Meiotic recombination is initiated by the programmed induction of DNA double-strand breaks (DSBs) catalyzed by the evolutionarily conserved Spo11 protein. Studies in yeast have shown that DSB formation requires several other proteins, the role and conservation of which remain unknown. Here we show that two of these Saccharomyces cerevisiae proteins, Mei4 and Rec114, are evolutionarily conserved in most eukaryotes. Mei4 À/À mice are deficient in meiotic DSB formation, thus showing the functional conservation of Mei4 in mice. Cytological analyses reveal that, in mice, MEI4 is localized in discrete foci on the axes of meiotic chromosomes that do not overlap with DMC1 and RPA foci. We thus propose that MEI4 acts as a structural component of the DSB machinery that ensures meiotic DSB formation on chromosome axes. We show that mouse MEI4 and REC114 proteins interact directly, and we identify conserved motifs as required for this interaction. Finally, the unexpected, concomitant absence of Mei4 and Rec114, as well as of Mnd1, Hop2, and Dmc1, in some eukaryotic species (particularly Neurospora crassa, Drosophila melanogaster, and Caenorhabditis elegans) suggests the existence of Mei4-Rec114-dependent and Mei4-Rec114-independent mechanisms for DSB formation, and a functional relationship between the chromosome axis and DSB formation. In sexually reproducing organisms, the chromosome content of diploid germ cells must be halved during gamete formation. This is achieved by the specialized meiotic cycle composed of one replication phase followed by two divisions, thereby leading to the formation of four haploid gametes from a diploid cell. During the first division (the reductional division), homologous chromosomes segregate from each other, whereas during the second division, sister chromatids segregate. The reductional division is unique to meiotic cells and requires specialized mechanisms to connect homologous chromosomes for their proper orientation at metaphase I and faithful separation by the segregation machinery. In most eukaryotes, these connections are established by crossovers (COs), which are reciprocal recombination events between homologous chromosomes, visualized in the cell as chiasmata (Petronczki et al. 2003). The mechanical role of COs implies a precise regulation of the frequency of these events, at least one per chromosome. These recombination events, in addition to ensuring the mechanism of reductional segregation, also have evolutionary consequences as they increase genome diversity (Coop and Przeworski 2007).The molecular mechanism of CO formation has been analyzed in details in Saccharomyces cerevisiae and Schizosaccharomyces pombe (Hunter 2007), and several of the main properties are conserved in mammals (Handel and Schimenti 2010) and other eukaryotes. Meiotic recombination is initiated by the formation of DNA doublestrand breaks (DSBs). DSBs are repaired by homologous recombination using the homologous chromosome as a template, leading to a gene conversion either without CO (NCO) or with CO (Baud...

Gene conversions resulting from meiotic recombination are critical in shaping genome diversification and evolution. How the extent of gene conversions is regulated is unknown. Here we show that the budding yeast mismatch repair related MutLβ complex, Mlh1-Mlh2, specifically interacts with the conserved meiotic Mer3 helicase, which recruits it to recombination hotspots, independently of mismatch recognition. This recruitment is essential to limit gene conversion tract lengths genome-wide, without affecting crossover formation. Contrary to expectations, Mer3 helicase activity, proposed to extend the displacement loop (D-loop) recombination intermediate, does not influence the length of gene conversion events, revealing non-catalytical roles of Mer3. In addition, both purified Mer3 and MutLβ preferentially recognize D-loops, providing a mechanism for limiting gene conversion in vivo. These findings show that MutLβ is an integral part of a new regulatory step of meiotic recombination, which has implications to prevent rapid allele fixation and hotspot erosion in populations.DOI: http://dx.doi.org/10.7554/eLife.21900.001