Meiotic chromosomes: it takes two to tango

Bypass of a meiotic checkpoint by overproduction of meiotic chromosomal proteins

The Saccharomyces cerevisiae zip1 mutant, which exhibits defects in synaptonemal complex formation and meiotic recombination, triggers a checkpoint that causes cells to arrest at the pachytene stage of meiotic prophase. Overproduction of either the meiotic chromosomal protein Red1 or the meiotic kinase Mek1 bypasses this checkpoint, allowing zip1 cells to sporulate. Red1 or Mek1 overproduction also promotes sporulation of other mutants (zip2, dmc1, hop2) that undergo checkpoint-mediated arrest at pachytene. In addition, Red1 overproduction antagonizes interhomolog interactions in the zip1 mutant, substantially decreasing double-strand break formation, meiotic recombination, and homologous chromosome pairing. Mek1 overproduction, in contrast, suppresses checkpoint-induced arrest without significantly decreasing meiotic recombination. Cooverproduction of Red1 and Mek1 fails to bypass the checkpoint; moreover, overproduction of the meiotic chromosomal protein Hop1 blocks the Red1 and Mek1 overproduction phenotypes. These results suggest that meiotic chromosomal proteins function in the signaling of meiotic prophase defects and that the correct stoichiometry of Red1, Mek1, and Hop1 is needed to achieve checkpoint-mediated cell cycle arrest at pachytene.

Recombination factors of Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been an excellent genetic and biochemical model for our understanding of homologous recombination. Central to the process of homologous recombination are the products of the RAD52 epistasis group of genes, whose functions we now know include the nucleolytic processing of DNA double-stand breaks, the ability to conduct a DNA homology search, and the capacity to promote the exchange of genetic information between homologous regions on recombining chromosomes. It is also clear that the basic functions of the RAD52 group of genes have been highly conserved among eukaryotes. Disruption of this important process causes genomic instability, which can result in a number of unsavory consequences, including tumorigenesis and cell death.

The many faces of mismatch repair in meiosis

Mismatches, and the proteins that repair them, play multiple roles during meiosis from generating the diversity upon which selection acts to preventing the intermingling of diverged populations and species. The mechanisms by which the mismatch repair proteins accomplish these many roles include gene conversion, reciprocal crossing over, mismatch repair-induced recombination and anti-recombination. This review focuses on recent studies, predominantly in Saccharomyces cerevisiae, that have advanced our understanding of the details of mismatch repair complexes and how they apply to the diverse roles these proteins play in meiosis. These studies have also revealed unexpected and novel functions for some of the mismatch repair proteins.

Mitotic recombination in yeast: elements controlling its incidence

Mitotic recombination is an important mechanism of DNA repair in eukaryotic cells. Given the redundancy of the eukaryotic genomes and the presence of repeated DNA sequences, recombination may also be an important source of genomic instability. Here we review the data, mainly from the budding yeast S. cerevisiae, that may help to understand the spontaneous origin of mitotic recombination and the different elements that may control its occurrence. We cover those observations suggesting a putative role of replication defects and DNA damage, including double-strand breaks, as sources of mitotic homologous recombination. An important part of the review is devoted to the experimental evidence suggesting that transcription and chromatin structure are important factors modulating the incidence of mitotic recombination. This is of great relevance in order to identify the causes and risk factors of genomic instability in eukaryotes.

Increased frequency of mutations in DNA from infertile men with meiotic arrest

In diverse organisms from yeast to mice, mutations in numerous genes required for DNA repair may lead to defects in meiosis. Although it is likely that meiosis is conserved throughout evolution, little is known about the genetics of meiosis in humans even though meiotic arrest associated with azoospermia is common. In this work, we compared the sequence fidelity of a polymorphic marker amplified from DNA of two groups of patients: those with testis biopsy suggesting meiotic arrest and those with normal spermatogenesis who were obstructed. We demonstrated that mutations are more common in DNA from testicular tissue derived from men with meiotic arrest than in DNA from testicular tissue derived from men with normal spermatogenesis and physical obstruction (P < 0.05). No mutations were observed in blood tissue from either group of men. This suggests the possibility that defects in genes required in DNA repair could contribute to meiotic arrest in men just as has been observed in other organisms.

Characterization of human crossover interference

We present an analysis of crossover interference over the entire human genome, on the basis of genotype data from more than 8,000 polymorphisms in eight CEPH families. Overwhelming evidence was found for strong positive crossover interference, with average strength lying between the levels of interference implied by the Kosambi and Carter-Falconer map functions. Five mathematical models of interference were evaluated: the gamma model and four versions of the count-location model. The gamma model fit the data far better than did any of the other four models. Analysis of intercrossover distances was greatly superior to the analysis of crossover counts, in both demonstrating interference and distinguishing between the five models. In contrast to earlier suggestions, interference was found to continue uninterrupted across the centromeres. No convincing differences in the levels of interference were found between the sexes or among chromosomes; however, we did detect possible individual variation in interference among the eight mothers. Finally, we present an equation that provides the probability of the occurrence of a double crossover between two nonrecombinant, informative polymorphisms.

DNA repair protein Rad55 is a terminal substrate of the DNA damage checkpoints

Checkpoints, which are integral to the cellular response to DNA damage, coordinate transient cell cycle arrest and the induced expression of DNA repair genes after genotoxic stress. DNA repair ensures cellular survival and genomic stability, utilizing a multipathway network. Here we report evidence that the two systems, DNA damage checkpoint control and DNA repair, are directly connected by demonstrating that the Rad55 double-strand break repair protein of the recombinational repair pathway is a terminal substrate of DNA damage and replication block checkpoints. Rad55p was specifically phosphorylated in response to DNA damage induced by the alkylating agent methyl methanesulfonate, dependent on an active DNA damage checkpoint. Rad55p modification was also observed after gamma ray and UV radiation. The rapid time course of phosphorylation and the recombination defects identified in checkpoint-deficient cells are consistent with a role of the DNA damage checkpoint in activating recombinational repair. Rad55p phosphorylation possibly affects the balance between different competing DNA repair pathways.

The nucleolus: the magician's hat for cell cycle tricks

The nucleolus, for decades considered a ribosome factory and site for ribosomal RNA synthesis and processing, has recently acquired new fame. Analyses of proteins important for cell-cycle regulation have shown that this organelle is used to sequester proteins, thereby inhibiting their activity.

