The DNA helicase FANCJ (BRIP1) functions in double strand break repair processing, but not crossover formation during prophase I of meiosis in male mice

Meiotic recombination between homologous chromosomes is initiated by the formation of hundreds of programmed double-strand breaks (DSBs). Approximately 10% of these DSBs result in crossovers (COs), sites of physical DNA exchange between homologs that are critical to correct chromosome segregation. Virtually all COs are formed by coordinated efforts of the MSH4/MSH5 and MLH1/MLH3 heterodimers, the latter representing the defining marks of CO sites. The regulation of CO number and position is poorly understood, but undoubtedly requires the coordinated action of multiple repair pathways. In a previous report, we found gene-trap disruption of the DNA helicase, FANCJ (BRIP1/BACH1), elicited elevated numbers of MLH1 foci and chiasmata. In somatic cells, FANCJ interacts with numerous DNA repair proteins including MLH1, and we hypothesized that FANCJ functions with MLH1 to regulate the major CO pathway. To further elucidate the meiotic function of FANCJ, we produced three new Fancj mutant mouse lines via CRISPR/Cas9 gene editing: a full-gene deletion, truncation of the N-terminal Helicase domain, and a C-terminal dual-tagged allele. We also generated an antibody against the C-terminus of the mouse FANCJ protein. Surprisingly, none of our Fancj mutants show any change in either MLH1 focus counts during pachynema or total CO number at diakinesis of prophase I. We find evidence that FANCJ and MLH1 do not interact in meiosis; further, FANCJ does not co-localize with MSH4, MLH1, or MLH3 in meiosis. Instead, FANCJ co-localizes with BRCA1 and TOPBP1, forming discrete foci along the chromosome cores beginning in early meiotic prophase I and densely localized to unsynapsed chromosome axes in late zygonema and to the XY chromosomes in early pachynema. Fancj mutants also exhibit a subtle persistence of DSBs in pachynema. Collectively, these data indicate a role for FANCJ in early DSB repair, but they rule out a role for FANCJ in MLH1-mediated CO events.

3D genome remodeling and homologous pairing during meiotic prophase of mouse oogenesis and spermatogenesis

During meiosis, the chromatin and transcriptome undergo prominent switches. Although recent studies have explored the genome reorganization during spermatogenesis, the chromatin remodeling in oogenesis and characteristics of homologous pairing remain largely elusive. We comprehensively compared chromatin structures and transcriptomes at successive substages of meiotic prophase in both female and male mice using low-input high-through chromosome conformation capture (Hi-C) and RNA sequencing (RNA-seq). Compartments and topologically associating domains (TADs) gradually disappeared and slowly recovered in both sexes. We found that homologs adopted different sex-conserved pairing strategies prior to and after the leptotene-to-zygotene transition, changing from long interspersed nuclear element (LINE)-enriched compartments B to short interspersed nuclear element (SINE)-enriched compartments A. We complemented marker genes and predicted the sex-specific meiotic sterile genes for each substage. This study provides valuable insights into the similarities and distinctions between sexes in chromosome architecture, homologous pairing, and transcriptome during meiotic prophase of both oogenesis and spermatogenesis.

Nonhomologous Chromosome Interactions in Prophase I: Dynamics of Bizarre Meiotic Contacts in the Alay Mole Vole Ellobius alaicus (Mammalia, Rodentia)

Nonhomologous chromosome interactions take place in both somatic and meiotic cells. Prior to this study, we had discovered special contacts through the SYCP3 (synaptonemal complex protein 3) filament between the short arms of nonhomologous acrocentrics at the pachytene stage in the Alay mole vole, and these contacts demonstrate several patterns from proximity to the complete fusion stage. Here, we investigated the nonhomologous chromosome contacts in meiotic prophase I. It turned out that such contacts do not introduce changes into the classic distribution of DNA double-strand breaks. It is noteworthy that not all meiotic contacts were localized in the H3k9me3-positive heterochromatic environment. Both in the mid zygotene and in the early-mid diplotene, three types of contacts (proximity, touching, and anchoring/tethering) were observed, whereas fusion seems to be characteristic only for pachytene. The number of contacts in the mid pachytene is significantly higher than that in the zygotene, and the distance between centromeres in nonhomologous contacts is also the smallest in mid pachytene for all types of contacts. Thus, this work provides a new insight into the behavior of meiotic contacts during prophase I and points to avenues of further research.

Dispersive forces and resisting spot welds by alternative homolog conjunction govern chromosome shape in Drosophila spermatocytes during prophase I

The bivalent chromosomes that are generated during prophase of meiosis I comprise a pair of homologous chromosomes. Homolog pairing during prophase I must include mechanisms that avoid or eliminate entanglements between non-homologous chromosomes. In Drosophila spermatocytes, non-homologous associations are disrupted by chromosome territory formation, while linkages between homologous chromosomes are maintained by special conjunction proteins. These proteins function as alternative for crossovers that link homologs during canonical meiosis but are absent during the achiasmate Drosophila male meiosis. How and where within bivalents the alternative homolog conjunction proteins function is still poorly understood. To clarify the rules that govern territory formation and alternative homolog conjunction, we have analyzed spermatocytes with chromosomal aberrations. We examined territory formation after acute chromosome cleavage by Cas9, targeted to the dodeca satellite adjacent to the centromere of chromosome 3 specifically in spermatocytes. Moreover, we studied territory organization, as well as the eventual orientation of chromosomes during meiosis I, in spermatocytes with stable structural aberrations, including heterozygous reciprocal autosomal translocations. Our observations indicate that alternative homolog conjunction is applied in a spatially confined manner. Comparable to crossovers, only a single conjunction spot per chromosome arm appears to be applied usually. These conjunction spots resist separation by the dispersing forces that drive apart homologous pericentromeric heterochromatin and embedded centromeres within territories, as well as the distinct chromosomal entities into peripheral, maximally separated territories within the spermatocyte nucleus.

Already the primordial eukaryote appears to have used meiosis for sexual reproduction, because this sophisticated process follows a canonical program in lineages ranging from unicellular organisms to plants and animals. The maternal and paternal copies of a particular chromosome, i.e., the homologs, are first physically linked into a bivalent before the first meiotic division. Linkage is essential for error-free chromosome segregation. In canonical meiosis, linkage is achieved by crossovers. These are regulated so that each chromosome pair is linked, but only by very few crossovers. Surprisingly, crossovers are absent during meiosis in males of the fruit fly Drosophila melanogaster. Instead, an alternative homolog conjunction system is used. It is not yet clear how this functions. Here, we demonstrate that the alternative chromosome glue appears to be applied in a locally restricted manner rather than all along the paired homologs. Just two spots of glue appear to conjoin the two homologous chromosomes usually, with one spot linking the left and another the right chromosome arm. Thus, number and location of linkages appear to be similar as crossovers, raising the possibility of mechanistic similarities in the establishment of the two distinct types of homolog linkage.

How do small chromosomes know they are small? Maximizing meiotic break formation on the shortest yeast chromosomes

The programmed formation of DNA double-strand breaks (DSBs) in meiotic prophase I initiates the homologous recombination process that yields crossovers between homologous chromosomes, a prerequisite to accurately segregating chromosomes during meiosis I (MI). In the budding yeast Saccharomyces cerevisiae, proteins required for meiotic DSB formation (DSB proteins) accumulate to higher levels specifically on short chromosomes to ensure that these chromosomes make DSBs. We previously demonstrated that as-yet undefined cis-acting elements preferentially recruit DSB proteins and promote higher levels of DSBs and recombination and that these intrinsic features are subject to selection pressure to maintain the hyperrecombinogenic properties of short chromosomes. Thus, this targeted boosting of DSB protein binding may be an evolutionarily recurrent strategy to mitigate the risk of meiotic mis-segregation caused by karyotypic constraints. However, the underlining mechanisms are still elusive. Here, we discuss possible scenarios in which components of the meiotic chromosome axis (Red1 and Hop1) bind to intrinsic features independent of the meiosis-specific cohesin subunit Rec8 and DNA replication, promoting preferential binding of DSB proteins to short chromosomes. We also propose a model where chromosome position in the nucleus, influenced by centromeres, promotes the short-chromosome boost of DSB proteins.

