Although it was not clear that Mnd1 knockout mice would be substantially different from Hop2 mice it has now become clear that these mice are a treasure trove of genetic information about mammalian meiosis. Unlike the Hop2 mice, the Mnd1 mice have a significant proportion of spermatocyte nuclei that show homologously paired chromosome synapsis, progress up to late pachytene but do not show any crossovers. These results strongly suggest that, as has been proposed for budding yeast, there are two main pathways for the repair of double-strand breaks in mice (one that leads to both crossover and non-crossovers and one that results in non-crossovers exclusively) and that Hop2 can act on both pathways. This mouse might also provide insights into how chromosome interactions are channeled primarily between homologs versus sisters, a fundamental requirement leading to the crossovers that ensure the proper segregation of chromosomes.