Multiple roles of Spo11 in meiotic chromosome behavior

Spo11, a type II topoisomerase, is likely to be required universally for initiation of meiotic recombination. However, a dichotomy exists between budding yeast and the animals Caenorhabditis elegans and Drosophila melanogaster with respect to additional roles of Spo11 in meiosis. In Saccharomyces cerevisiae, Spo11 is required for homolog pairing, as well as axial element (AE) and synaptonemal complex (SC) formation. All of these functions are Spo11 independent in C.elegans and D.melanogaster. We examined Spo11 function in a multicellular fungus, Coprinus cinereus. The C.cinereus spo11-1 mutant shows high levels of homolog pairing and occasionally forms full-length AEs, but no SC. In C.cinereus, Spo11 is also required for maintenance of meiotic chromosome condensation and proper spindle formation. Meiotic progression in spo11-1 is aberrant; late in meiosis basidia undergo programmed cell death (PCD). To our knowledge, this is the first example of meiotic PCD outside the animal kingdom. Ionizing radiation can partially rescue spo11-1 for both AE and SC formation and viable spore production, suggesting that the double-strand break function of Spo11 is conserved and is required for these functions.

Life history and developmental processes in the basidiomycete Coprinus cinereus

Coprinus cinereus has two main types of mycelia, the asexual monokaryon and the sexual dikaryon, formed by fusion of compatible monokaryons. Syngamy (plasmogamy) and karyogamy are spatially and temporally separated, which is typical for basidiomycetous fungi. This property of the dikaryon enables an easy exchange of nuclear partners in further dikaryotic-monokaryotic and dikaryotic-dikaryotic mycelial fusions. Fruiting bodies normally develop on the dikaryon, and the cytological process of fruiting-body development has been described in its principles. Within the specialized basidia, present within the gills of the fruiting bodies, karyogamy occurs in a synchronized manner. It is directly followed by meiosis and by the production of the meiotic basidiospores. The synchrony of karyogamy and meiosis has made the fungus a classical object to study meiotic cytology and recombination. Several genes involved in these processes have been identified. Both monokaryons and dikaryons can form multicellular resting bodies (sclerotia) and different types of mitotic spores, the small uninucleate aerial oidia, and, within submerged mycelium, the large thick-walled chlamydospores. The decision about whether a structure will be formed is made on the basis of environmental signals (light, temperature, humidity, and nutrients). Of the intrinsic factors that control development, the products of the two mating type loci are most important. Mutant complementation and PCR approaches identified further genes which possibly link the two mating-type pathways with each other and with nutritional regulation, for example with the cAMP signaling pathway. Among genes specifically expressed within the fruiting body are those for two galectins, beta-galactoside binding lectins that probably act in hyphal aggregation. These genes serve as molecular markers to study development in wild-type and mutant strains. The isolation of genes for potential non-DNA methyltransferases, needed for tissue formation within the fruiting body, promises the discovery of new signaling pathways, possibly involving secondary fungal metabolites.

MutS homolog 4 localization to meiotic chromosomes is required for chromosome pairing during meiosis in male and female mice

Msh4 (MutS homolog 4) is a member of the mammalian mismatch repair gene family whose members are involved in postreplicative DNA mismatch repair as well as in the control of meiotic recombination. In this report we show that MSH4 has an essential role in the control of male and female meiosis. We demonstrate that MSH4 is present in the nuclei of spermatocytes early in prophase I and that it forms discrete foci along meiotic chromosomes during the zygotene and pachytene stages of meiosis. Disruption of the Msh4 gene in mice results in male and female sterility due to meiotic failure. Although meiosis is initiated in Msh4 mutant male and female mice, as indicated by the chromosomal localization of RAD51 and COR1 during leptonema/zygonema, the chromosomes fail to undergo normal pairing. Our results show that MSH4 localization on chromosomes during the early stages of meiosis is essential for normal chromosome synapsis in prophase I and that it acts in the same pathway as MSH5.

Meiotic DNA breaks associated with recombination in S. pombe

In the fission yeast Schizosaccharomyces pombe, we have detected prominent DNA breaks that appeared shortly after premeiotic DNA replication. These breaks, like meiotic recombination, required the products of the six rec genes tested. Prominent breaks were detected at widely separated sites, about 100-300 kb apart, equivalent to about 50-150 sites per genome or approximately the number of meiotic recombination events. Certain features of these breaks are similar to those in the distantly related yeast Saccharomyces cerevisiae, the only other organism in which meiotic DNA breaks have been reported. Other features, however, appear to be different. These results suggest that, although DNA breaks may be a general feature of meiotic recombination, the breaks in S. pombe may play a role different from those in S. cerevisiae.

Pachytene exit controlled by reversal of Mek1-dependent phosphorylation

During yeast meiosis, a checkpoint prevents exit from pachytene in response to defects in meiotic recombination and chromosome synapsis. This pachytene checkpoint requires two meiotic chromosomal proteins, Red1 and Mek1; Mek1 is a kinase that phosphorylates Red1. In mutants that undergo checkpoint-mediated pachytene arrest, Mek1 is active and Red1 remains phosphorylated. Activation of Mek1 requires the initiation of meiotic recombination and certain DNA damage checkpoint proteins. Mek1 kinase activity and checkpoint-induced pachytene arrest are counteracted by protein phosphatase type 1 (Glc7). Glc7 coimmunoprecipitates with Red1, colocalizes with Red1 on chromosomes, and dephosphorylates Red1 in vitro. We speculate that phosphorylated Red1 prevents exit from pachytene and that completion of meiotic recombination triggers Glc7-dependent dephosphorylation of Red1.

Molecular characterisation of two paralogous SPO11 homologues in Arabidopsis thaliana

The Spo11 protein of yeast has been found to be covalently bound to double-strand breaks in meiosis, demonstrating a unique role of the protein in the formation of these breaks. Homologues of the SPO11 gene have been found in various eukaryotes, indicating that the machinery involved in meiotic recombination is conserved in eukaryotes. Here we report on SPO11 homologues in plants. In contrast to what is known from other eukaryotes, Arabidopsis thaliana carries in its genome at least two SPO11 homologues, AtSPO11-1 and AtSPO11-2. Both genes are not more closely related to each other than to other eukaryotic SPO11 homologues, indicating that they did not arise via a recent duplication event during higher plant evolution. For both genes three different poly-adenylation sites were found. AtSPO11-1 is expressed not only in generative but also to a lesser extent in somatic tissues. We were able to detect in different organs various AtSPO11-1 cDNAs in which introns were differently spliced-a surprising phenomenon also reported for SPO11 homologues in mammals. In the case of AtSPO11-2 we found that the 3' end of the mRNA is overlapping with a mRNA produced by a gene located in inverse orientation next to it. This points to a possible antisense regulation mechanism. Our findings hint to the intriguing possibility that, at least for plants, Spo11-like proteins might have more and possibly other biological functions than originally anticipated for yeast.