Multilayered mechanisms ensure that short chromosomes recombine in meiosis

In most species, homologous chromosomes must recombine in order to segregate accurately during meiosis1. Because small chromosomes would be at risk of missegregation if recombination were randomly distributed, the double-strand breaks (DSBs) that initiate recombination are not located arbitrarily2. How the nonrandomness of DSB distributions is controlled is not understood, although several pathways are known to regulate the timing, location and number of DSBs. Meiotic DSBs are generated by Spo11 and accessory DSB proteins, including Rec114 and Mer2, which assemble on chromosomes3-7 and are nearly universal in eukaryotes8-11. Here we demonstrate how Saccharomyces cerevisiae integrates multiple temporally distinct pathways to regulate the binding of Rec114 and Mer2 to chromosomes, thereby controlling the duration of a DSB-competent state. The engagement of homologous chromosomes with each other regulates the dissociation of Rec114 and Mer2 later in prophase I, whereas the timing of replication and the proximity to centromeres or telomeres influence the accumulation of Rec114 and Mer2 early in prophase I. Another early mechanism enhances the binding of Rec114 and Mer2 specifically on the shortest chromosomes, and is subject to selection pressure to maintain the hyperrecombinogenic properties of these chromosomes. Thus, the karyotype of an organism and its risk of meiotic missegregation influence the shape and evolution of its recombination landscape. Our results provide a cohesive view of a multifaceted and evolutionarily constrained system that allocates DSBs to all pairs of homologous chromosomes.

SKP1 drives the prophase I to metaphase I transition during male meiosis

The meiotic prophase I to metaphase I (PI/MI) transition requires chromosome desynapsis and metaphase competence acquisition. However, control of these major meiotic events is poorly understood. Here, we identify an essential role for SKP1, a core subunit of the SKP1-Cullin-F-box (SCF) ubiquitin E3 ligase, in the PI/MI transition. SKP1 localizes to synapsed chromosome axes and evicts HORMAD proteins from these regions in meiotic spermatocytes. SKP1-deficient spermatocytes display premature desynapsis, precocious pachytene exit, loss of PLK1 and BUB1 at centromeres, but persistence of HORMAD, γH2AX, RPA2, and MLH1 in diplonema. Strikingly, SKP1-deficient spermatocytes show sharply reduced MPF activity and fail to enter MI despite treatment with okadaic acid. SKP1-deficient oocytes exhibit desynapsis, chromosome misalignment, and progressive postnatal loss. Therefore, SKP1 maintains synapsis in meiosis of both sexes. Furthermore, our results support a model where SKP1 functions as the long-sought intrinsic metaphase competence factor to orchestrate MI entry during male meiosis.

CRL4 regulates recombination and synaptonemal complex aggregation in the Caenorhabditis elegans germline

To maintain the integrity of the genome, meiotic DNA double strand breaks (DSBs) need to form by the meiosis-specific nuclease Spo11 and be repaired by homologous recombination. One class of products formed by recombination are crossovers, which are required for proper chromosome segregation in the first meiotic division. The synaptonemal complex (SC) is a protein structure that connects homologous chromosomes during meiotic prophase I. The proper assembly of the SC is important for recombination, crossover formation, and the subsequent chromosome segregation. Here we identify the components of Cullin RING E3 ubiquitin ligase 4 (CRL4) that play a role in SC assembly in Caenorhabditis elegans. Mutants of the CRL4 complex (cul-4, ddb-1, and gad-1) show defects in SC assembly manifested in the formation of polycomplexes (PCs), impaired progression of meiotic recombination, and reduction in crossover numbers. PCs that are formed in cul-4 mutants lack the mobile properties of wild type SC, but are likely not a direct target of ubiquitination. In C. elegans, SC assembly does not require recombination and there is no evidence that PC formation is regulated by recombination as well. However, in one cul-4 mutant PC formation is dependent upon early meiotic recombination, indicating that proper assembly of the SC can be diminished by recombination in some scenarios. Lastly, our studies suggest that CUL-4 deregulation leads to transposition of the Tc3 transposable element, and defects in formation of SPO-11-mediated DSBs. Our studies highlight previously unknown functions of CRL4 in C. elegans meiosis and show that CUL-4 likely plays multiple roles in meiosis that are essential for maintaining genome integrity.

Defects in the formation of the structure named the synaptonemal complex (SC) lead to the missegregation of chromosomes in the divisions that generate sperm and egg cells. In humans, this chromosome missegregation is associated with infertility and developmental disabilities of the surviving progeny. Abnormal SC structures composed of misfolded and aggregated SC proteins are associated with an inability to properly repair DNA damage and accurately segregate meiotic chromosomes. How SC proteins assemble such that they do not form misfolded protein aggregates is poorly understood. The germlines of nematodes (Caenorhabditis elegans) that lack protein components of the Cullin 4 E3 Ubiquitin ligase complex (CRL4), have defects in the formation of the SC that can be due to misfolding of SC proteins and their aggregation. CRL4 appears to be involved in other germline functions that directly affect chromosome stability (DNA damage repair and transposition), indicating that CRL4 has a central function in the formation of functional sperm and egg cells.

Dibutyl phthalate exposure disrupts the progression of meiotic prophase I by interfering with homologous recombination in fetal mouse oocytes

Dibutyl phthalate (DBP), one of the most widely used plasticizers, is a known environmental endocrine disruptor that impairs male and female fertility. In this study, oral administration of DBP was given to pregnant mice on 14.5 days post coitus (dpc) for 3 days; and additionally, DBP was added into the culture of 14.5 dpc fetal ovaries for 3 days. DBP exposure during gestation disturbed the progression of meiotic prophase I of mouse oocytes, specifically from the zygotene to pachytene stages. Meanwhile, the DBP-exposed pachytene oocytes showed increased homologous recombination sites and unrepaired DNA damage. Furthermore, DBP caused DNA damage by increasing oxidative stress, decreased the expression of multiple critical meiotic regulators, and consequently induced oocyte apoptosis. Moreover, the effect of DBP on meiosis I prophase involved estrogen receptors α and β. Collectively, these results demonstrated a set of meiotic defects in DBP-exposed fetal oocytes. As aberrations in homologous recombination can result in aneuploid gametes and embryos, this study provides new support for the deleterious effects of phthalates.

Defective Leptotene Chromosome 1 (DLC1) encodes a type-B response regulator and is required for rice meiosis

Meiosis is critical for sexual reproduction and the generation of new allelic variations in most eukaryotes. In this study, we report the isolation of a meiotic gene, DLC1, using a map-based cloning strategy. The dlc1 mutant is sterile in both male and female gametophytes due to an earlier defect in the leptotene chromosome and subsequent abnormalities at later stages. DLC1 is strongly expressed in the pollen mother cells (PMCs) and tapetum and encodes a nucleus-located rice type-B response regulator (RR) with transcriptional activity. Further investigations showed that DLC1 interacts with all five putative rice histidine phosphotransfer proteins (HPs) in yeast and planta cells, suggesting a possible participation of the two-component signalling systems (TCS) in rice meiosis. Our results demonstrated that DLC1 is required for rice meiosis and fertility, providing useful information for the role of TCS in rice meiosis.

The histone modification reader ZCWPW1 is required for meiosis prophase I in male but not in female mice

Meiosis is a specialized type of cell division that creates haploid germ cells and ensures their genetic diversity through homologous recombination. We show that the H3K4me3 reader ZCWPW1 is specifically required for meiosis prophase I progression in male but not in female germ cells in mice. Loss of Zcwpw1 in male mice caused a complete failure of synapsis, resulting in meiotic arrest at the zygotene to pachytene stage, accompanied by incomplete DNA double-strand break repair and lack of crossover formation, leading to male infertility. In oocytes, deletion of Zcwpw1 only somewhat slowed down meiosis prophase I progression; Zcwpw1^-/- oocytes were able to complete meiosis, and Zcwpw1^-/- female mice had normal fertility until mid-adulthood. We conclude that the H3K4me3 reader ZCWPW1 is indispensable for meiosis synapsis in males but is dispensable for females. Our results suggest that ZCWPW1 may represent a previously unknown, sex-dependent epigenetic regulator of germ cell meiosis in mammals.

A mutation in the endonuclease domain of mouse MLH3 reveals novel roles for MutLγ during crossover formation in meiotic prophase I

During meiotic prophase I, double-strand breaks (DSBs) initiate homologous recombination leading to non-crossovers (NCOs) and crossovers (COs). In mouse, 10% of DSBs are designated to become COs, primarily through a pathway dependent on the MLH1-MLH3 heterodimer (MutLγ). Mlh3 contains an endonuclease domain that is critical for resolving COs in yeast. We generated a mouse (Mlh3DN/DN) harboring a mutation within this conserved domain that is predicted to generate a protein that is catalytically inert. Mlh3DN/DN males, like fully null Mlh3-/- males, have no spermatozoa and are infertile, yet spermatocytes have grossly normal DSBs and synapsis events in early prophase I. Unlike Mlh3-/- males, mutation of the endonuclease domain within MLH3 permits normal loading and frequency of MutLγ in pachynema. However, key DSB repair factors (RAD51) and mediators of CO pathway choice (BLM helicase) persist into pachynema in Mlh3DN/DN males, indicating a temporal delay in repair events and revealing a mechanism by which alternative DSB repair pathways may be selected. While Mlh3DN/DN spermatocytes retain only 22% of wildtype chiasmata counts, this frequency is greater than observed in Mlh3-/- males (10%), suggesting that the allele may permit partial endonuclease activity, or that other pathways can generate COs from these MutLγ-defined repair intermediates in Mlh3DN/DN males. Double mutant mice homozygous for the Mlh3DN/DN and Mus81-/- mutations show losses in chiasmata close to those observed in Mlh3-/- males, indicating that the MUS81-EME1-regulated crossover pathway can only partially account for the increased residual chiasmata in Mlh3DN/DN spermatocytes. Our data demonstrate that mouse spermatocytes bearing the MLH1-MLH3DN/DN complex display the proper loading of factors essential for CO resolution (MutSγ, CDK2, HEI10, MutLγ). Despite these functions, mice bearing the Mlh3DN/DN allele show defects in the repair of meiotic recombination intermediates and a loss of most chiasmata.

