Synaptonemal complex formation: where does it start?

The synaptonemal complex is a prominent, evolutionarily conserved feature of meiotic prophase. The assembly of this structure is closely linked to meiotic recombination. A recent study in budding yeast reveals an unexpected role in centromere pairing for a protein component of the synaptonemal complex, Zip1. These findings have implications for synaptonemal complex formation.     

Meiotic Cohesin Promotes Pairing of Nonhomologous Centromeres in Early Meiotic Prophase

A period of pairing between nonhomologous centromeres occurs early in meiosis in a diverse collection of organisms. This early, homology-independent, centromere pairing, referred to as centromere coupling in budding yeast, gives way to an alignment of homologous centromeres as homologues synapse later in meiotic prophase. The regulation of centromere coupling and its underlying mechanism have not been elucidated. In budding yeast, the protein Zip1p is a major component of the central element of the synaptonemal complex in pachytene of meiosis, and earlier, is essential for centromere coupling. The experiments reported here demonstrate that centromere coupling is mechanistically distinct from synaptonemal complex assembly. Zip2p, Zip3p, and Red1p are all required for the assembly of Zip1 into the synaptonemal complex but are dispensable for centromere coupling. However, the meiotic cohesin Rec8p is required for centromere coupling. Loading of meiotic cohesins to centromeres and cohesin-associated regions is required for the association of Zip1 with these sites, and the association of Zip1 with the centromeres then promotes coupling. These findings reveal a mechanism that promotes associations between centromeres before the assembly of the synaptonemal complex, and they demonstrate that chromosomes are preloaded with Zip1p in a manner that may promote synapsis.     

SUMO Localizes to the Central Element of Synaptonemal Complex and Is Required for the Full Synapsis of Meiotic Chromosomes in Budding Yeast

The synaptonemal complex (SC) is a widely conserved structure that mediates the intimate alignment of homologous chromosomes during meiotic prophase and is required for proper homolog segregation at meiosis I. However, fundamental details of SC architecture and assembly remain poorly understood. The coiled-coil protein, Zip1, is the only component whose arrangement within the mature SC of budding yeast has been extensively characterized. It has been proposed that the Small Ubiquitin-like MOdifier, SUMO, plays a role in SC assembly by linking chromosome axes with Zip1''s C termini. The role of SUMO in SC structure has not been directly tested, however, because cells lacking SUMO are inviable. Here, we provide direct evidence for SUMO''s function in SC assembly. A meiotic smt3 reduction-of-function strain displays reduced sporulation, abnormal levels of crossover recombination, and diminished SC assembly. SC structures are nearly absent when induced at later meiotic time points in the smt3 reduction-of-function background. Using Structured Illumination Microscopy we furthermore determine the position of SUMO within budding yeast SC structure. In contrast to previous models that positioned SUMO near Zip1''s C termini, we demonstrate that SUMO lies at the midline of SC central region proximal to Zip1''s N termini, within a subdomain called the “central element”. The recently identified SUMOylated SC component, Ecm11, also localizes to the SC central element. Finally, we show that SUMO, Ecm11, and even unSUMOylatable Ecm11 exhibit Zip1-like ongoing incorporation into previously established SCs during meiotic prophase and that the relative abundance of SUMO and Ecm11 correlates with Zip1''s abundance within SCs of varying Zip1 content. We discuss a model in which central element proteins are core building blocks that stabilize the architecture of SC near Zip1''s N termini, and where SUMOylation may occur subsequent to the incorporation of components like Ecm11 into an SC precursor structure.     

A component of C. elegans meiotic chromosome axes at the interface of homolog alignment, synapsis, nuclear reorganization, and recombination

A universal feature of meiotic prophase is the pairing of homologous chromosomes, a fundamental prerequisite for the successful completion of all subsequent meiotic events. HIM-3 is a Caenorhabditis elegans meiosis-specific non-cohesin component of chromosome axes that is required for synapsis. Our characterization of new him-3 alleles reveals previously unknown functions for the protein. HIM-3 is required for the establishment of initial contacts between homologs, for the nuclear reorganization characteristic of early meiotic prophase, and for the coordination of these events with synaptonemal complex (SC) assembly. Despite the absence of homolog alignment, we find that recombination is initiated efficiently, indicating that initial pairing is not a prerequisite for early steps of the recombination pathway. Surprisingly, RAD-51-marked recombination intermediates disappear with apparent wild-type kinetics in him-3 null mutants in which homologs are spatially unavailable for recombination, raising the possibility that HIM-3's presence at chromosome axes inhibits the use of sister chromatids as templates for repair. We propose that HIM-3 is a molecular link between multiple landmark events of meiotic prophase; it is critical for establishing chromosome identity by configuring homologs to facilitate their recognition while simultaneously imposing structural constraints that later promote the formation of the crossover essential for proper segregation.     