Meiotic chromosomes: integrating structure and function

Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, a feature which greatly facilitates the study of chromosome behavior. The complex events involved in homolog juxtaposition necessitate prolongation of prophase, thus permitting resolution of events that are temporally compressed in the mitotic cycle. Furthermore, once homologs are paired, the chromosomes are connected by a specific structure: the synaptonemal complex. Finally, interaction of homologs includes recombination at the DNA level, which is intimately linked to structural features of the chromosomes. In consequence, recombination-related events report on diverse aspects of chromosome morphogenesis, notably relationships between sisters, development of axial structure, and variations in chromatin status. The current article reviews recent information on these topics in an historical context. This juxtaposition has suggested new relationships between structure and function. Additional issues were addressed in a previous chapter (551).

Evidence for the coincident initiation of homolog pairing and synapsis during the telomere-clustering (bouquet) stage of meiotic prophase

To improve knowledge of the prerequisites for meiotic chromosome segregation in higher eukaryotes, we analyzed the spatial distribution of a pair of homologs before and during early meiotic prophase. Three-dimensional images of fluorescence in situ hybridization (FISH) were used to localize a single pair of homologs in diploid nuclei of a chromosome-addition line of oat, oat-maize9b. The system provided a robust assay for pairing based on cytological colocalization of FISH signals. Using a triple labeling scheme for simultaneous imaging of chromatin, telomeres and the homolog pair, we determined the timing of pairing in relation to the onset of three sequential hallmarks of early meiotic prophase: chromatin condensation (the leptotene stage), meiotic telomere clustering (the bouquet stage) and the initiation of synapsis (the zygotene stage). We found that the two homologs were mostly unpaired up through middle leptotene, at which point their spherical cloud-like domains began to transform into elongated and stretched-out domains. At late leptotene, the homologs had completely reorganized into long extended fibers, and the beginning of the bouquet stage was conspicuously marked by the de novo clustering of telomeres at the nuclear periphery. The homologs paired and synapsed during the bouquet stage, consistent with the timing of pairing observed for several oat 5S rDNA loci. In summary, results from analysis of more than 100 intact nuclei lead us to conclude that pairing and synapsis of homologous chromosomes are largely coincident processes, ruling out a role for premeiotic pairing in this system. These findings suggest that the genome-wide remodeling of chromatin and telomere-mediated nuclear reorganization are prerequisite steps to the DNA sequence-based homology-search process in higher eukaryotes.

A conserved checkpoint pathway mediates DNA damage--induced apoptosis and cell cycle arrest in C. elegans

To maintain genomic stability following DNA damage, multicellular organisms activate checkpoints that induce cell cycle arrest or apoptosis. Here we show that genotoxic stress blocks cell proliferation and induces apoptosis of germ cells in the nematode C. elegans. Accumulation of recombination intermediates similarly leads to the demise of affected cells. Checkpoint-induced apoptosis is mediated by the core apoptotic machinery (CED-9/CED-4/CED-3) but is genetically distinct from somatic cell death and physiological germ cell death. Mutations in three genes--mrt-2, which encodes the C. elegans homolog of the S. pombe rad1 checkpoint gene, rad-5, and him-7-block both DNA damage-induced apoptosis and cell proliferation arrest. Our results implicate rad1 homologs in DNA damage-induced apoptosis in animals.

Progression of meiotic DNA replication is modulated by interchromosomal interaction proteins, negatively by Spo11p and positively by Rec8p

Spo11p is a key mediator of interhomolog interactions during meiosis. Deletion of the SPO11 gene decreases the length of S phase by ∼25%. Rec8p is a key coordinator of meiotic interhomolog and intersister interactions. Deletion of the REC8 gene increases S-phase length, by ∼10% in wild-type and ∼30% in aspo11Δ background. Thus, the progression of DNA replication is modulated by interchromosomal interaction proteins. Thespo11–Y135F DSB (double strand break) catalysis-defective mutant is normal for S-phase modulation and DSB-independent homolog pairing but is defective for later events, formation of DSBs, and synaptonemal complexes. Thus, earlier and later functions of Spo11 are defined. We propose that meiotic S-phase progression is linked directly to development of specific chromosomal features required for meiotic interhomolog interactions and that this feedback process is built upon a more fundamental mechanism, common to all cell types, by which S-phase progression is coupled to development of nascent intersister connections and/or related aspects of chromosome morphogenesis. Roles for Rec8 and/or Spo11 in progression through other stages of meiosis are also revealed.

Association of mammalian SMC1 and SMC3 proteins with meiotic chromosomes and synaptonemal complexes

In somatic cells, the heterodimeric Structural Maintenance of Chromosomes (SMC) proteins are involved in chromosome condensation and gene dosage compensation (SMC2 and 4), and sister chromatid cohesion and DNA recombination (SMC1 and 3). We report here evidence for an involvement of mammalian SMC1 and SMC3 proteins in meiosis. Immunofluorescence analysis of testis sections showed intense chromatin association in meiotic prophase cells, weaker staining in round spermatids and absence of the SMC proteins in elongated spermatids. In spermatocyte nuclei spreads, the SMC1 and SMC3 proteins localize in a beaded structure along the axial elements of synaptonemal complexes of pachytene and diplotene chromosomes. Both SMC proteins are present in rat spermatocytes and enriched in preparations of synaptonemal complexes. Several independent experimental approaches revealed interactions of the SMC proteins with synaptonemal complex-specific proteins SCP2 and SCP3. These results suggest a model for the arrangement of SMC proteins in mammalian meiotic chromatin.