Dynamic reorganization of the genome shapes the recombination landscape in meiotic prophase

In meiotic prophase, chromosomes are organized into compacted loop arrays to promote homolog pairing and recombination. Here, we probe the architecture of the mouse spermatocyte genome in early and late meiotic prophase using chromosome conformation capture (Hi-C). Our data support the established loop array model of meiotic chromosomes, and infer loops averaging 0.8-1.0 megabase pairs (Mb) in early prophase and extending to 1.5-2.0 Mb in late prophase as chromosomes compact and homologs undergo synapsis. Topologically associating domains (TADs) are lost in meiotic prophase, suggesting that assembly of the meiotic chromosome axis alters the activity of chromosome-associated cohesin complexes. While TADs are lost, physically separated A and B compartments are maintained in meiotic prophase. Moreover, meiotic DNA breaks and interhomolog crossovers preferentially form in the gene-dense A compartment, revealing a role for chromatin organization in meiotic recombination. Finally, direct detection of interhomolog contacts genome-wide reveals the structural basis for homolog alignment and juxtaposition by the synaptonemal complex.

The OsRR24/LEPTO1 Type-B Response Regulator is Essential for the Organization of Leptotene Chromosomes in Rice Meiosis

Response regulators play significant roles in controlling various biological processes; however, their roles in plant meiosis remain unclear. Here, we report the identification of OsRR24/LEPTOTENE1 (LEPTO1), a rice (Oryza sativa) type-B response regulator that participates in the establishment of key molecular and morphological features of chromosomes in leptotene, an early stage of prophase I in meiosis. Although meiosis initiates normally, as indicated by staining of the centromere-specific histone CENH3, the meiotic chromosomes in lepto1 mutant pollen mother cells fail to form the thin thread-like structures that are typical of leptotene chromosomes in wild-type pollen mother cells. Furthermore, lepto1 mutants fail to form chromosomal double-strand breaks, do not recruit meiosis-specific proteins to the meiotic chromosomes, and show disrupted callose deposition. LEPTO1 also is essential for programmed cell death in tapetal cells. LEPTO1 contains a conserved signal receiver domain (DDK) and a myb-like DNA binding domain at the N terminus. LEPTO1 interacts with two authentic histidine phosphotransfer (AHP) proteins, OsAHP1 and OsAHP2, via the DDK domain, and a phosphomimetic mutation of the DDK domain relieves its repression of LEPTO1 transactivation activity. Collectively, our results show that OsRR24/LEPTO1 plays a significant role in the leptotene phase of meiotic prophase I.

Identification of ASYNAPTIC4, a Component of the Meiotic Chromosome Axis

During the leptotene stage of prophase I of meiosis, chromatids become organized into a linear looped array via a protein axis that forms along the loop bases. Establishment of the axis is essential for the subsequent synapsis of the homologous chromosome pairs and the progression of recombination to form genetic crossovers. Here, we describe ASYNAPTIC4 (ASY4), a meiotic axis protein in Arabidopsis (Arabidopsis thaliana). ASY4 is a small coiled-coil protein that exhibits limited sequence similarity with the carboxyl-terminal region of the axis protein ASY3. We used enhanced yellow fluorescent protein-tagged ASY4 to show that ASY4 localizes to the chromosome axis throughout prophase I. Bimolecular fluorescence complementation revealed that ASY4 interacts with ASY1 and ASY3, and yeast two-hybrid analysis confirmed a direct interaction between ASY4 and ASY3. Mutants lacking full-length ASY4 exhibited defective axis formation and were unable to complete synapsis. Although the initiation of recombination appeared to be unaffected in the asy4 mutant, the number of crossovers was reduced significantly, and crossovers tended to group in the distal parts of the chromosomes. We conclude that ASY4 is required for normal axis and crossover formation. Furthermore, our data suggest that ASY3/ASY4 are the functional homologs of the mammalian SYCP2/SYCP3 axial components.

ATR is a multifunctional regulator of male mouse meiosis

Meiotic cells undergo genetic exchange between homologs through programmed DNA double-strand break (DSB) formation, recombination and synapsis. In mice, the DNA damage-regulated phosphatidylinositol-3-kinase-like kinase (PIKK) ATM regulates all of these processes. However, the meiotic functions of the PIKK ATR have remained elusive, because germline-specific depletion of this kinase is challenging. Here we uncover roles for ATR in male mouse prophase I progression. ATR deletion causes chromosome axis fragmentation and germ cell elimination at mid pachynema. This elimination cannot be rescued by deletion of ATM and the third DNA damage-regulated PIKK, PRKDC, consistent with the existence of a PIKK-independent surveillance mechanism in the mammalian germline. ATR is required for synapsis, in a manner genetically dissociable from DSB formation. ATR also regulates loading of recombinases RAD51 and DMC1 to DSBs and recombination focus dynamics on synapsed and asynapsed chromosomes. Our studies reveal ATR as a critical regulator of mouse meiosis.

NHEJ Contributes to the Fast Repair of Radiation-induced DNA Double-strand Breaks at Late Prophase I Telomeres

Exposure of cells to ionizing radiation induces DNA double-strand breaks. To repair double-strand breaks correctly, cells must distinguish between the ends of chromosomes (telomeres) and DNA double-strand breaks within chromosomes. Double-strand breaks in telomeric DNA may lead to telomere shortening and mutagenesis. Eukaryotic cells repair double-strand breaks primarily by two mechanisms: error-free homologous recombination and error-prone nonhomologous end joining, of which homologous recombination is used in early meiotic prophase I to create recombined haploid gametes by two meiotic cell divisions lacking an intervening S-phase. Genotoxic exposures put meiosis at risk to transmit mutations, and ionizing radiation is known to induce large double-strand break-marking phospho (gamma)-H2AX foci along the cores and ends of mouse meiotic chromosomes. However, it remained unclear through which repair pathway the ionizing radiation-induced telomeric double-strand breaks are repaired in late prophase I spermatocytes. Using male wild-type and nonhomologous end joining-deficient (severe combined immunodeficient) mice, this study investigated the kinetics of in vivo double-strand break formation and repair at telomeres of late prophase I chromosomes up to 12 h after 0.5 Gy of whole-body gamma irradiation. Late pachytene and diplotene spermatocytes revealed overlapping gamma-H2AX and telomere repeat signal foci, indicating telomeric DNA damage. The comparison of double-strand break repair rates at telomeres and internal prophase chromosome sites revealed a more rapid double-strand break repair at wild-type telomeres during the first hour after irradiation. Increased double-strand break foci numbers at nonhomologous end joining-deficient telomeres and chromosomes and a slowed repair rate in this DNA-dependent protein kinase catalytic subunit mutant suggest that the fast repair of double-strand breaks in telomeric DNA repeats during late prophase I is largely mediated by canonical nonhomologous end joining.

Tex19.1 promotes Spo11-dependent meiotic recombination in mouse spermatocytes

Meiosis relies on the SPO11 endonuclease to generate the recombinogenic DNA double strand breaks (DSBs) required for homologous chromosome synapsis and segregation. The number of meiotic DSBs needs to be sufficient to allow chromosomes to search for and find their homologs, but not excessive to the point of causing genome instability. Here we report that the mammal-specific gene Tex19.1 promotes Spo11-dependent recombination in mouse spermatocytes. We show that the chromosome asynapsis previously reported in Tex19.1-/- spermatocytes is preceded by reduced numbers of recombination foci in leptotene and zygotene. Tex19.1 is required for normal levels of early Spo11-dependent recombination foci during leptotene, but not for upstream events such as MEI4 foci formation or accumulation of H3K4me3 at recombination hotspots. Furthermore, we show that mice carrying mutations in Ubr2, which encodes an E3 ubiquitin ligase that interacts with TEX19.1, phenocopy the Tex19.1-/- recombination defects. These data suggest that Tex19.1 and Ubr2 are required for mouse spermatocytes to accumulate sufficient Spo11-dependent recombination to ensure that the homology search is consistently successful, and reveal a hitherto unknown genetic pathway promoting meiotic recombination in mammals.

