Parallel detection of crossovers and noncrossovers in mouse germ cells

The recombination between homologous chromosomes during the prophase of the first meiotic division plays an essential role in the formation of euploid gametes, as well as contributing to genetic diversity through the generation of new allele combinations. Two types of products are formed, crossovers (CO) and gene conversions not associated with a crossover, also called noncrossovers (NCO). They result from the repair through differentiated pathways of a common initiating event, DNA double-strand breaks, formation of which is programmed during early meiotic prophase. In contrast to CO, little is known on the frequency, distribution, and regulation of NCO in mammals, mostly due to the technical challenge represented by their detection. However, the development of batch sperm-typing methods allowed for the detection of meiotic NCO products at meiotic recombination hotspots in human and mice. Several recent studies based on this technique demonstrated that mammalian CO hotspots are sites of recombination initiation and that the formation of CO and NCO has different genetic requirements. Here, we describe a method for detecting and characterizing CO and NCO, which has been applied to the analysis of the mouse Psmb9 recombination hotspot.