Expression and nuclear localization of BLM, a chromosome stability protein mutated in Bloom's syndrome, suggest a role in recombination during meiotic prophase

Bloom's syndrome (BS) is a recessive human genetic disorder characterized by short stature, immunodeficiency and elevated risk of malignancy. BS cells have genomic instability and an increased frequency of sister chromatid exchange. The gene mutated in BS, BLM, encodes a 3′-5′ helicase (BLM) with homology to bacterial recombination factor, RecQ. Human males homozygous for BLM mutations are infertile and heterozygous individuals display increased frequencies of structural chromosome abnormalities in their spermatozoa. Also, mutations in the Saccharomyces cerevisiae homolog of BLM, Sgs1, cause a delay in meiotic nuclear division and a reduction in spore viability. These observations suggest that BLM may play a role during meiosis. Our antibodies raised against the C terminus of the human protein specifically recognize both mouse and human BLM in western blots of cell lines and in successive developmental stages of spermatocytes, but fail to detect BLM protein in a cell line with a C-terminally truncated protein. BLM protein expression and location are detected by immunofluorescence and immunoelectron microscopy as discrete foci that are sparsely present on early meiotic prophase chromosome cores, later found abundantly on synapsed cores, frequently in combination with the recombinases RAD51 and DMC1, and eventually as pure BLM foci. The colocalization of RAD51/DMC1 with BLM and the statistically significant excess of BLM signals in the synapsed pseudoautosomal region of the X-Y chromosomes, which is a recombinational hot spot, provide indications that BLM protein may function in the meiotic recombination process.

Organization of the yeast Zip1 protein within the central region of the synaptonemal complex

The yeast Zip1 protein is a component of the central region of the synaptonemal complex (SC). Zip1 is predicted to form an alpha-helical coiled coil, flanked by globular domains at the NH(2) and COOH termini. Immunogold labeling with domain-specific anti-Zip1 antibodies demonstrates that the NH(2)-terminal domain of Zip1 is located in the middle of the central region of the SC, whereas the COOH-terminal domain is embedded in the lateral elements of the complex. Previous studies have shown that overproduction of Zip1 results in the assembly of two types of aggregates, polycomplexes and networks, that are unassociated with chromatin. Our epitope mapping data indicate that the organization of Zip1 within polycomplexes is similar to that of the SC, whereas the organization of Zip1 within networks is fundamentally different. Zip1 protein purified from bacteria assembles into dimers in vitro, and electron microscopic analysis demonstrates that the two monomers within a dimer are arranged in parallel and in register. Together, these results suggest that two Zip1 dimers, lying head-to-head, span the width of the SC.

Suppressed recombination and a pairing anomaly on the mating-type chromosome of Neurospora tetrasperma

Neurospora crassa and related heterothallic ascomycetes produce eight homokaryotic self-sterile ascospores per ascus. In contrast, asci of N. tetrasperma contain four self-fertile ascospores each with nuclei of both mating types (matA and mata). The self-fertile ascospores of N. tetrasperma result from first-division segregation of mating type and nuclear spindle overlap at the second meiotic division and at a subsequent mitotic division. Recently, Merino et al. presented population-genetic evidence that crossing over is suppressed on the mating-type chromosome of N. tetrasperma, thereby preventing second-division segregation of mating type and the formation of self-sterile ascospores. The present study experimentally confirmed suppressed crossing over for a large segment of the mating-type chromosome by examining segregation of markers in crosses of wild strains. Surprisingly, our study also revealed a region on the far left arm where recombination is obligatory. In cytological studies, we demonstrated that suppressed recombination correlates with an extensive unpaired region at pachytene. Taken together, these results suggest an unpaired region adjacent to one or more paired regions, analogous to the nonpairing and pseudoautosomal regions of animal sex chromosomes. The observed pairing and obligate crossover likely reflect mechanisms to ensure chromosome disjunction.

Yeast meiosis-specific protein Hop1 binds to G4 DNA and promotes its formation

DNA molecules containing stretches of contiguous guanine residues can assume a stable configuration in which planar quartets of guanine residues joined by Hoogsteen pairing appear in a stacked array. This conformation, called G4 DNA, has been implicated in several aspects of chromosome behavior including immunoglobulin gene rearrangements, promoter activation, and telomere maintenance. Moreover, the ability of the yeast SEP1 gene product to cleave DNA in a G4-DNA-dependent fashion, as well as that of the SGS1 gene product to unwind G4 DNA, has suggested a crucial role for this structure in meiotic synapsis and recombination. Here, we demonstrate that the HOP1 gene product, which plays a crucial role in the formation of synaptonemal complex in Saccharomyces cerevisiae, binds robustly to G4 DNA. The apparent dissociation constant for interaction with G4 DNA is 2 x 10(-10), indicative of binding that is about 1,000-fold stronger than to normal duplex DNA. Oligonucleotides of appropriate sequence bound Hop1 protein maximally if the DNA was first subjected to conditions favoring the formation of G4 DNA. Furthermore, incubation of unfolded oligonucleotides with Hop1 led to their transformation into G4 DNA. Methylation interference experiments confirmed that modifications blocking G4 DNA formation inhibit Hop1 binding. In contrast, neither bacterial RecA proteins that preferentially interact with GT-rich DNA nor histone H1 bound strongly to G4 DNA or induced its formation. These findings implicate specific interactions of Hop1 protein with G4 DNA in the pathway to chromosomal synapsis and recombination in meiosis.

Down syndrome: genetic recombination and the origin of the extra chromosome 21

Despite the clinical importance of trisomy 21, we have been ignorant of the causes of meiotic nondisjunction of chromosome 21. Recently, however, genetic mapping studies of trisomy 21 families have led to the identification of the first molecular correlate of human nondisjunction; i.e. altered levels and positioning of meiotic recombinational events. Specifically, increases in 0 exchange events or in distal-only or pericentromeric exchanges are significantly increased in trisomy 21-generating meioses. These observations have led to the idea that chromosome 21 nondisjunction requires 'two hits': first, the establishment in prophase I of a 'vulnerable' bivalent and second, abnormal processing of the bivalent at metaphase I or II.

An integration of old and new perspectives of mammalian meiotic sterility

The many events of meiotic prophase can now be viewed as a series of specialized incidents that are monitored by meiotic checkpoints, some of which are similar to their mitotic counterparts, and some of which are probably unique to meiosis. This shift in perspective means that meiotic sterility in mammals must be reexamined and viewed as the result of errors subject to meiotic checkpoint controls. Like their mitotic counterparts, the meiotic checkpoints detect defects and halt normal progression until these mistakes can be repaired. Some of these checkpoints utilize mitotic checkpoint proteins, others may involve meiotic-specific proteins, or splice forms. If repair is impossible, the checkpoints then either trigger immediate apoptosis or cause an arrest of meiotic progression followed by eventual cell death. If a sufficient number of spermatocytes are involved, either alternative results in sterility. Identification of these meiotic checkpoints and delineation of the signal transduction cascades involved has only just begun. While yeast, or other model organisms, may provide clues to some of these pathways, others appears to have arisen during vertebrate evolution. The study of mammalian meiosis has entered a new era and the foundations are being laid for a growing understanding of the many problems that may contribute to sterility.