Differential cohesin loading marks paired and unpaired regions of platypus sex chromosomes at prophase I

Cohesins are vital for chromosome organisation during meiosis and mitosis. In addition to the important function in sister chromatid cohesion, these complexes play key roles in meiotic recombination, DSB repair, homologous chromosome pairing and segregation. Egg-laying mammals (monotremes) feature an unusually complex sex chromosome system, which raises fundamental questions about organisation and segregation during meiosis. We discovered a dynamic and differential accumulation of cohesins on sex chromosomes during platypus prophase I and specific reorganisation of the sex chromosome complex around a large nucleolar body. Detailed analysis revealed a differential loading of SMC3 on the chromatin and chromosomal axis of XY shared regions compared with the chromatin and chromosomal axes of asynapsed X and Y regions during prophase I. At late prophase I, SMC3 accumulation is lost from both the chromatin and chromosome axes of the asynaptic regions of the chain and resolves into subnuclear compartments. This is the first report detailing unpaired DNA specific SMC3 accumulation during meiosis in any species and allows speculation on roles for cohesin in monotreme sex chromosome organisation and segregation.

Meiotic recombination modulates the structure and dynamics of the synaptonemal complex during C. elegans meiosis

During meiotic prophase, a structure called the synaptonemal complex (SC) assembles at the interface between aligned pairs of homologous chromosomes, and crossover recombination events occur between their DNA molecules. Here we investigate the inter-relationships between these two hallmark features of the meiotic program in the nematode C. elegans, revealing dynamic properties of the SC that are modulated by recombination. We demonstrate that the SC incorporates new subunits and switches from a more highly dynamic/labile state to a more stable state as germ cells progress through the pachytene stage of meiotic prophase. We further show that the more dynamic state of the SC is prolonged in mutants where meiotic recombination is impaired. Moreover, in meiotic mutants where recombination intermediates are present in limiting numbers, SC central region subunits become preferentially stabilized on the subset of chromosome pairs that harbor a site where pro-crossover factors COSA-1 and MutSγ are concentrated. Polo-like kinase PLK-2 becomes preferentially localized to the SCs of chromosome pairs harboring recombination sites prior to the enrichment of SC central region proteins on such chromosomes, and PLK-2 is required for this enrichment to occur. Further, late pachytene nuclei in a plk-2 mutant exhibit the more highly dynamic SC state. Together our data demonstrate that crossover recombination events elicit chromosome-autonomous stabilizing effects on the SC and implicate PLK-2 in this process. We discuss how this recombination-triggered modulation of SC state might contribute to regulatory mechanisms that operate during meiosis to ensure the formation of crossovers while at the same time limiting their numbers.

Distinct Roles of Meiosis-Specific Cohesin Complexes in Mammalian Spermatogenesis

Mammalian meiocytes feature four meiosis-specific cohesin proteins in addition to ubiquitous ones, but the roles of the individual cohesin complexes are incompletely understood. To decipher the functions of the two meiosis-specific kleisins, REC8 or RAD21L, together with the only meiosis-specific SMC protein SMC1β, we generated Smc1β-/-Rec8-/- and Smc1β-/-Rad21L-/- mouse mutants. Analysis of spermatocyte chromosomes revealed that besides SMC1β complexes, SMC1α/RAD21 and to a small extent SMC1α/REC8 contribute to chromosome axis length. Removal of SMC1β and RAD21L almost completely abolishes all chromosome axes. The sex chromosomes do not pair in single or double mutants, and autosomal synapsis is impaired in all mutants. Super resolution microscopy revealed synapsis-associated SYCP1 aberrantly deposited between sister chromatids and on single chromatids in Smc1β-/-Rad21L-/- cells. All mutants show telomere length reduction and structural disruptions, while wild-type telomeres feature a circular TRF2 structure reminiscent of t-loops. There is no loss of centromeric cohesion in both double mutants at leptonema/early zygonema, indicating that, at least in the mutant backgrounds, an SMC1α/RAD21 complex provides centromeric cohesion at this early stage. Thus, in early prophase I the most prominent roles of the meiosis-specific cohesins are in axis-related features such as axis length, synapsis and telomere integrity rather than centromeric cohesion.

The distribution of α-kleisin during meiosis in the holocentromeric plant Luzula elegans

Holocentric chromosomes occur in a number of independent eukaryotic lineages, and they form holokinetic kinetochores along the entire poleward chromatid surfaces. Due to this alternative chromosome structure, Luzula elegans sister chromatids segregate already in anaphase I followed by the segregation of the homologues in anaphase II. However, not yet known is the localization and dynamics of cohesin and the structure of the synaptonemal complex (SC) during meiosis. We show here that the α-kleisin subunit of cohesin localizes at the centromeres of both mitotic and meiotic metaphase chromosomes and that it, thus, may contribute to assemble the centromere in L. elegans. This localization and the formation of a tripartite SC structure indicate that the prophase I behaviour of L. elegans is similar as in monocentric species.

Early programming of the oocyte epigenome temporally controls late prophase I transcription and chromatin remodelling

Oocytes are arrested for long periods of time in the prophase of the first meiotic division (prophase I). As chromosome condensation poses significant constraints to gene expression, the mechanisms regulating transcriptional activity in the prophase I-arrested oocyte are still not entirely understood. We hypothesized that gene expression during the prophase I arrest is primarily epigenetically regulated. Here we comprehensively define the Drosophila female germ line epigenome throughout oogenesis and show that the oocyte has a unique, dynamic and remarkably diversified epigenome characterized by the presence of both euchromatic and heterochromatic marks. We observed that the perturbation of the oocyte's epigenome in early oogenesis, through depletion of the dKDM5 histone demethylase, results in the temporal deregulation of meiotic transcription and affects female fertility. Taken together, our results indicate that the early programming of the oocyte epigenome primes meiotic chromatin for subsequent functions in late prophase I.

Kdm5/Lid Regulates Chromosome Architecture in Meiotic Prophase I Independently of Its Histone Demethylase Activity

During prophase of the first meiotic division (prophase I), chromatin dynamically reorganises to recombine and prepare for chromosome segregation. Histone modifying enzymes are major regulators of chromatin structure, but our knowledge of their roles in prophase I is still limited. Here we report on crucial roles of Kdm5/Lid, one of two histone demethylases in Drosophila that remove one of the trimethyl groups at Lys4 of Histone 3 (H3K4me3). In the absence of Kdm5/Lid, the synaptonemal complex was only partially formed and failed to be maintained along chromosome arms, while localisation of its components at centromeres was unaffected. Kdm5/Lid was also required for karyosome formation and homologous centromere pairing in prophase I. Although loss of Kdm5/Lid dramatically increased the level of H3K4me3 in oocytes, catalytically inactive Kdm5/Lid can rescue the above cytological defects. Therefore Kdm5/Lid controls chromatin architecture in meiotic prophase I oocytes independently of its demethylase activity.

Accurate transmission of chromosomes carrying genetic materials from generation to generation is essential for life. Cell divisions that generate gametes, such as eggs and sperm, are critical, as chromosomes inherited from both parents recombine and are accurately sorted into gametes. Errors in these cell divisions often result in infertility, miscarriages or birth defects such as Down syndrome in humans. During these divisions, chromosomes undergo dramatic reorganisation but the molecular mechanisms are not well understood. Chromosome organisation is known to be regulated by various epigenetic marks, which are chemical marks on chromatin crucial for regulating gene expression. We found that an enzyme (Kdm5/Lid) that erases a mark linked to active gene expression regulates multiple aspects of meiotic chromatin organisation in oocytes, including stability of the recombination machinery. Unexpectedly, this function does not require its enzymatic activity. Our findings provide novel insights into how chromosomes are reorganised during reproduction and prompt re-evaluation of the role of this eraser enzyme.

Histone H2AFX Links Meiotic Chromosome Asynapsis to Prophase I Oocyte Loss in Mammals

Chromosome abnormalities are common in the human population, causing germ cell loss at meiotic prophase I and infertility. The mechanisms driving this loss are unknown, but persistent meiotic DNA damage and asynapsis may be triggers. Here we investigate the contribution of these lesions to oocyte elimination in mice with chromosome abnormalities, e.g. Turner syndrome (XO) and translocations. We show that asynapsed chromosomes trigger oocyte elimination at diplonema, which is linked to the presence of phosphorylated H2AFX (γH2AFX). We find that DNA double-strand break (DSB) foci disappear on asynapsed chromosomes during pachynema, excluding persistent DNA damage as a likely cause, and demonstrating the existence in mammalian oocytes of a repair pathway for asynapsis-associated DNA DSBs. Importantly, deletion or point mutation of H2afx restores oocyte numbers in XO females to wild type (XX) levels. Unexpectedly, we find that asynapsed supernumerary chromosomes do not elicit prophase I loss, despite being enriched for γH2AFX and other checkpoint proteins. These results suggest that oocyte loss cannot be explained simply by asynapsis checkpoint models, but is related to the gene content of asynapsed chromosomes. A similar mechanistic basis for oocyte loss may operate in humans with chromosome abnormalities.