Three Distinct Modes of Mec1/ATR and Tel1/ATM Activation Illustrate Differential Checkpoint Targeting during Budding Yeast Early Meiosis

Recombination and synapsis of homologous chromosomes are hallmarks of meiosis in many organisms. Meiotic recombination is initiated by Spo11-induced DNA double-strand breaks (DSBs), whereas chromosome synapsis is mediated by a tripartite structure named the synaptonemal complex (SC). Previously, we proposed that budding yeast SC is assembled via noncovalent interactions between the axial SC protein Red1, SUMO chains or conjugates, and the central SC protein Zip1. Incomplete synapsis and unrepaired DNA are monitored by Mec1/Tel1-dependent checkpoint responses that prevent exit from the pachytene stage. Here, our results distinguished three distinct modes of Mec1/Tec1 activation during early meiosis that led to phosphorylation of three targets, histone H2A at S129 (γH2A), Hop1, and Zip1, which are involved, respectively, in DNA replication, the interhomolog recombination and chromosome synapsis checkpoint, and destabilization of homology-independent centromere pairing. γH2A phosphorylation is Red1 independent and occurs prior to Spo11-induced DSBs. DSB- and Red1-dependent Hop1 phosphorylation is activated via interaction of the Red1-SUMO chain/conjugate ensemble with the Ddc1-Rad17-Mec3 (9-1-1) checkpoint complex and the Mre11-Rad50-Xrs2 complex. During SC assembly, Zip1 outcompetes 9-1-1 from the Red1-SUMO chain ensemble to attenuate Hop1 phosphorylation. In contrast, chromosome synapsis cannot attenuate DSB-dependent and Red1-independent Zip1 phosphorylation. These results reveal how DNA replication, DSB repair, and chromosome synapsis are differentially monitored by the meiotic checkpoint network.     

A novel nonnull ZIP1 allele triggers meiotic arrest with synapsed chromosomes in Saccharomyces cerevisiae

During meiotic prophase, assembly of the synaptonemal complex (SC) brings homologous chromosomes into close apposition along their lengths. The Zip1 protein is a major building block of the SC in Saccharomyces cerevisiae. In the absence of Zip1, SC fails to form, cells arrest or delay in meiotic prophase (depending on strain background), and crossing over is reduced. We created a novel allele of ZIP1, zip1-4LA, in which four leucine residues in the central coiled-coil domain have been replaced by alanines. In the zip1-4LA mutant, apparently normal SC assembles with wild-type kinetics; however, crossing over is delayed and decreased compared to wild type. The zip1-4LA mutant undergoes strong checkpoint-induced arrest in meiotic prophase; the defect in cell cycle progression is even more severe than that of the zip1 null mutant. When the zip1-4LA mutation is combined with the pch2 checkpoint mutation, cells sporulate with wild-type efficiency and crossing over occurs at wild-type levels. This result suggests that the zip1-4LA defect in recombination is an indirect consequence of cell cycle arrest. Previous studies have suggested that the Pch2 protein acts in a checkpoint pathway that monitors chromosome synapsis. We hypothesize that the zip1-4LA mutant assembles aberrant SC that triggers the synapsis checkpoint.     

The budding yeast Msh4 protein functions in chromosome synapsis and the regulation of crossover distribution.

The budding yeast MSH4 gene encodes a MutS homolog produced specifically in meiotic cells. Msh4 is not required for meiotic mismatch repair or gene conversion, but it is required for wild-type levels of crossing over. Here, we show that a msh4 null mutation substantially decreases crossover interference. With respect to the defect in interference and the level of crossing over, msh4 is similar to the zip1 mutant, which lacks a structural component of the synaptonemal complex (SC). Furthermore, epistasis tests indicate that msh4 and zip1 affect the same subset of meiotic crossovers. In the msh4 mutant, SC formation is delayed compared to wild type, and full synapsis is achieved in only about half of all nuclei. The simultaneous defects in synapsis and interference observed in msh4 (and also zip1 and ndj1/tam1) suggest a role for the SC in mediating interference. The Msh4 protein localizes to discrete foci on meiotic chromosomes and colocalizes with Zip2, a protein involved in the initiation of chromosome synapsis. Both Zip2 and Zip1 are required for the normal localization of Msh4 to chromosomes, raising the possibility that the zip1 and zip2 defects in crossing over are indirect, resulting from the failure to localize Msh4 properly.     