The murine SCP3 gene is required for synaptonemal complex assembly, chromosome synapsis, and male fertility

During meiosis, the homologous chromosomes pair and recombine. An evolutionarily conserved protein structure, the synaptonemal complex (SC), is located along the paired meiotic chromosomes. We have studied the function of a structural component in the axial/lateral element of the SC, the synaptonemal complex protein 3 (SCP3). A null mutation in the SCP3 gene was generated, and we noted that homozygous mutant males were sterile due to massive apoptotic cell death during meiotic prophase. The SCP3-deficient male mice failed to form axial/lateral elements and SCs, and the chromosomes in the mutant spermatocytes did not synapse. While the absence of SCP3 affected the nuclear distribution of DNA repair and recombination proteins (Rad51 and RPA), as well as synaptonemal complex protein 1 (SCP1), a residual chromatin organization remained in the mutant meiotic cells.

Splicing of the meiosis-specific HOP2 transcript utilizes a unique 5' splice site

The Saccharomyces cerevisiae HOP2 gene is required to prevent formation of synaptonemal complex between nonhomologous chromosomes during meiosis. The HOP2 gene is expressed specifically in meiotic cells, with the transcript reaching maximum abundance early in meiotic prophase. The HOP2 coding region is interrupted by an intron located near the 5' end of the gene. This intron contains a nonconsensus 5' splice site (GUUAAGU) that differs from the consensus 5' splice signal (GUAPyGU) by the insertion of a nucleotide and by a single nucleotide substitution. Bases flanking the HOP2 5' splice site have the potential to pair with sequences in U1 small nuclear RNA, and mutations disrupting this pairing reduce splicing efficiency. HOP2 pre-mRNA is spliced efficiently in the absence of the Mer1 and Nam8 proteins, which are required for splicing the transcripts of two other meiosis-specific genes.

Functional specificity of MutL homologs in yeast: evidence for three Mlh1-based heterocomplexes with distinct roles during meiosis in recombination and mismatch correction

The yeast genome encodes four proteins (Pms1 and Mlh1-3) homologous to the bacterial mismatch repair component, MutL. Using two hybrid-interaction and coimmunoprecipitation studies, we show that these proteins can form only three types of complexes in vivo. Mlh1 is the common component of all three complexes, interacting with Pms1, Mlh2, and Mlh3, presumptively as heterodimers. The phenotypes of single deletion mutants reveal distinct functions for the three heterodimers during meiosis: in a pms1 mutant, frequent postmeiotic segregation indicates a defect in the correction of heteroduplex DNA, whereas the frequency of crossing-over is normal. Conversely, crossing-over in the mlh3 mutant is reduced to approximately 70% of wild-type levels but correction of heteroduplex is normal. In a mlh2 mutant, crossing-over is normal and postmeiotic segregation is not observed but non-Mendelian segregation is elevated and altered with respect to parity. Finally, to a first approximation, the mlh1 mutant represents the combined single mutant phenotypes. Taken together, these data imply modulation of a basic Mlh1 function via combination with the three other MutL homologs and suggest specifically that Mlh1 combines with Mlh3 to promote meiotic crossing-over.

Crossing over during Caenorhabditis elegans meiosis requires a conserved MutS-based pathway that is partially dispensable in budding yeast

Formation of crossovers between homologous chromosomes during Caenorhabditis elegans meiosis requires the him-14 gene. Loss of him-14 function severely reduces crossing over, resulting in lack of chiasmata between homologs and consequent missegregation. Cytological analysis showing that homologs are paired and aligned in him-14 pachytene nuclei, together with temperature-shift experiments showing that him-14 functions during the pachytene stage, indicate that him-14 is not needed to establish pairing or synapsis and likely has a more direct role in crossover formation. him-14 encodes a germline-specific member of the MutS family of DNA mismatch repair (MMR) proteins. him-14 has no apparent role in MMR, but like its Saccharomyces cerevisiae ortholog MSH4, has a specialized role in promoting crossing over during meiosis. Despite this conservation, worms and yeast differ significantly in their reliance on this pathway: whereas worms use this pathway to generate most, if not all, crossovers, yeast still form 30-50% of their normal number of crossovers when this pathway is absent. This differential reliance may reflect differential stability of crossover-competent recombination intermediates, or alternatively, the presence of two different pathways for crossover formation in yeast, only one of which predominates during nematode meiosis. We discuss a model in which HIM-14 promotes crossing over by interfering with Holliday junction branch migration.

The pachytene checkpoint in S. cerevisiae depends on Swe1-mediated phosphorylation of the cyclin-dependent kinase Cdc28

Mutants defective in meiotic recombination and synaptonemal complex formation undergo checkpoint-mediated arrest in mid-meiotic prophase. In S. cerevisiae, this checkpoint requires Swe1, which phosphorylates and inactivates the cyclin-dependent kinase Cdc28. A swe1 deletion allows mutants that normally arrest in meiotic prophase to sporulate at wild-type levels, though sporulation is delayed. This delay is eliminated by overproducing Clb1, the major cyclin required for meiosis I. The Swe1 protein accumulates and is hyperphosphorylated in checkpoint-arrested cells. Our results suggest that meiotic arrest is mediated both by increasing Swe1 activity and limiting cyclin production, with Swe1 being the primary downstream target of checkpoint control. The requirement for Swe1 distinguishes the pachytene checkpoint from the DNA damage checkpoints operating in vegetative cells.

Control of meiotic recombination and gene expression in yeast by a simple repetitive DNA sequence that excludes nucleosomes

Tandem repeats of the pentanucleotide 5'-CCGNN (where N indicates any base) were previously shown to exclude nucleosomes in vitro (Y. -H. Wang and J. D. Griffith, Proc. Natl. Acad. Sci. USA 93:8863-8867, 1996). To determine the in vivo effects of these sequences, we replaced the upstream regulatory sequences of the HIS4 gene of Saccharomyces cerevisiae with either 12 or 48 tandem copies of CCGNN. Both tracts activated HIS4 transcription. We found that (CCGNN)(12) tracts elevated meiotic recombination (hot spot activity), whereas the (CCGNN)(48) tract repressed recombination (cold spot activity). In addition, a "pure" tract of (CCGAT)(12) activated both transcription and meiotic recombination. We suggest that the cold spot activity of the (CCGNN)(48) tract is related to the phenomenon of the suppressive interactions of adjacent hot spots previously described in yeast (Q.-Q. Fan, F. Xu, and T. D. Petes, Mol. Cell. Biol. 15:1679-1688, 1995; Q.-Q. Fan, F. Xu, M. A. White, and T. D. Petes, Genetics 145:661-670, 1997; T.-C. Wu and M. Lichten, Genetics 140:55-66, 1995; L. Xu and N. Kleckner, EMBO J. 16:5115-5128, 1995).