Chromosome abnormalities, such as aneuploidies and structural variants (i.e. translocations, inversions), are strikingly common in the human population, causing disorders such as Down syndrome and Turner syndrome. One important consequence of chromosome abnormalities in mammals is errors during meiosis, the specialized cell division that generates sperm and eggs for reproduction. As a result of these meiotic errors, patients with chromosome abnormalities oftentimes suffer from infertility due to loss of developing germ cells. The precise molecular mechanism for germ cell losses and infertility due to chromosome abnormalities is not well understood, but is hypothesized to result from a surveillance mechanism, which has evolved to prevent aneuploidies from developing from abnormal germ cells. In mammals, meiotic surveillance mechanisms have been hypothesized to monitor for unrepaired DNA double-strand breaks (DSB) and/or chromosome pairing/synapsis errors. Here we test these hypotheses using a variety of chromosomally variant mouse models. We find that germ cell loss in female mice with chromosome abnormalities is dependent on phosphorylation of the histone variant H2AFX, an epigenetic mark involved in the transcriptional silencing of asynapsed chromosomes during meiosis. These data inform a silencing-based mechanism of germ cell loss in patients with chromosome abnormalities and for the prophase I surveillance system which safeguards against aneuploidy.

Rejuvenation of meiotic cohesion in oocytes during prophase I is required for chiasma maintenance and accurate chromosome segregation

Chromosome segregation errors in human oocytes are the leading cause of birth defects, and the risk of aneuploid pregnancy increases dramatically as women age. Accurate segregation demands that sister chromatid cohesion remain intact for decades in human oocytes, and gradual loss of the original cohesive linkages established in fetal oocytes is proposed to be a major cause of age-dependent segregation errors. Here we demonstrate that maintenance of meiotic cohesion in Drosophila oocytes during prophase I requires an active rejuvenation program, and provide mechanistic insight into the molecular events that underlie rejuvenation. Gal4/UAS inducible knockdown of the cohesion establishment factor Eco after meiotic S phase, but before oocyte maturation, causes premature loss of meiotic cohesion, resulting in destabilization of chiasmata and subsequent missegregation of recombinant homologs. Reduction of individual cohesin subunits or the cohesin loader Nipped B during prophase I leads to similar defects. These data indicate that loading of newly synthesized replacement cohesin rings by Nipped B and establishment of new cohesive linkages by the acetyltransferase Eco must occur during prophase I to maintain cohesion in oocytes. Moreover, we show that rejuvenation of meiotic cohesion does not depend on the programmed induction of meiotic double strand breaks that occurs during early prophase I, and is therefore mechanistically distinct from the DNA damage cohesion re-establishment pathway identified in G2 vegetative yeast cells. Our work provides the first evidence that new cohesive linkages are established in Drosophila oocytes after meiotic S phase, and that these are required for accurate chromosome segregation. If such a pathway also operates in human oocytes, meiotic cohesion defects may become pronounced in a woman's thirties, not because the original cohesive linkages finally give out, but because the rejuvenation program can no longer supply new cohesive linkages at the same rate at which they are lost.

OsHUS1 facilitates accurate meiotic recombination in rice

Meiotic recombination normally takes place between allelic sequences on homologs. This process can also occur between non-allelic homologous sequences. Such ectopic interaction events can lead to chromosome rearrangements and are normally avoided. However, much remains unknown about how these ectopic interaction events are sensed and eliminated. In this study, using a screen in rice, we characterized a homolog of HUS1 and explored its function in meiotic recombination. In Oshus1 mutants, in conjunction with nearly normal homologous pairing and synapsis, vigorous, aberrant ectopic interactions occurred between nonhomologous chromosomes, leading to multivalent formation and subsequent chromosome fragmentation. These ectopic interactions relied on programed meiotic double strand breaks and were formed in a manner independent of the OsMER3-mediated interference-sensitive crossover pathway. Although early homologous recombination events occurred normally, the number of interference-sensitive crossovers was reduced in the absence of OsHUS1. Together, our results indicate that OsHUS1 might be involved in regulating ectopic interactions during meiosis, probably by forming the canonical RAD9-RAD1-HUS1 (9-1-1) complex.

Meiosis is a special type of cell division that generates gametes for sexual reproduction. During meiosis, recombination not only occurs between allelic sequences on homologs, but also between non-allelic homologous sequences at dispersed loci. Such ectopic recombination is the main cause of chromosomal alterations and accounts for numerous genomic disorders in humans. To ensure genomic integrity, those ectopic recombinations must be quickly resolved. Despite the importance of ectopic recombination suppression, the mechanism underlying this process still remains largely unknown. Here, using rice as a model system, we identified the rice HUS1 homolog, a member of the RAD9-RAD1-HUS1 (9-1-1) complex, and elucidated its roles in meiotic recombination. In Oshus1, vigorous ectopic interactions occur between nonhomologous chromosomes, and the number of crossovers is reduced. We suspect that OsHUS1 participates in regulating ectopic interactions during meiosis, probably by forming the canonical RAD9-RAD1-HUS1 (9-1-1) complex.

Meiotic cohesin SMC1β provides prophase I centromeric cohesion and is required for multiple synapsis-associated functions

Cohesin subunit SMC1β is specific and essential for meiosis. Previous studies showed functions of SMC1β in determining the axis-loop structure of synaptonemal complexes (SCs), in providing sister chromatid cohesion (SCC) in metaphase I and thereafter, in protecting telomere structure, and in synapsis. However, several central questions remained unanswered and concern roles of SMC1β in SCC and synapsis and processes related to these two processes. Here we show that SMC1β substantially supports prophase I SCC at centromeres but not along chromosome arms. Arm cohesion and some of centromeric cohesion in prophase I are provided by non-phosphorylated SMC1α. Besides supporting synapsis of autosomes, SMC1β is also required for synapsis and silencing of sex chromosomes. In absence of SMC1β, the silencing factor γH2AX remains associated with asynapsed autosomes and fails to localize to sex chromosomes. Microarray expression studies revealed up-regulated sex chromosome genes and many down-regulated autosomal genes. SMC1β is further required for non-homologous chromosome associations observed in absence of SPO11 and thus of programmed double-strand breaks. These breaks are properly generated in Smc1β^−/− spermatocytes, but their repair is delayed on asynapsed chromosomes. SMC1α alone cannot support non-homologous associations. Together with previous knowledge, three main functions of SMC1β have emerged, which have multiple consequences for spermatocyte biology: generation of the loop-axis architecture of SCs, homologous and non-homologous synapsis, and SCC starting in early prophase I.

The generation of mammalian gametes through meiosis comprises two subsequent cell divisions. The first division, meiosis I, features highly specific chromosome structures, and behavior, and requires distinct sets of chromosome-associated proteins. Cohesin proteins, of which some are meiosis-specific, are essential for meiosis, but their particular roles in meiosis are incompletely understood. We show here that SMC1β, a meiosis-specific cohesin, serves key functions already in prophase of meiosis I: SMC1β contributes to keeping sister chromatids in cohesion at their centromeres and supports synapsis of the four sister chromatids present in these cells. SMC1β is required for the synapsis of the X and Y sex chromosomes. The failure of autosomes to properly synapse in absence of SMC1β causes extensive alterations in gene expression. This leads to expression of sex chromosome-linked genes, which are lethal at this stage, explaining the death of spermatocytes in mid-prophase I. Together with the analyses of other cohesin proteins and of phosphorylated forms of SMC3 and SMC1α, this paper describes hitherto undescribed properties and functions of meiotic cohesin in sister chromatid cohesion and synapsis.

The DNA damage checkpoint protein RAD9A is essential for male meiosis in the mouse

In mitotic cells, RAD9A functions in repairing DNA double-strand breaks (DSBs) by homologous recombination and facilitates the process by cell cycle checkpoint control in response to DNA damage. DSBs occur naturally in the germline during meiosis but whether RAD9A participates in repairing such breaks is not known. In this study, we determined that RAD9A is indeed expressed in the male germ line with a peak of expression in late pachytene and diplotene stages, and the protein was found associated with the XY body. As complete loss of RAD9A is embryonic lethal, we constructed and characterized a mouse strain with Stra8-Cre driven germ cell-specific ablation of Rad9a beginning in undifferentiated spermatogonia in order to assess its role in spermatogenesis. Adult mutant male mice were infertile or sub-fertile due to massive loss of spermatogenic cells. The onset of this loss occurs during meiotic prophase, and there was an increase in the numbers of apoptotic spermatocytes as determined by TUNEL. Spermatocytes lacking RAD9A usually arrested in meiotic prophase, specifically in pachytene. The incidence of unrepaired DNA breaks increased, as detected by accumulation of γH2AX and DMC1 foci on the axes of autosomal chromosomes in pachytene spermatocytes. The DNA topoisomerase IIβ-binding protein 1 (TOPBP1) was still localized to the sex body, albeit with lower intensity, suggesting that RAD9A may be dispensable for sex body formation. We therefore show for the first time that RAD9A is essential for male fertility and for repair of DNA DSBs during meiotic prophase I.