A Quality Control Mechanism Coordinates Meiotic Prophase Events to Promote Crossover Assurance

Meiotic chromosome segregation relies on homologous chromosomes being linked by at least one crossover, the obligate crossover. Homolog pairing, synapsis and meiosis specific DNA repair mechanisms are required for crossovers but how they are coordinated to promote the obligate crossover is not well understood. PCH-2 is a highly conserved meiotic AAA+-ATPase that has been assigned a variety of functions; whether these functions reflect its conserved role has been difficult to determine. We show that PCH-2 restrains pairing, synapsis and recombination in C. elegans. Loss of pch-2 results in the acceleration of synapsis and homolog-dependent meiotic DNA repair, producing a subtle increase in meiotic defects, and suppresses pairing, synapsis and recombination defects in some mutant backgrounds. Some defects in pch-2 mutants can be suppressed by incubation at lower temperature and these defects increase in frequency in wildtype worms grown at higher temperature, suggesting that PCH-2 introduces a kinetic barrier to the formation of intermediates that support pairing, synapsis or crossover recombination. We hypothesize that this kinetic barrier contributes to quality control during meiotic prophase. Consistent with this possibility, defects in pch-2 mutants become more severe when another quality control mechanism, germline apoptosis, is abrogated or meiotic DNA repair is mildly disrupted. PCH-2 is expressed in germline nuclei immediately preceding the onset of stable homolog pairing and synapsis. Once chromosomes are synapsed, PCH-2 localizes to the SC and is removed in late pachytene, prior to SC disassembly, correlating with when homolog-dependent DNA repair mechanisms predominate in the germline. Indeed, loss of pch-2 results in premature loss of homolog access. Altogether, our data indicate that PCH-2 coordinates pairing, synapsis and recombination to promote crossover assurance. Specifically, we propose that the conserved function of PCH-2 is to destabilize pairing and/or recombination intermediates to slow their progression and ensure their fidelity during meiotic prophase.     

The CSN/COP9 Signalosome Regulates Synaptonemal Complex Assembly during Meiotic Prophase I of Caenorhabditis elegans

The synaptonemal complex (SC) is a conserved protein structure that holds homologous chromosome pairs together throughout much of meiotic prophase I. It is essential for the formation of crossovers, which are required for the proper segregation of chromosomes into gametes. The assembly of the SC is likely to be regulated by post-translational modifications. The CSN/COP9 signalosome has been shown to act in many pathways, mainly via the ubiquitin degradation/proteasome pathway. Here we examine the role of the CSN/COP9 signalosome in SC assembly in the model organism C. elegans. Our work shows that mutants in three subunits of the CSN/COP9 signalosome fail to properly assemble the SC. In these mutants, SC proteins aggregate, leading to a decrease in proper pairing between homologous chromosomes. The reduction in homolog pairing also results in an accumulation of recombination intermediates and defects in repair of meiotic DSBs to form the designated crossovers. The effect of the CSN/COP9 signalosome mutants on synapsis and crossover formation is due to increased neddylation, as reducing neddylation in these mutants can partially suppress their phenotypes. We also find a marked increase in apoptosis in csn mutants that specifically eliminates nuclei with aggregated SC proteins. csn mutants exhibit defects in germline proliferation, and an almost complete pachytene arrest due to an inability to activate the MAPK pathway. The work described here supports a previously unknown role for the CSN/COP9 signalosome in chromosome behavior during meiotic prophase I.     

