Meiotic DSB patterning: A multifaceted process

Meiosis is a specialized two-step cell division responsible for genome haploidization and the generation of genetic diversity during gametogenesis. An integral and distinctive feature of the meiotic program is the evolutionarily conserved initiation of homologous recombination (HR) by the developmentally programmed induction of DNA double-strand breaks (DSBs). The inherently dangerous but essential act of DSB formation is subject to multiple forms of stringent and self-corrective regulation that collectively ensure fruitful and appropriate levels of genetic exchange without risk to cellular survival. Within this article we focus upon an emerging element of this control—spatial regulation—detailing recent advances made in understanding how DSBs are evenly distributed across the genome, and present a unified view of the underlying patterning mechanisms employed

Repair of exogenous DNA double-strand breaks promotes chromosome synapsis in SPO11-mutant mouse meiocytes, and is altered in the absence of HORMAD1

Repair of SPO11-dependent DNA double-strand breaks (DSBs) via homologous recombination (HR) is essential for stable homologous chromosome pairing and synapsis during meiotic prophase. Here, we induced radiation-induced DSBs to study meiotic recombination and homologous chromosome pairing in mouse meiocytes in the absence of SPO11 activity (Spo11YF/YF model), and in the absence of both SPO11 and HORMAD1 (Spo11/Hormad1 dko). Within 30 min after 5 Gy irradiation of Spo11YF/YF mice, 140–160 DSB repair foci were detected, which specifically localized to the synaptonemal complex axes. Repair of radiation-induced DSBs was incomplete in Spo11YF/YF compared to Spo11+/YF meiocytes. Still, repair of exogenous DSBs promoted partial recovery of chromosome pairing and synapsis in Spo11YF/YF meiocytes. This indicates that at least part of the exogenous DSBs can be processed in an interhomolog recombination repair pathway. Interestingly, in a seperate experiment, using 3 Gy of irradiation, we observed that Spo11/Hormad1 dko spermatocytes contained fewer remaining DSB repair foci at 48 h after irradiation compared to irradiated Spo11 knockout spermatocytes. Together, these results show that recruitment of exogenous DSBs to the synaptonemal complex, in conjunction with repair of exogenous DSBs via the homologous chromosome, contributes to homology recognition. In addition, the data suggest a role for HORMAD1 in DNA repair pathway choice in mouse meiocytes

Cohesin dynamics during meiotic prophase

For faithful segregation during meiosis, chromosomes must be physically linked by both sister chromatid cohesion (SCC), provided by cohesin, and at least one crossover (CO). In mitosis, cohesin is dynamically associated with chromatin and this has been shown to be crucial for the repair of DSBs. Although DSBs are purposely made to start meiotic recombination, it is unknown if meiotic cohesin is dynamically associated with chromatin. However, cohesin loss or degradation is thought to be involved in the high incidence of aneuploidy observed in human eggs. In Caenorhabditis elegans (C. elegans), the cohesin loader SCC-2 remains associated with the axial element of meiotic chromosomes following the completion of S-phase, hinting that cohesin may be reloaded during meiotic prophase. To confirm this, I investigated if depleting SCC-2 by RNAi after entrance into meiotic prophase had an effect on cohesin association with chromosomes. This revealed loss of the cohesin subunit REC-8 from late prophase nuclei, suggesting that without reloading cohesin is removed from chromatin. Furthermore, scc-2 RNAi also resulted in the impairment of chiasmata, raising the possibility that cohesin reloading plays a role in CO formation or in chiasma maintenance. Two key mediators of cohesin removal are known to operate during the G2 phase of the mitotic cell cycle: the presence of DSBs and the cohesion anti-establishment factor Wapl1. Here I show for the first time that WAPL-1 modulates the cohesiveness of complexes containing the meiosis-specific kleisins COH-3 and COH-4. Furthermore, cohesin complexes containing different kleisins are differentially modulated by DSBs, and only REC-8-containing cohesin complexes can undertake the repair of DNA damage. Finally, I have developed several genetic tools to allow the visualization of cohesin turnover during meiosis. These findings show the exceptional complexity of cohesin dynamics during meiotic prophase, as well as demonstrating roles for cohesin outside of the provision of SCC.Open Acces

A spontaneous mutation in MutL-Homolog 3 (HvMLH3) affects synapsis and crossover resolution in the barley desynaptic mutant des10