Combinatorial regulation of meiotic holliday junction resolution in C. elegans by HIM-6 (BLM) helicase, SLX-4, and the SLX-1, MUS-81 and XPF-1 nucleases

Holliday junctions (HJs) are cruciform DNA structures that are created during recombination events. It is a matter of considerable importance to determine the resolvase(s) that promote resolution of these structures. We previously reported that C. elegans GEN-1 is a symmetrically cleaving HJ resolving enzyme required for recombinational repair, but we could not find an overt role in meiotic recombination. Here we identify C. elegans proteins involved in resolving meiotic HJs. We found no evidence for a redundant meiotic function of GEN-1. In contrast, we discovered two redundant HJ resolution pathways likely coordinated by the SLX-4 scaffold protein and also involving the HIM-6/BLM helicase. SLX-4 associates with the SLX-1, MUS-81 and XPF-1 nucleases and has been implicated in meiotic recombination in C. elegans. We found that C. elegans [mus-81; xpf-1], [slx-1; xpf-1], [mus-81; him-6] and [slx-1; him-6] double mutants showed a similar reduction in survival rates as slx-4. Analysis of meiotic diakinesis chromosomes revealed a distinct phenotype in these double mutants. Instead of wild-type bivalent chromosomes, pairs of "univalents" linked by chromatin bridges occur. These linkages depend on the conserved meiosis-specific transesterase SPO-11 and can be restored by ionizing radiation, suggesting that they represent unresolved meiotic HJs. This suggests the existence of two major resolvase activities, one provided by XPF-1 and HIM-6, the other by SLX-1 and MUS-81. In all double mutants crossover (CO) recombination is reduced but not abolished, indicative of further redundancy in meiotic HJ resolution. Real time imaging revealed extensive chromatin bridges during the first meiotic division that appear to be eventually resolved in meiosis II, suggesting back-up resolution activities acting at or after anaphase I. We also show that in HJ resolution mutants, the restructuring of chromosome arms distal and proximal to the CO still occurs, suggesting that CO initiation but not resolution is likely to be required for this process.

Dynamics of cohesin subunits in grasshopper meiotic divisions

The cohesin complex plays a key role for the maintenance of sister chromatid cohesion and faithful chromosome segregation in both mitosis and meiosis. This complex is formed by two structural maintenance of chromosomes protein family (SMC) subunits and two non-SMC subunits: an α-kleisin subunit SCC1/RAD21/REC8 and an SCC3-like protein. Several studies carried out in different species have revealed that the distribution of the cohesin subunits along the chromosomes during meiotic prophase I is not regular and that some subunits are distinctly incorporated at different cell stages. However, the accurate distribution of the different cohesin subunits in condensed meiotic chromosomes is still controversial. Here, we describe the dynamics of the cohesin subunits SMC1α, SMC3, RAD21 and SA1 during both meiotic divisions in grasshoppers. Although these subunits show a similar patched labelling at the interchromatid domain of metaphase I bivalents, SMCs and non-SMCs subunits do not always colocalise. Indeed, SA1 is the only cohesin subunit accumulated at the centromeric region of all metaphase I chromosomes. Additionally, non-SMC subunits do not appear at the interchromatid domain in either single X or B chromosomes. These data suggest the existence of several cohesin complexes during metaphase I. The cohesin subunits analysed are released from chromosomes at the beginning of anaphase I, with the exception of SA1 which can be detected at the centromeres until telophase II. These observations indicate that the cohesin components may be differentially loaded and released from meiotic chromosomes during the first and second meiotic divisions. The roles of these cohesin complexes for the maintenance of chromosome structure and their involvement in homologous segregation at first meiotic division are proposed and discussed.

Multiple opposing constraints govern chromosome interactions during meiosis

Homolog pairing and crossing over during meiosis I prophase is required for accurate chromosome segregation to form euploid gametes. The repair of Spo11-induced double-strand breaks (DSB) using a homologous chromosome template is a major driver of pairing in many species, including fungi, plants, and mammals. Inappropriate pairing and crossing over at ectopic loci can lead to chromosome rearrangements and aneuploidy. How (or if) inappropriate ectopic interactions are disrupted in favor of allelic interactions is not clear. Here we used an in vivo "collision" assay in budding yeast to test the contributions of cohesion and the organization and motion of chromosomes in the nucleus on promoting or antagonizing interactions between allelic and ectopic loci at interstitial chromosome sites. We found that deletion of the cohesin subunit Rec8, but not other chromosome axis proteins (e.g. Red1, Hop1, or Mek1), caused an increase in homolog-nonspecific chromosome interaction, even in the absence of Spo11. This effect was partially suppressed by expression of the mitotic cohesin paralog Scc1/Mdc1, implicating Rec8's role in cohesion rather than axis integrity in preventing nonspecific chromosome interactions. Disruption of telomere-led motion by treating cells with the actin polymerization inhibitor Latrunculin B (Lat B) elevated nonspecific collisions in rec8Δ spo11Δ. Next, using a visual homolog-pairing assay, we found that the delay in homolog pairing in mutants defective for telomere-led chromosome motion (ndj1Δ or csm4Δ) is enhanced in Lat B-treated cells, implicating actin in more than one process promoting homolog juxtaposition. We suggest that multiple, independent contributions of actin, cohesin, and telomere function are integrated to promote stable homolog-specific interactions and to destabilize weak nonspecific interactions by modulating the elastic spring-like properties of chromosomes.

Phosphorylation of chromosome core components may serve as axis marks for the status of chromosomal events during mammalian meiosis

Meiotic recombination and chromosome synapsis between homologous chromosomes are essential for proper chromosome segregation at the first meiotic division. While recombination and synapsis, as well as checkpoints that monitor these two events, take place in the context of a prophase I-specific axial chromosome structure, it remains unclear how chromosome axis components contribute to these processes. We show here that many protein components of the meiotic chromosome axis, including SYCP2, SYCP3, HORMAD1, HORMAD2, SMC3, STAG3, and REC8, become post-translationally modified by phosphorylation during the prophase I stage. We found that HORMAD1 and SMC3 are phosphorylated at a consensus site for the ATM/ATR checkpoint kinase and that the phosphorylated forms of HORMAD1 and SMC3 localize preferentially to unsynapsed chromosomal regions where synapsis has not yet occurred, but not to synapsed or desynapsed regions. We investigated the genetic requirements for the phosphorylation events and revealed that the phosphorylation levels of HORMAD1, HORMAD2, and SMC3 are dramatically reduced in the absence of initiation of meiotic recombination, whereas BRCA1 and SYCP3 are required for normal levels of phosphorylation of HORMAD1 and HORMAD2, but not of SMC3. Interestingly, reduced HORMAD1 and HORMAD2 phosphorylation is associated with impaired targeting of the MSUC (meiotic silencing of unsynapsed chromatin) machinery to unsynapsed chromosomes, suggesting that these post-translational events contribute to the regulation of the synapsis surveillance system. We propose that modifications of chromosome axis components serve as signals that facilitate chromosomal events including recombination, checkpoint control, transcription, and synapsis regulation.

[Morphological manifestation of a unique DNA segment in human meiotic prophase I]

The commercial sample of human DNA fragment from the choromosome 17 was used as the probe for FISH to study of the mode of its attachment to the lateral elements of synaptonemal complex (SC) in human spermatocytes. It was a 160 kb probe from the band 17p1.2, containing RAI1 gene with D17S620 marker (the probe for deletion causing Smith-Magenis syndrome). The probe made lateral chromatin protrusions, contacting with SC stained with anty-SYCP3. Different morphological configuration of lateral chromatin protrusions where observed. They depended on substages of meiotic prophase I. At zygotene, FISH probe form two sticks, c. a. 6 micro long, which was perpendicular to SC longitudinal axe, one stick at each SC side. At early pachytene, each stick transforms into a globule, one globule at each SC side again. At late pachytene each globule transformed into two crumbly globules containing short threads and clumps. At diplotene, globules finally transformed into thin DNA (chromatin) loops up to 10 micro long from the base to top with periodical thickenings (beads) along their length. As the result of this dynamics of transformation, two chromatin loops with beads were found on each side of SC of the chromosome 17. These loops most probably were the loops of sister chromatides, the full set of chromatide loops at the particular SC (bivalent) site being four in number, i. e. representing of two pair of chromatides. This study is the first one in which lateral chromatin loops in human mail meiotic prophase I are visualized as true open loop instead of that usually postulated "loops" after observation of condensed road-like or brush-like chromatin protrusion attached to the lateral elements of synaptonemal complexes. Open configuration of the loops, presumably, depends on activation of transcription during late pachytene-early diplotene. They resemble lateral loops of mini lampbrush chromosomes.