Synaptonemal complex-dependent centromeric clustering and the initiation of synapsis in Drosophila oocytes

The pairing of homologous chromosomes and the intimate synapsis of the paired homologs by the synaptonemal complex (SC) are essential for subsequent meiotic processes including recombination and chromosome segregation. Here we show that the centromere clustering plays an important role in initiating homolog synapsis during meiosis in Drosophila females. Although centromeres are not clustered prior to the onset of meiosis, all four pairs of centromeres are actively clustered into one or two masses during early meiotic prophase. Within the 16-cell cyst, centromeric clustering appears to define the first step in the initiation of synapsis. Clustering is restricted to the nuclei that form the SC and is dependent on all known SC proteins. Surprisingly, both centromeric clusters and the SC components associated with them persist long after the disassembly of the euchromatic SC at the end of pachytene. The initiation of homologous recombination through the formation of programmed double-strand breaks (DSBs) is not required for either the formation or the maintenance of the centromeric clusters. Our data support a view in which the SC-mediated clustering at the centromeres is the initiating event for meiotic synapsis.     

A Role for SUMO in meiotic chromosome synapsis

During meiotic prophase, homologous chromosomes engage in a complex series of interactions that ensure their proper segregation at meiosis I. A central player in these interactions is the synaptonemal complex (SC), a proteinaceous structure elaborated along the lengths of paired homologs. In mutants that fail to make SC, crossing over is decreased, and chromosomes frequently fail to recombine; consequently, many meiotic products are inviable because of aneuploidy. Here, we have investigated the role of the small ubiquitin-like protein modifier (SUMO) in SC formation during meiosis in budding yeast. We show that SUMO localizes specifically to synapsed regions of meiotic chromosomes and that this localization depends on Zip1, a major building block of the SC. A non-null allele of the UBC9 gene, which encodes the SUMO-conjugating enzyme, impairs Zip1 polymerization along chromosomes. The Ubc9 protein localizes to meiotic chromosomes, coincident with SUMO staining. In the zip1 mutant, SUMO localizes to discrete foci on chromosomes. These foci coincide with axial associations, where proteins involved in synapsis initiation are located. Our data suggest a model in which SUMO modification of chromosomal proteins promotes polymerization of Zip1 along chromosomes. The ubc9 mutant phenotype provides the first evidence for a cause-and-effect relationship between sumoylation and synapsis.     

Zip3 provides a link between recombination enzymes and synaptonemal complex proteins

In budding yeast, absence of the meiosis-specific Zip3 protein (also known as Cst9) causes synaptonemal complex formation to be delayed and incomplete. The Zip3 protein colocalizes with Zip2 at discrete foci on meiotic chromosomes, corresponding to the sites where synapsis initiates. Observations suggest that Zip3 promotes synapsis by recruiting the Zip2 protein to chromosomes and/or stabilizing the association of Zip2 with chromosomes. Zip3 interacts with a number of gene products involved in meiotic recombination, including proteins that act at both early (Mre11, Rad51, and Rad57) and late (Msh4 and Msh5) steps in the exchange process. We speculate that Zip3 is a component of recombination nodules and serves to link the initiation of synapsis to meiotic recombination.     

Immunofluorescent synaptonemal complex analysis in azoospermic men

The molecular cause of germ cell meiotic defects in azoospermic men is rarely known. During meiotic prophase I, a proteinaceous structure called the synaptonemal complex (SC) appears along the pairing axis of homologous chromosomes and meiotic recombination takes place. Newly-developed immunofluorescence techniques for SC proteins (SCP1 and SCP3) and for a DNA mismatch repair protein (MLH1) present in late recombination nodules allow simultaneous analysis of synapsis, and of meiotic recombination, during the first meiotic prophase in spermatocytes. This immunofluorescent SC analysis enables accurate meiotic prophase substaging and the identification of asynaptic pachytene spermatocytes. Spermatogenic defects were examined in azoospermic men using immunofluorescent SC and MLH1 analysis. Five males with obstructive azoospermia, 18 males with nonobstructive azoospermia and 11 control males with normal spermatogenesis were recruited for the study. In males with obstructive azoospermia, the fidelity of chromosome pairing (determined by the percentage of cells with gaps [discontinuities]/splits [unpaired chromosome regions] in the SCs, and nonexchange SCs [bivalents with 0 MLH1 foci]) was similar to those in normal males. The recombination frequencies (determined by the mean number of MLH1 foci per cell at the pachytene stage) were significantly reduced in obstructive azoospermia compared to that in controls. In men with nonobstructive azoospermia, a marked heterogeneity in spermatogenesis was found: 45% had a complete absence of meiotic cells; 5% had germ cells arrested at the zygotene stage of meiotic prophase; the rest had impaired fidelity of chromosome synapsis and significantly reduced recombination in pachytene. In addition, significantly more cells were in the leptotene and zygotene meiotic prophase stages in nonobstructive azoospermic patients, compared to controls. Defects in chromosome pairing and decreased recombination during meiotic prophase may have led to spermatogenesis arrest and contributed in part to this unexplained infertility.     