Although meiosis is evolutionarily conserved, many of the underlying mechanisms show species-specific differences. These are poorly understood in large genome plant species such as barley (Hordeum vulgare) where meiotic recombination is very heavily skewed to the ends of chromosomes. The characterization of mutant lines can help elucidate how recombination is controlled. We used a combination of genetic segregation analysis, cytogenetics, immunocytology and 3D imaging to genetically map and characterize the barley meiotic mutant DESYNAPTIC 10 (des10). We identified a spontaneous exonic deletion in the orthologue of MutL-Homolog 3 (HvMlh3) as the causal lesion. Compared with wild-type, des10 mutants exhibit reduced recombination and fewer chiasmata, resulting in the loss of obligate crossovers and leading to chromosome mis-segregation. Using 3D structured illumination microscopy (3D-SIM), we observed that normal synapsis progression was also disrupted in des10, a phenotype that was not evident with standard confocal microscopy and that has not been reported with Mlh3 knockout mutants in Arabidopsis. Our data provide new insights on the interplay between synapsis and recombination in barley and highlight the need for detailed studies of meiosis in nonmodel species. This study also confirms the importance of early stages of prophase I for the control of recombination in large genome cereals.Isabelle Colas, Malcolm Macaulay, James D. Higgins, Dylan Phillips, Abdellah Barakate ... Robbie Waugh ... et al

ATR is required to complete meiotic recombination in mice

Precise execution of recombination during meiosis is essential for forming chromosomally-balanced gametes. Meiotic recombination initiates with the formation and resection of DNA double-strand breaks (DSBs). Cellular responses to meiotic DSBs are critical for efficient repair and quality control, but molecular features of these remain poorly understood, particularly in mammals. Here we report that the DNA damage response protein kinase ATR is crucial for meiotic recombination and completion of meiotic prophase in mice. Using a hypomorphic Atr mutation and pharmacological inhibition of ATR in vivo and in cultured spermatocytes, we show that ATR, through its effector kinase CHK1, promotes efficient RAD51 and DMC1 assembly at RPA-coated resected DSB sites and establishment of interhomolog connections during meiosis. Furthermore, our findings suggest that ATR promotes local accumulation of recombination markers on unsynapsed axes during meiotic prophase to favor homologous chromosome synapsis. These data reveal that ATR plays multiple roles in mammalian meiotic recombination.We thank M. A. Handel (The Jackson Laboratory, Bar Harbor, USA) for the anti-H1T antibody; E. Marcon for the anti-RPA antibody (University of Toronto, Canada); A. Toth for the anti-pHORMAD2 antibody (U. Dresden, Germany) and N. Hunter for the anti- RNF212 antibody (UC Davis, USA); J. Turner (National Institute for Medical Research,London, UK) for assistance in the RNA-FISH experiments, for the X chromosome probe,for providing AtrFL/−testis samples and for sharing unpublished data; L. Kauppi(University of Helsinki, Finland) for providing us with protocols for the testis cultures;and members of the Roig lab and the Spanish Ministerio de Ciencia e Innovación-funded Network of Spanish groups working on Meiosis (MeioNet, BFU201‐71786‐REDT) and Enrique Martínez Pérez (Imperial College, London, UK) for helpful discussions. M.M.O. was supported by a FPI fellowship from the Ministerio de Ciencia e Innovación (BES-2011-045381). J.L. was supported in part by American Cancer Society post-doctoral fellowship (PF-12-157-01-DMC). S.K. is an Investigator of the Howard Hughes Medical Institute. This work was supported by the Ministerio de Ciencia e Innovación (BFU2010-18965, BFU2013-43965-P and BFU2016-80370-P, I.R.), by the UAB-Aposta award to young investigators (APOSTA2011-03, I.R.) and by the NIH (R35 GM118175, to M.J.and R35 GM118092 to S.K.).S

Meiotic Recombination: The Essence of Heredity.

The study of homologous recombination has its historical roots in meiosis. In this context, recombination occurs as a programmed event that culminates in the formation of crossovers, which are essential for accurate chromosome segregation and create new combinations of parental alleles. Thus, meiotic recombination underlies both the independent assortment of parental chromosomes and genetic linkage. This review highlights the features of meiotic recombination that distinguish it from recombinational repair in somatic cells, and how the molecular processes of meiotic recombination are embedded and interdependent with the chromosome structures that characterize meiotic prophase. A more in-depth review presents our understanding of how crossover and noncrossover pathways of meiotic recombination are differentiated and regulated. The final section of this review summarizes the studies that have defined defective recombination as a leading cause of pregnancy loss and congenital disease in humans