Chromosome organization and dynamics in plants

The past few years have brought renewed interest in understanding the dynamics of chromosomes in interphase cells as well as during cell division, particularly meiosis. This research has been fueled by new imaging methods, particularly three-dimensional, high-resolution, and live microscopy. Major contributors are also new genetic tools that allow elucidation of mechanisms controlling chromosome behavior. Recent studies in plants have explored chromatin arrangement in interphase nuclei, chromosome interactions and movement during meiotic prophase I, and mechanisms that ensure correct segregation of chromosomes during anaphase. These studies shed light on chromosome dynamics in a small-genome plant Arabidopsis thaliana, as well as in plants with large and complex genomes of polyploid origin, such as wheat and maize.

DNA double-strand breaks, but not crossovers, are required for the reorganization of meiotic nuclei in Tetrahymena

During meiosis, the micronuclei of the ciliated protist Tetrahymena thermophila elongate dramatically. Within these elongated nuclei, chromosomes are arranged in a bouquet-like fashion and homologous pairing and recombination takes place. We studied meiotic chromosome behavior in Tetrahymena in the absence of two genes, SPO11 and a homolog of HOP2 (HOP2A), which have conserved roles in the formation of meiotic DNA double-strand breaks (DSBs) and their repair, respectively. Single-knockout mutants for each gene display only a moderate reduction in chromosome pairing, but show a complete failure to form chiasmata and exhibit chromosome missegregation. The lack of SPO11 prevents the elongation of meiotic nuclei, but it is restored by the artificial induction of DSBs. In the hop2ADelta mutant, the transient appearance of gamma-H2A.X and Rad51p signals indicates the formation and efficient repair of DSBs; but this repair does not occur by interhomolog crossing over. In the absence of HOP2A, the nuclei are elongated, meaning that DSBs but not their conversion to crossovers are required for the development of this meiosis-specific morphology. In addition, by in silico homology searches, we compiled a list of likely Tetrahymena meiotic proteins as the basis for further studies of the unusual synaptonemal complex-less meiosis in this phylogenetically remote model organism.

Small RNAs and RNAi pathways in meiotic prophase I

RNA interference is involved in many aspects of cell biology, and the recent identification of germ-cell specific small RNAs has led to speculation that RNAi might also be involved in gametogenesis. Work in yeast indicates that RNAi is involved in establishing and maintaining heterochromatin at centromeres, an important component of yeast and mammalian meiosis. Here we review developments in the field of RNAi and relate these to possible roles in mammalian gametogenesis.

Potential functions of retinoic acid receptor A in Sertoli cells and germ cells during spermatogenesis

Elucidation of the retinoid signaling circuitry in the testis is critical to understanding how male germ cells develop to spermatozoa. Retinoic acid receptor A protein (RARA) is an essential mediator of retinoid signaling in the testis, as shown by a sterility phenotype observed for retinoic acid receptor A gene (Rara) knockout male mice. The seminiferous tubules of Rara knockout mice showed varying degrees of germ-cell degeneration. A dramatic increase in apoptosis of early meiotic prophase spermatocytes was observed in these tubules compared to the wild-type tubules. Germ-cell loss was dependent on the stages of the spermatogenic cycle: germ-cell loss was negligible in stages I-V, but severe after stages VIII and IX of the spermatogenic cycle. Using spermatogonial transplantation, the individual function of RARA in Sertoli cells or germ cells was determined. The wild-type donor germ cells, transplanted into Rara knockout testes, colonized and proliferated in the RARA-deficient microenvironment. The donor-derived cells were mostly early meiotic prophase spermatocytes, with few more advanced germ cells detected. Conversely, when Rara-deficient germ cells were injected into the microenvironment that express RARA, establishment of donor-derived germ-cell colonies was rare, but remarkably, once colonized, Rara-deficient germ cells progressed normally through spermatogenesis. These results together suggest that RARA may function in Sertoli cells to promote the survival and development of early meiotic prophase spermatocytes, whereas RARA in germ cells functions to increase the proliferation and differentiation of spermatogonia, prior to meiotic prophase.

Mutation in mouse hei10, an e3 ubiquitin ligase, disrupts meiotic crossing over

Crossing over during meiotic prophase I is required for sexual reproduction in mice and contributes to genome-wide genetic diversity. Here we report on the characterization of an N-ethyl-N-nitrosourea-induced, recessive allele called mei4, which causes sterility in both sexes owing to meiotic defects. In mutant spermatocytes, chromosomes fail to congress properly at the metaphase plate, leading to arrest and apoptosis before the first meiotic division. Mutant oocytes have a similar chromosomal phenotype but in vitro can undergo meiotic divisions and fertilization before arresting. During late meiotic prophase in mei4 mutant males, absence of cyclin dependent kinase 2 and mismatch repair protein association from chromosome cores is correlated with the premature separation of bivalents at diplonema owing to lack of chiasmata. We have identified the causative mutation, a transversion in the 5′ splice donor site of exon 1 in the mouse ortholog of Human Enhancer of Invasion 10 (Hei10; also known as Gm288 in mouse and CCNB1IP1 in human), a putative B-type cyclin E3 ubiquitin ligase. Importantly, orthologs of Hei10 are found exclusively in deuterostomes and not in more ancestral protostomes such as yeast, worms, or flies. The cloning and characterization of the mei4 allele of Hei10 demonstrates a novel link between cell cycle regulation and mismatch repair during prophase I.

Human infertility and reproductive complications have devastating social and monetary costs. Errors in meiosis during reproduction may lead to birth defects, spontaneous abortion, or infertility. Many of the genes essential for meiosis function in DNA repair and mutations in several of these genes have been shown to contribute to cancer. The identification of the genes necessary for normal meiosis is an important goal and will potentially influence the fields of reproductive and cancer biology. In this study, genetic screens in mice have generated the mutation mei4. mei4 causes male and female sterility by disrupting meiosis and altering the function of the DNA repair system known as mismatch repair. We have identified the causative mutation behind the mei4 phenotype in a gene called Human Enhancer of Invasion 10 or Hei10. This work demonstrates that Hei10 is essential for the completion of meiosis and that it functions to coordinate the DNA repair system and the progression of the cell cycle during meiosis.

Characterization of Spo11-dependent and independent phospho-H2AX foci during meiotic prophase I in the male mouse

Meiotic DNA double strand breaks (DSBs) are indicated at leptotene by the phosphorylated form of histone H2AX (gamma-H2AX). In contrast to previous studies, we identified on both zygotene and pachytene chromosomes two distinct types of gamma-H2AX foci: multiple small (S) foci located along autosomal synaptonemal complexes (SCs) and larger signals on chromatin loops (L-foci). The S-foci number gradually declined throughout pachytene, in parallel with the repair of DSBs monitored by repair proteins suggesting that S-foci mark DSB repair events. We validated this interpretation by showing the absence of S-foci in Spo11(-/-) spermatocytes. By contrast, the L-foci number was very low through pachytene. Based on the analysis of gamma-H2AX labeling after irradiation of spermatocytes, the formation of DSBs clearly induced L-foci formation. Upon DSB repair, these foci appear to be processed and lead to the above mentioned S-foci. The presence of L-foci in wild-type pachytene and diplotene could therefore reflect delayed or unregulated DSB repair events. Interestingly, their distribution was different in Spo11(+/-) spermatocytes compared with Spo11(+/+) spermatocytes, where DSB repair might be differently regulated as a response to homeostatic control of crossing-over. The presence of these L-foci in Spo11(-/-) spermatocytes raises the interesting possibility of yet uncharacterized alterations in DNA or chromosome structure in Spo11(-/-) cells.

RAD51C deficiency in mice results in early prophase I arrest in males and sister chromatid separation at metaphase II in females

RAD51C is a member of the RecA/RAD51 protein family, which is known to play an important role in DNA repair by homologous recombination. In mice, it is essential for viability. Therefore, we have generated a hypomorphic allele of Rad51c in addition to a null allele. A subset of mice expressing the hypomorphic allele is infertile. This infertility is caused by sexually dimorphic defects in meiotic recombination, revealing its two distinct functions. Spermatocytes undergo a developmental arrest during the early stages of meiotic prophase I, providing evidence for the role of RAD51C in early stages of RAD51-mediated recombination. In contrast, oocytes can progress normally to metaphase I after superovulation but display precocious separation of sister chromatids, aneuploidy, and broken chromosomes at metaphase II. These defects suggest a possible late role of RAD51C in meiotic recombination. Based on the marked reduction in Holliday junction (HJ) resolution activity in Rad51c-null mouse embryonic fibroblasts, we propose that this late function may be associated with HJ resolution.