Separable Crossover-Promoting and Crossover-Constraining Aspects of Zip1 Activity during Budding Yeast Meiosis

Accurate chromosome segregation during meiosis relies on the presence of crossover events distributed among all chromosomes. MutSγ and MutLγ homologs (Msh4/5 and Mlh1/3) facilitate the formation of a prominent group of meiotic crossovers that mature within the context of an elaborate chromosomal structure called the synaptonemal complex (SC). SC proteins are required for intermediate steps in the formation of MutSγ-MutLγ crossovers, but whether the assembled SC structure per se is required for MutSγ-MutLγ-dependent crossover recombination events is unknown. Here we describe an interspecies complementation experiment that reveals that the mature SC is dispensable for the formation of Mlh3-dependent crossovers in budding yeast. Zip1 forms a major structural component of the budding yeast SC, and is also required for MutSγ and MutLγ-dependent crossover formation. Kluyveromyces lactis ZIP1 expressed in place of Saccharomyces cerevisiae ZIP1 in S. cerevisiae cells fails to support SC assembly (synapsis) but promotes wild-type crossover levels in those nuclei that progress to form spores. While stable, full-length SC does not assemble in S. cerevisiae cells expressing K. lactis ZIP1, aggregates of K. lactis Zip1 displayed by S. cerevisiae meiotic nuclei are decorated with SC-associated proteins, and K. lactis Zip1 promotes the SUMOylation of the SC central element protein Ecm11, suggesting that K. lactis Zip1 functionally interfaces with components of the S. cerevisiae synapsis machinery. Moreover, K. lactis Zip1-mediated crossovers rely on S. cerevisiae synapsis initiation proteins Zip3, Zip4, Spo16, as well as the Mlh3 protein, as do the crossovers mediated by S. cerevisiae Zip1. Surprisingly, however, K. lactis Zip1-mediated crossovers are largely Msh4/Msh5 (MutSγ)-independent. This separation-of-function version of Zip1 thus reveals that neither assembled SC nor MutSγ is required for Mlh3-dependent crossover formation per se in budding yeast. Our data suggest that features of S. cerevisiae Zip1 or of the assembled SC in S. cerevisiae normally constrain MutLγ to preferentially promote resolution of MutSγ-associated recombination intermediates.     

Synaptonemal complex damage induced by clastogenic and anti-mitotic chemicals: implications for non-disjunction and aneuploidy

Mice were treated with mitomycin C, cyclophosphamide, amsacrine, colchicine, or vinblastine sulfate, and meiotic prophase cells analyzed for synaptonemal complex (SC) damage. All test agents caused synaptonemal complex breakage and synapsis irregularities, although propensities for inducing specific types of damage at S-phase or prophase stages varied among the chemicals. The data indicate that SC analysis can reveal chemical-specific alterations to meiotic homologue pairing/synapsis which have not generally been recognized, and which theoretically may be implicated in non-disjunction.     

Protein Phosphatase 4 Promotes Chromosome Pairing and Synapsis,and Contributes to Maintaining Crossover Competence with Increasing Age

Prior to the meiotic divisions, dynamic chromosome reorganizations including pairing, synapsis, and recombination of maternal and paternal chromosome pairs must occur in a highly regulated fashion during meiotic prophase. How chromosomes identify each other''s homology and exclusively pair and synapse with their homologous partners, while rejecting illegitimate synapsis with non-homologous chromosomes, remains obscure. In addition, how the levels of recombination initiation and crossover formation are regulated so that sufficient, but not deleterious, levels of DNA breaks are made and processed into crossovers is not understood well. We show that in Caenorhabditis elegans, the highly conserved Serine/Threonine protein phosphatase PP4 homolog, PPH-4.1, is required independently to carry out four separate functions involving meiotic chromosome dynamics: (1) synapsis-independent chromosome pairing, (2) restriction of synapsis to homologous chromosomes, (3) programmed DNA double-strand break initiation, and (4) crossover formation. Using quantitative imaging of mutant strains, including super-resolution (3D-SIM) microscopy of chromosomes and the synaptonemal complex, we show that independently-arising defects in each of these processes in the absence of PPH-4.1 activity ultimately lead to meiotic nondisjunction and embryonic lethality. Interestingly, we find that defects in double-strand break initiation and crossover formation, but not pairing or synapsis, become even more severe in the germlines of older mutant animals, indicating an increased dependence on PPH-4.1 with increasing maternal age. Our results demonstrate that PPH-4.1 plays multiple, independent roles in meiotic prophase chromosome dynamics and maintaining meiotic competence in aging germlines. PP4''s high degree of conservation suggests it may be a universal regulator of meiotic prophase chromosome dynamics.     