Coordination of meiotic recombination in diploid and tetraploid arabidopsis

Homologous recombination is an integral part of meiosis and is essential for generating crossovers that ensure balanced segregation of homologous chromosomes and establish genetic variation within offspring. It is therefore exceedingly important that meiotic cells employ stringent control mechanisms to safeguard crossover formation. Work in yeast has indicated that the meiotic axis, a proteinaceous structure that tethers meiotic chromosomes into looped arrays, plays a crucial role in many aspects of homologous recombination, from double strand break formation to crossover interference. It has also been suggested that increased crossover interference helps to establish meiotic stability by inhibiting multivalent formation during autopolyploid meiosis. Using immunocytochemistry coupled with super-resolution microscopy, we have further investigated the role played by the meiotic axis protein ASY1 in stabilising meiosis in the established autotetraploid Arabidopsis arenosa. We have also used Arabidopsis arenosa as a model for studying how meiotic interference might operate within an autopolyploid context. Alongside this, experiments using transgenic lines of the model plant Arabidopsis thaliana have helped to shed light on how crossover formation and synapsis are affected by reduced expression of ASY1 and ASY3 and to determine what effect limiting meiotic crossover numbers might have on neopolyploid meiotic stabilisation

Differentiated function and localisation of SPO11-1 and PRD3 on the chromosome axis during meiotic DSB formation in Arabidopsis thaliana

During meiosis, DNA double-strand breaks (DSBs) occur throughout the genome, a subset of which are repaired to form reciprocal crossovers between chromosomes. Crossovers are essential to ensure balanced chromosome segregation and to create new combinations of genetic variation. Meiotic DSBs are formed by a topoisomerase-VI-like complex, containing catalytic (e.g. SPO11) proteins and auxiliary (e.g. PRD3) proteins. Meiotic DSBs are formed in chromatin loops tethered to a linear chromosome axis, but the interrelationship between DSB-promoting factors and the axis is not fully understood. Here, we study the localisation of SPO11-1 and PRD3 during meiosis, and investigate their respective functions in relation to the chromosome axis. Using immunocytogenetics, we observed that the localisation of SPO11-1 overlaps relatively weakly with the chromosome axis and RAD51, a marker of meiotic DSBs, and that SPO11-1 recruitment to chromatin is genetically independent of the axis. In contrast, PRD3 localisation correlates more strongly with RAD51 and the chromosome axis. This indicates that PRD3 likely forms a functional link between SPO11-1 and the chromosome axis to promote meiotic DSB formation. We also uncovered a new function of SPO11-1 in the nucleation of the synaptonemal complex protein ZYP1. We demonstrate that chromosome co-alignment associated with ZYP1 deposition can occur in the absence of DSBs, and is dependent on SPO11-1, but not PRD3. Lastly, we show that the progression of meiosis is influenced by the presence of aberrant chromosomal connections, but not by the absence of DSBs or synapsis. Altogether, our study provides mechanistic insights into the control of meiotic DSB formation and reveals diverse functional interactions between SPO11-1, PRD3 and the chromosome axis

C. elegans Germ Cells Switch between Distinct Modes of Double-Strand Break Repair During Meiotic Prophase Progression

Chromosome inheritance during sexual reproduction relies on deliberate induction of double-strand DNA breaks (DSBs) and repair of a subset of these breaks as interhomolog crossovers (COs). Here we provide a direct demonstration, based on our analysis of rad-50 mutants, that the meiotic program in Caenorhabditis elegans involves both acquisition and loss of a specialized mode of double-strand break repair (DSBR). In premeiotic germ cells, RAD-50 is not required to load strand-exchange protein RAD-51 at sites of spontaneous or ionizing radiation (IR)-induced DSBs. A specialized meiotic DSBR mode is engaged at the onset of meiotic prophase, coincident with assembly of meiotic chromosome axis structures. This meiotic DSBR mode is characterized both by dependence on RAD-50 for rapid accumulation of RAD-51 at DSB sites and by competence for converting DSBs into interhomolog COs. At the mid-pachytene to late pachytene transition, germ cells undergo an abrupt release from the meiotic DSBR mode, characterized by reversion to RAD-50-independent loading of RAD-51 and loss of competence to convert DSBs into interhomolog COs. This transition in DSBR mode is dependent on MAP kinase-triggered prophase progression and coincides temporally with a major remodeling of chromosome architecture. We propose that at least two developmentally programmed switches in DSBR mode, likely conferred by changes in chromosome architecture, operate in the C. elegans germ line to allow formation of meiotic crossovers without jeopardizing genomic integrity. Our data further suggest that meiotic cohesin component REC-8 may play a role in limiting the activity of SPO-11 in generating meiotic DSBs and that RAD-50 may function in counteracting this inhibition