RAD51 and DMC1 form mixed complexes associated with mouse meiotic chromosome cores and synaptonemal complexes

The eukaryotic RecA homologues RAD51 and DMC1 function in homology recognition and formation of joint-molecule recombination intermediates during yeast meiosis. The precise immunolocalization of these two proteins on the meiotic chromosomes of plants and animals has been complicated by their high degree of identity at the amino acid level. With antibodies that have been immunodepleted of cross-reactive epitopes, we demonstrate that RAD51 and DMC1 have identical distribution patterns in extracts of mouse spermatocytes in successive prophase I stages, suggesting coordinate functionality. Immunofluorescence and immunoelectron microscopy with these antibodies demonstrate colocalization of the two proteins on the meiotic chromosome cores at early prophase I. We also show that mouse RAD51 and DMC1 establish protein-protein interactions with each other and with the chromosome core component COR1(SCP3) in a two-hybrid system and in vitro binding analyses. These results suggest that the formation of a multiprotein recombination complex associated with the meiotic chromosome cores is essential for the development and fulfillment of the meiotic recombination process.

A mouse homolog of the Saccharomyces cerevisiae meiotic recombination DNA transesterase Spo11p

The Saccharomyces cerevisiae Spo11 protein is thought to catalyze formation of the DNA double-strand breaks that initiate meiotic recombination. We have cloned cDNA and genomic DNA for a mouse gene encoding a protein with significant sequence similarity to conserved domains found in proteins of the Spo11p family. This putative mouse Spo11 gene maps to the distal region of chromosome 2 (homologous to human chromosome 20q13.2-q13.3) and comprises at least 12 exons, spanning approximately 15-18 kb. Strong expression of the Spo11 message is seen in juvenile and adult testis by RNA in situ hybridization, RT-PCR, and Northern blot, with much weaker expression in thymus and brain. In situ hybridization detects expression in oocytes of embryonic ovary, but not of adult ovary. RT-PCR and in situ hybridization analyses of a time course of juvenile testis development indicate that Spo11 expression begins in early meiotic Prophase I, prior to the pachytene stage, with increasing accumulation of mRNA through the pachytene stage. Taken together, these results strongly suggest that this gene encodes the functional homolog of yeast Spo11p, which in turn suggests that the mechanism of meiotic recombination initiation is conserved between yeast and mammals.

Genetic control of recombination partner preference in yeast meiosis. Isolation and characterization of mutants elevated for meiotic unequal sister-chromatid recombination

Meiotic exchange occurs preferentially between homologous chromatids, in contrast to mitotic recombination, which occurs primarily between sister chromatids. To identify functions that direct meiotic recombination events to homologues, we screened for mutants exhibiting an increase in meiotic unequal sister-chromatid recombination (SCR). The msc (meiotic sister-chromatid recombination) mutants were quantified in spo13 meiosis with respect to meiotic unequal SCR frequency, disome segregation pattern, sporulation frequency, and spore viability. Analysis of the msc mutants according to these criteria defines three classes. Mutants with a class I phenotype identified new alleles of the meiosis-specific genes RED1 and MEK1, the DNA damage checkpoint genes RAD24 and MEC3, and a previously unknown gene, MSC6. The genes RED1, MEK1, RAD24, RAD17, and MEC1 are required for meiotic prophase arrest induced by a dmc1 mutation, which defines a meiotic recombination checkpoint. Meiotic unequal SCR was also elevated in a rad17 mutant. Our observation that meiotic unequal SCR is elevated in meiotic recombination checkpoint mutants suggests that, in addition to their proposed monitoring function, these checkpoint genes function to direct meiotic recombination events to homologues. The mutants in class II, including a dmc1 mutant, confer a dominant meiotic lethal phenotype in diploid SPO13 meiosis in our strain background, and they identify alleles of UBR1, INP52, BUD3, PET122, ELA1, and MSC1-MSC3. These results suggest that DMC1 functions to bias the repair of meiosis-specific double-strand breaks to homologues. We hypothesize that the genes identified by the class II mutants function in or are regulators of the DMC1-promoted interhomologue recombination pathway. Class III mutants may be elevated for rates of both SCR and homologue exchange.

Meiotic DNA replication checkpoint control in fission yeast

In eukaryotes, the DNA replication checkpoint prevents entry into mitosis when DNA replication is incomplete and is crucial for maintaining genomic integrity. Much less is known about equivalent controls that operate during meiosis. Here, we show that a DNA replication checkpoint control operates during meiosis in fission yeast. The mitotic checkpoint Rad genes and the Cds1 protein kinase are required for the DNA replication checkpoint during meiosis, with Cds1 playing a more prominent role than it does during mitosis. When DNA replication is blocked, the checkpoint maintains Cdc2 tyrosine 15 phosphorylation keeping Cdc2 protein kinase activity low and preventing onset of meiosis I. Additionally, there is a second checkpoint acting during meiosis that is revealed if cells are prevented from maintaining Cdc2 tyrosine 15 phosphorylation when DNA replication is blocked. Such cells arrest with high Cdc2 protein kinase activity and separated spindle pole bodies, an arrest state similar to that observed in mitotic budding yeast cells when DNA replication is incomplete. This second checkpoint is meiosis specific and may reflect processes occurring only during meiosis such as increased recombination rates, an extended duration of nuclear division, or homolog chromosome pairing.

Initiation and progression of homologous chromosome synapsis in Crepis capillaris: Variations on a theme

The model cytogenetic plant species Crepis capillaris (2x = 6), in which all 3 chromosomes are readily distinguished, was used to analyse the initiation and progression of meiotic synapsis in a large sample of spread and silver-stained pollen mother cells. Particular emphasis was placed on detecting general patterns or trends of synaptic order, both among different bivalents and within (along) individual bivalents, and investigating the consistency or otherwise of these synaptic patterns. The order of synaptic progression and completion was partly related to chromosome length; as in other species, shorter bivalents tended to complete synapsis ahead of longer ones. Individual bivalents also showed distinct patterns of synapsis, with a tendency for subterminal regions to initiate synapsis early, followed by multiple synaptic initiations in internal bivalent regions. However, the analysis showed that these synaptic patterns are only general trends and significant variations in synaptic order and pattern, among and within bivalents, occur in individual cells.Key words: meiosis, synapsis, synaptonemal complex, Crepis.