When specialized sites are important for synapsis and the distribution of crossovers

In C. elegans and D. melanogaster, specialized sites have an important role in meiotic recombination. Recent evidence has shown that these sites in C. elegans have a role in synapsis. Here we compare the initiation of synapsis in organisms with specialized sites and those without. We propose that, early in prophase, synapsis requires an initiator to overcome inhibitory factors that function to prevent synaptonemal complex (SC) formation between nonhomologous sequences. These initiators of SC formation can be stimulated by crossover sites, possibly other types of recombination sites and also specialized sites where recombination does not occur.     

A Mec1- and PP4-dependent checkpoint couples centromere pairing to meiotic recombination

The faithful alignment of homologous chromosomes during meiotic prophase requires the coordination of DNA double-strand break (DSB) repair with large-scale chromosome reorganization. Here we identify the phosphatase PP4 (Pph3/Psy2) as a mediator of this process in Saccharomyces cerevisiae. In pp4 mutants, early stages of crossover repair and homology-independent pairing of centromeres are coordinately blocked. We traced the loss of centromere pairing to the persistent phosphorylation of the chromosomal protein Zip1 on serine 75. Zip1-S75 is a consensus site for the ATR-like checkpoint kinase Mec1, and centromere pairing is restored in mec1 mutants. Importantly, Zip1-S75 phosphorylation does not alter chromosome synapsis or DSB repair, indicating that Mec1 separates centromere pairing from the other functions of Zip1. The centromeric localization and persistent activity of PP4 during meiotic prophase suggest a model whereby Zip1-S75 phosphorylation dynamically destabilizes homology-independent centromere pairing in response to recombination initiation, thereby coupling meiotic chromosome dynamics to DSB repair.     

Separation of roles of Zip1 in meiosis revealed in heterozygous mutants of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Synapsis of homologs during meiotic prophase I is associated with a protein complex built along the bivalents—the synaptonemal complex (SC). Mutations in the SC-component gene ZIP1 diminish SC formation, leading to reduced recombination levels and low spore viability. Here we show that in SK1 strains heterozygous for a deletion of ZIP1 in certain regions meiotic interference are impaired with no decrease in recombination levels. The extent of synapsis is over all reduced and NDJ levels of a large endogenous chromosome and of artificial chromosomes (YACs) rise to twice the level of wild type strains. A substantial proportion of mis-segregating YACs had undergone crossing over. This demonstrates that different functions of Zip1 display differential sensitivities to changes in expression levels.     

Disruption of pairing and synapsis of chromosomes causes stage-specific apoptosis of male meiotic cells

During meiosis, DNA replication is followed by two successive rounds of chromosome segregation (meiosis I and II), which give rise to genetically diverse haploid gametes. The prophase of the first meiotic division is highly regulated and alignment and synapsis of the homologous chromosomes during this stage are mediated by the synaptonemal complex. Incorrect assembly of the synaptonemal complex results in cell death, impaired meiotic recombination and aneuploidy. Oocytes with meiotic defects often survive the first meiotic prophase and give rise to aneuploid gametes. Similarly affected spermatocytes, on the other hand, almost always undergo apoptosis at a male-specific meiotic checkpoint, located specifically at epithelial stage IV during spermatogenesis. Many examples of this stage IV-specific arrest have been described for several genetic mouse models in which DNA repair or meiotic recombination are abrogated. Interestingly, in C. elegans, meiotic recombination and synapsis are monitored by two separate checkpoint pathways. Therefore we studied spermatogenesis in several knockout mice (Sycp1(-/-), Sycp3(-/-), Smc1beta(-/-) and Sycp3/Sycp1 and Sycp3/Smc1beta double-knockouts) that are specifically defective in meiotic pairing and synapsis. Like for recombination defects, we found that all these genotypes also specifically arrest at epithelial stage IV. It seems that the epithelial stage IV checkpoint eliminates spermatocytes that fail a certain quality check, being either synapsis or DNA damage related.