanism of generation.