RNA Interference as a tool to study the function of MAD2 in mouse oocyte meiotic maturation

Spindle checkpoint proteins control entry into anaphase and chromosome segregation. As a member of spindle checkpoint proteins, MAD2 takes a central role in the regulation of anaphase onset and genome integrity. Here, we used MAD2 siRNA transfection approach to study MAD2 functions during mouse oocyte meiotic maturation in vitro. Real-time PCR and laser scanning confocal microscopy showed that we successfully downregulated MAD2 transcript and protein expression. We further demonstrated that MAD2 downregulation resulted in a shortened duration of meiosis I and meiotic spindle abnormality, suggesting the function of MAD2 in mouse oocyte meiotic maturation. We also showed that MAD2 interference to some extent decreased GVBD rate, but increased apoptosis in mouse oocytes. In conclusion, our study shows that siRNA transfection is an effective tool to study MAD2 functions, and our results provide further evidence for the role of MAD2 as a spindle checkpoint protein in mouse oocytes.

Alleles of afd1 dissect REC8 functions during meiotic prophase I

REC8 is a master regulator of chromatin structure and function during meiosis. Here, we dissected the functions of absence of first division (afd1), a maize rec8/alpha-kleisin homolog, using a unique afd1 allelic series. The first observable defect in afd1 mutants is the inability to make a leptotene chromosome. AFD1 protein is required for elongation of axial elements but not for their initial recruitment, thus showing that AFD1 acts downstream of ASY1/HOP1. AFD1 is associated with the axial and later the lateral elements of the synaptonemal complex. Rescuing 50% of axial element elongation in the weakest afd1 allele restored bouquet formation demonstrating that extent of telomere clustering depends on axial element elongation. However, rescuing bouquet formation was not sufficient for either proper RAD51 distribution or homologous pairing. It provides the basis for a model in which AFD1/REC8 controls homologous pairing through its role in axial element elongation and the subsequent distribution of the recombination machinery independent of bouquet formation.

Genetic analysis of chromosome pairing, recombination, and cell cycle control during first meiotic prophase in mammals

Meiosis is a double-division process that is preceded by only one DNA replication event to produce haploid gametes. The defining event in meiosis is prophase I, during which chromosome pairs locate each other, become physically connected, and exchange genetic information. Although many aspects of this process have been elucidated in lower organisms, there has been scant information available until now about the process in mammals. Recent advances in genetic analysis, especially in mice and humans, have revealed many genes that play essential roles in meiosis in mammals. These include cell cycle-regulatory proteins that couple the exit from the premeiotic DNA synthesis to the progression through prophase I, the chromosome structural proteins involved in synapsis, and the repair and recombination proteins that process the recombination events. Failure to adequately repair the DNA damage caused by recombination triggers meiotic checkpoints that result in ablation of the germ cells by apoptosis. These analyses have revealed surprising sexual dimorphism in the requirements of different gene products and a much less stringent checkpoint regulation in females. This may provide an explanation for the 10-fold increase in meiotic errors in females compared with males. This review provides a comprehensive analysis of the use of genetic manipulation, particularly in mice, but also of the analysis of mutations in humans, to elucidate the mechanisms that are required for traverse through prophase I.

Cell cycle control by daf-21/Hsp90 at the first meiotic prophase/metaphase boundary during oogenesis in Caenorhabditis elegans

DAF-21, a Caenorhabditis elegans homologue of Hsp90, is expressed primarily in germline cells. Although mutations in the daf-21 gene affect animal fertility, its cellular roles have remained elusive. To phenocopy daf-21 mutations, we impaired the daf-21 function by RNA interference (RNAi), and found that oocytes skipped the diakinesis arrest and displayed a defective diakinesis arrest, which led to the production of endomitotic oocytes with polyploid chromosomes (Emo phenotype). The same Emo phenotype was also observed with RNAi against wee-1.3. To identify a cause for Emo, we examined the CDK-1 (Cdc2) phosphorylation status in Emo animals, since CDK-1 is a key regulator of the prophase/metaphase transition and is kept inactivated by WEE-1.3 kinase during prophase. We immunostained both daf-21(RNAi) and wee-1.3(RNAi) animals with anti-phosphorylated-CDK-1 antibody and observed no detectable phosphates on CDK-1 in either of the animals. We also examined WEE-1.3 expression in daf-21(RNAi) and found a significant reduction of WEE-1.3. These results indicate that CDK-1 was not phosphorylated in either daf-21(RNAi) or wee-1.3(RNAi) animals, and suggest that daf-21 was necessary for producing functional WEE-1.3. Thus, all together, we propose that DAF-21 indirectly regulates the meiotic prophase/metaphase transition during oocyte development by ensuring the normal function of WEE-1.3.

Chromosome sites play dual roles to establish homologous synapsis during meiosis in C. elegans

We have investigated the role of pairing centers (PCs), cis-acting sites required for accurate segregation of homologous chromosomes during meiosis in C. elegans. We find that these sites play two distinct roles that contribute to proper segregation. Chromosomes lacking PCs usually fail to synapse and also lack a synapsis-independent stabilization activity. The presence of a PC on just one copy of a chromosome pair promotes synapsis but does not support synapsis-independent pairing stabilization, indicating that these functions are separable. Once initiated, synapsis is highly processive, even between nonhomologous chromosomes of disparate lengths, elucidating how translocations suppress meiotic recombination in C. elegans. These findings suggest a multistep pathway for chromosome synapsis in which PCs impart selectivity and efficiency through a "kinetic proofreading" mechanism. We speculate that concentration of these activities at one region per chromosome may have coevolved with the loss of a point centromere to safeguard karyotype stability.

HIM-8 binds to the X chromosome pairing center and mediates chromosome-specific meiotic synapsis

The him-8 gene is essential for proper meiotic segregation of the X chromosomes in C. elegans. Here we show that loss of him-8 function causes profound X chromosome-specific defects in homolog pairing and synapsis. him-8 encodes a C2H2 zinc-finger protein that is expressed during meiosis and concentrates at a site on the X chromosome known as the meiotic pairing center (PC). A role for HIM-8 in PC function is supported by genetic interactions between PC lesions and him-8 mutations. HIM-8 bound chromosome sites associate with the nuclear envelope (NE) throughout meiotic prophase. Surprisingly, a point mutation in him-8 that retains both chromosome binding and NE localization fails to stabilize pairing or promote synapsis. These observations indicate that stabilization of homolog pairing is an active process in which the tethering of chromosome sites to the NE may be necessary but is not sufficient.

Genetics of meiotic prophase I in plants

During meiotic prophase I, traits are reassorted as a result of a highly organized process involving sister chromatid cohesion, homologous chromosome alignment, pairing, synapsis, and recombination. In the past two years, a number of components involved in this pathway, including Structure Maintenance of Chromosomes (SMC), MRE11, the RAD51 homologs, BRCA2, MSH4, MER3, and ZIP1, have been characterized in plants; in addition, several genes that encode components unique to plants, such as POOR HOMOLOGOUS SYNAPSIS 1 and AMEIOTIC 1, have been cloned. Based on these recent data, essentially from maize and Arabidopsis, we discuss the conserved and plant-specific aspects of meiosis commitment and meiotic prophase I features.

DNA double-strand breaks and homology search: inferences from a species with incomplete pairing and synapsis

The relationship between meiotic recombination events and different patterns of pairing and synapsis has been analysed in prophase I spermatocytes of the grasshopper Stethophyma grossum, which exhibit very unusual meiotic characteristics, namely (1) the three shortest bivalents achieve full synapsis and do not show chiasma localisation; (2) the remaining eight bivalents show restricted synapsis and proximal chiasma localisation, and (3) the X chromosome remains unsynapsed. We have studied by means of immunofluorescence the localisation of the phosphorylated histone H2AX (gamma-H2AX), which marks the sites of double-strand breaks; the SMC3 cohesin subunit, which is thought to have a close relationship to the development of the axial element (a synaptonemal complex component); and the recombinase RAD51. We observed a marked nuclear polarization of both the maturation of SMC3 cohesin axis and the ulterior appearance of gamma-H2AX and RAD51 foci, these being exclusively restricted to those chromosomal regions that first form cohesin axis stretches. This polarised distribution of recombination events is maintained throughout prophase I over those autosomal regions that are undergoing, or about to undergo, synapsis. We propose that the restricted distribution of recombination events along the chromosomal axes in the spermatocytes is responsible for the incomplete presynaptic homologous alignment and, hence, for the partial synaptonemal complex formation displayed by most bivalents.