Activation of a meiotic checkpoint regulates translation of Gurken during Drosophila oogenesis

The genes okra and spindle-B act during meiosis in Drosophila to repair double-stranded DNA breaks (DSBs) associated with meiotic recombination. Unexpectedly, mutations in these genes cause dorsoventral patterning defects during oogenesis. These defects result from a failure to accumulate Gurken protein, which is required to initiate dorsoventral patterning during oogenesis. Here we find that the block in Gurken accumulation in the oocyte cytoplasm reflects activation of a meiotic checkpoint in response to the persistence of DSBs in the nucleus. We also show that Vasa is a target of this meiotic checkpoint, and so may mediate the checkpoint-dependent translational regulation of Gurken.

Saccharomyces cerevisiae checkpoint genes MEC1, RAD17 and RAD24 are required for normal meiotic recombination partner choice

Checkpoint gene function prevents meiotic progression when recombination is blocked by mutations in the recA homologue DMC1. Bypass of dmc1 arrest by mutation of the DNA damage checkpoint genes MEC1, RAD17, or RAD24 results in a dramatic loss of spore viability, suggesting that these genes play an important role in monitoring the progression of recombination. We show here that the role of mitotic checkpoint genes in meiosis is not limited to maintaining arrest in abnormal meioses; mec1-1, rad24, and rad17 single mutants have additional meiotic defects. All three mutants display Zip1 polycomplexes in two- to threefold more nuclei than observed in wild-type controls, suggesting that synapsis may be aberrant. Additionally, all three mutants exhibit elevated levels of ectopic recombination in a novel physical assay. rad17 mutants also alter the fraction of recombination events that are accompanied by an exchange of flanking markers. Crossovers are associated with up to 90% of recombination events for one pair of alleles in rad17, as compared with 65% in wild type. Meiotic progression is not required to allow ectopic recombination in rad17 mutants, as it still occurs at elevated levels in ndt80 mutants that arrest in prophase regardless of checkpoint signaling. These observations support the suggestion that MEC1, RAD17, and RAD24, in addition to their proposed monitoring function, act to promote normal meiotic recombination.

The Arabidopsis SKP1-LIKE1 gene is essential for male meiosis and may control homologue separation

The yeast and human SKP1 genes regulate the mitotic cell cycle but are not yet known to be required for meiosis. Nine Arabidopsis SKP1 homologues have been uncovered and are named ASK1 through ASK9. Here, we report the isolation and characterization of a male sterile Arabidopsis mutant and show that the mutant defect was caused by a Ds transposon insertion into the ASK1 gene. In the ask1-1 mutant, abnormal microspores exhibit a range of sizes. Furthermore, during mutant male meiosis, although homologous chromosome pairing appeared normal at metaphase I, chromosome segregation at anaphase I is unequal, and some chromosomes are abnormally extended. Therefore, in ask1-1, at least some homologues remain associated after metaphase I. In addition, immunofluorescence microscopy indicates that the mutant spindle morphology at both metaphase I and early anaphase I is normal; thus, the abnormal chromosome segregation is not likely caused by a spindle defect. Because the yeast Skp1p is required for targeting specific proteins for ubiquitin-mediated proteolysis, we propose that ASK1 controls homologue separation by degrading or otherwise removing a protein that is required directly or indirectly for homologue association before anaphase I.

Synapsis and chiasma formation in Caenorhabditis elegans require HIM-3, a meiotic chromosome core component that functions in chromosome segregation

Meiotic chromosomes are organized about a proteinaceous core that forms between replicated sister chromatids. We have isolated a Caenorhabditis elegans gene, him-3, which encodes a meiosis-specific component of chromosome cores with some similarity to the yeast lateral element protein Hop1p. Antibodies raised against HIM-3 localize the protein to condensing chromosomes in early prophase I and to the cores of both synapsed and desynapsed chromosomes. In RNA interference experiments, chromosomes appear to condense normally in the absence of detectable protein but fail to synapse and form chiasmata, indicating that HIM-3 is essential for these processes. Hypomorphs of him-3, although being synapsis proficient, show severe reductions in the frequency of crossing-over, demonstrating that HIM-3 has a role in establishing normal levels of interhomolog exchange. Him-3 mutants also show defects in meiotic chromosome segregation and the persistence of the protein at the chromosome core until the metaphase I-anaphase I transition suggests that HIM-3 may play a role in sister chromatid cohesion. The analysis of him-3 provides the first functional description of a chromosome core component in a multicellular organism and suggests that a mechanistic link exists between the early meiotic events of synapsis and recombination, and later events such as segregation.

Meiotic chromosome dynamics dependent upon the rec8(+), rec10(+) and rec11(+) genes of the fission yeast Schizosaccharomyces pombe

During meiosis homologous chromosomes replicate once, pair, experience recombination, and undergo two rounds of segregation to produce haploid meiotic products. The rec8(+), rec10(+), and rec11(+) genes of the fission yeast Schizosaccharomyces pombe exhibit similar specificities for meiotic recombination and rec8(+) is required for sister chromatid cohesion and homolog pairing. We applied cytological and genetic approaches to identify potential genetic interactions and to gauge the fidelity of meiotic chromosome segregation in the mutants. The rec8(+) gene was epistatic to rec10(+) and to rec11(+), but there was no clear epistatic relationship between rec10(+) and rec11(+). Reciprocal (crossover) recombination in the central regions of all three chromosomes was compromised in the rec mutants, but recombination near the telomeres was nearly normal. Each of the mutants also exhibited a high rate of aberrant segregation for all three chromosomes. The rec8 mutations affected mainly meiosis I segregation. Remarkably, the rec10 and rec11 mutations, which compromised recombination during meiosis I, affected mainly meiosis II segregation. We propose that these genes encode regulators or components of a "meiotic chromatid cohesion" pathway involved in establishing, maintaining, and appropriately releasing meiotic interactions between chromosomes. A model of synergistic interactions between sister chromatid cohesion and crossover position suggests how crossovers and cohesion help ensure the proper segregation of chromosomes in each of the meiotic divisions.

