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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 MutSγ (MSH4/ MSH5) and MutLγ (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 MutSγ/ MutLγ, and is instead dependent on MUS81-EME1. For class I events, loading of MutLγ is thought to be dependent on MutSγ, however MutSγ loads very early in prophase I at a frequency that far exceeds the final number of class I COs. Moreover, loss of MutSγ 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 (Msh5GA). While this mutation was not expected to affect MutSγ complex formation, MutSγ foci do not accumulate during prophase I. However, most spermatocytes from Msh5GA/GA mice progress to late pachynema and beyond, considerably further than meiosis in Msh5−/− animals. At pachynema, Msh5GA/GA spermatocytes show persistent DSBs, incomplete homolog pairing, and fail to accumulate MutLγ. Unexpectedly, Msh5GA/GA diakinesis-staged spermatocytes have no chiasmata at all from any CO pathway, indicating that a functional MutSγ complex is critical for all CO events regardless of their mechanism of generation.

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 (Msh5GA). While this mutation was not expected to affect MutSg complex formation, MutSg foci do not accumulate during prophase I. However, most spermatocytes from Msh5GA/GA mice progress to late pachynema and beyond, considerably further than meiosis in Msh52/2 animals. At pachynema, Msh5GA/GA spermatocytes show persistent DSBs, incomplete homolog pairing, and fail to accumulate MutLg. Unexpectedly, Msh5GA/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. [ABSTRACT FROM AUTHOR]