Centromere mapping functions for aneuploid meiotic products: Analysis of rec8, rec10 and rec11 mutants of the fission yeast Schizosaccharomyces pombe

Recent evidence suggests that the position of reciprocal recombination events (crossovers) is important for the segregation of homologous chromosomes during meiosis I and sister chromatids during meiosis II. We developed genetic mapping functions that permit the simultaneous analysis of centromere-proximal crossover recombination and the type of segregation error leading to aneuploidy. The mapping functions were tested in a study of the rec8, rec10, and rec11 mutants of fission yeast. In each mutant we monitored each of the three chromosome pairs. Between 38 and 100% of the chromosome segregation errors in the rec8 mutants were due to meiosis I nondisjunction of homologous chromosomes. The remaining segregation errors were likely the result of precocious separation of sister chromatids, a previously described defect in the rec8 mutants. Between 47 and 100% of segregation errors in the rec10 and rec11 mutants were due to nondisjunction of sister chromatids during meiosis II. In addition, centromere-proximal recombination was reduced as much as 14-fold or more on chromosomes that had experienced nondisjunction. These results demonstrate the utility of the new mapping functions and support models in which sister chromatid cohesion and crossover position are important determinants for proper chromosome segregation in each meiotic division.

Cohesin Rec8 is required for reductional chromosome segregation at meiosis

When cells exit from mitotic cell division, their sister chromatids lose cohesion and separate to opposite poles of the dividing cell, resulting in equational chromosome segregation. In contrast, the reductional segregation of the first stage of meiotic cell division (meiosis I) requires that sister chromatids remain associated through their centromeres and move together to the same pole. Centromeric cohesion is lost as cells exit from meiosis II and sister chromatids can then separate. The fission yeast cohesin protein Rec8 is specific to and required for meiosis. Here we show that Rec8 appears in the centromeres and adjacent chromosome arms during the pre-meiotic S phase. Centromeric Rec8 persists throughout meiosis I and disappears at anaphase of meiosis II. When the rec8 gene is deleted, sister chromatids separate at meiosis I, resulting in equational rather than reductional chromosome segregation. We propose that the persistence of Rec8 at centromeres during meiosis I maintains sister-chromatid cohesion, and that its presence in the centromere-adjacent regions orients the kinetochores so that sister chromatids move to the same pole. This results in the reductional pattern of chromosome segregation necessary to reduce a diploid zygote to haploid gametes.

Spo76p is a conserved chromosome morphogenesis protein that links the mitotic and meiotic programs

Spo76p is conserved and related to the fungal proteins Pds5p and BIMD and the human AS3 prostate proliferative shutoff-associated protein. Spo76p localizes to mitotic and meiotic chromosomes, except at metaphase(s) and anaphase(s). During meiotic prophase, Spo76p assembles into strong lines in correlation with axial element formation. As inferred from spo76-1 mutant phenotypes, Spo76p is required for sister chromatid cohesiveness, chromosome axis morphogenesis, and chromatin condensation during critical transitions at mitotic prometaphase and meiotic midprophase. Spo76p is also required for meiotic interhomolog recombination, likely at postinitiation stage(s). We propose that a disruptive force coordinately promotes chromosomal axial compaction and destabilization of sister connections and that Spo76p restrains and channels the effects of this force into appropriate morphogenetic mitotic and meiotic outcomes.

A central role for cohesins in sister chromatid cohesion, formation of axial elements, and recombination during yeast meiosis

A multisubunit complex, called cohesin, containing Smc1p, Smc3p, Scc1p, and Scc3p, is required for sister chromatid cohesion in mitotic cells. We show here that Smc3p and a meiotic version of Scc1p called Rec8p are required for cohesion between sister chromatids, for formation of axial elements, for reciprocal recombination, and for preventing hyperresection of double-strand breaks during meiosis. Both Rec8p and Smc3p colocalize with chromosome cores independently of synapsis during prophase I and largely disappear from chromosome arms after pachytene but persist in the neighborhood of centromeres until the onset of anaphase II. The eukaryotic cell's cohesion apparatus is required both for the repair of recombinogenic lesions and for chromosome segregation and therefore appears to lie at the heart of the meiotic process.

Use of a recombination reporter insert to define meiotic recombination domains on chromosome III of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, meiotic recombination is initiated by DNA double-strand breaks (DSBs). DSBs usually occur in intergenic regions that display nuclease hypersensitivity in digests of chromatin. DSBs are distributed nonuniformly across chromosomes; on chromosome III, DSBs are concentrated in two "hot" regions, one in each chromosome arm. DSBs occur rarely in regions within about 40 kb of each telomere and in an 80-kb region in the center of the chromosome, just to the right of the centromere. We used recombination reporter inserts containing arg4 mutant alleles to show that the "cold" properties of the central DSB-deficient region are imposed on DNA inserted in the region. Cold region inserts display DSB and recombination frequencies that are substantially less than those seen with similar inserts in flanking hot regions. This occurs without apparent change in chromatin structure, as the same pattern and level of DNase I hypersensitivity is seen in chromatin of hot and cold region inserts. These data are consistent with the suggestion that features of higher-order chromosome structure or chromosome dynamics act in a target sequence-independent manner to control where recombination events initiate during meiosis.

Atm inactivation results in aberrant telomere clustering during meiotic prophase

A-T (ataxia telangiectasia) individuals frequently display gonadal atrophy, and Atm-/- mice show spermatogenic failure due to arrest at prophase of meiosis I. Chromosomal movements take place during meiotic prophase, with telomeres congregating on the nuclear envelope to transiently form a cluster during the leptotene/zygotene transition (bouquet arrangement). Since the ATM protein has been implicated in telomere metabolism of somatic cells, we have set out to investigate the effects of Atm inactivation on meiotic telomere behavior. Fluorescent in situ hybridization and synaptonemal complex (SC) immunostaining of structurally preserved spermatocytes I revealed that telomere clustering occurs aberrantly in Atm-/- mice. Numerous spermatocytes of Atm-/- mice displayed locally accumulated telomeres with stretches of SC near the clustered chromosome ends. This contrasted with spermatogenesis of normal mice, where only a few leptotene/zygotene spermatocytes I with clustered telomeres were detected. Pachytene nuclei, which were much more abundant in normal mice, displayed telomeres scattered over the nuclear periphery. It appears that the timing and occurrence of chromosome polarization is altered in Atm-/- mice. When we examined telomere-nuclear matrix interactions in spermatocytes I, a significant difference was observed in the ratio of soluble versus matrix-associated telomeric DNA sequences between meiocytes of Atm-/- and control mice. We propose that the severe disruption of spermatogenesis during early prophase I in the absence of functional Atm may be partly due to altered interactions of telomeres with the nuclear matrix and distorted meiotic telomere clustering.