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Several protein ensembles facilitate crossover recombination and the associated assembly of synaptonemal complex (SC) during meiosis. In yeast, meiosis-specific factors including the DNA helicase Mer3, the "ZZS" complex consisting of Zip4, Zip2, and Spo16, the RING-domain protein Zip3, and the MutSγ heterodimer collaborate with crossover-promoting activity of the SC component, Zip1, to generate crossover-designated recombination intermediates. These ensembles also promote SC formation - the organized assembly of Zip1 with other structural proteins between aligned chromosome axes. We used proximity labeling to investigate spatial relationships between meiotic recombination and SC proteins in S. cerevisiae. We find that recombination initiation and SC factors are dispensable for proximity labeling of Zip3 by ZZS components, but proteins associated with early steps in recombination are required for Zip3 proximity labeling by MutSγ, suggesting that MutSγ joins Zip3 only after a recombination intermediate has been generated. We also find that zip1 separation-of-function mutants that are crossover deficient but still assemble SC fail to generate protein ensembles where Zip3 can engage ZZS and/or MutSγ. The SC structural protein Ecm11 is proximity labeled by ZZS proteins in a Zip4-dependent and Zip1-independent manner, but labeling of Ecm11 by Zip3 and MutSγ requires, at least in part, Zip1. Finally, mass spectrometry analysis of biotinylated proteins in eleven proximity labeling strains uncovered shared proximity targets of SC and crossover-associated proteins, some of which have not previously been implicated in meiotic recombination or SC formation, highlighting the potential of proximity labeling as a discovery tool., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Voelkel-Meiman et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

A central feature of meiosis is the pairing of homologous maternal and paternal chromosomes ('homologues') along their lengths 1-3 . Recognition between homologues and their juxtaposition in space is mediated by axis-associated recombination complexes. Also, pairing must occur without entanglements among unrelated chromosomes. Here we examine homologue juxtaposition in real time by four-dimensional fluorescence imaging of tagged chromosomal loci at high spatio-temporal resolution in budding yeast. We discover that corresponding loci come together from a substantial distance (1.8 µm) and complete pairing in a very short time, about 6 min (thus, rapid homologue juxtaposition or RHJ). Homologue loci first move rapidly together (in 30 s, at speeds of roughly 60 nm s -1 ) into an intermediate stage corresponding to canonical 400 nm axis coalignment. After a short pause, crossover/non-crossover differentiation (crossover interference) mediates a second short, rapid transition that ultimately gives close pairing of axes at 100 nm by means of synaptonemal complex formation. Furthermore, RHJ (1) occurs after chromosomes acquire prophase chromosome organization, (2) is nearly synchronous over thirds of chromosome lengths, but (3) is asynchronous throughout the genome. Finally, cytoskeleton-mediated movement is important for the timing and distance of RHJ onset and for ensuring its normal progression. General implications for local and global aspects of pairing are discussed., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Meiosis is a form of cell division that is essential to sexually reproducing organisms and is therefore highly regulated. Each event of meiosis must occur at the correct developmental stage to ensure that chromosomes are segregated properly during both meiotic divisions. One unique meiosis-specific structure that is tightly regulated in terms of timing of assembly and disassembly is the synaptonemal complex (SC). While the mechanism(s) for assembly and disassembly of the SC are poorly understood in Drosophila melanogaster , posttranslational modifications, including ubiquitination and phosphorylation, are known to play a role. Here, we identify a role for the deubiquitinase Usp7 in the maintenance of the SC in early prophase and show that its function in SC maintenance is independent of the meiotic recombination process. Using two usp7 shRNA constructs that result in different knockdown levels, we have shown that the presence of SC through early/mid-pachytene is critical for normal levels and placement of crossovers., Competing Interests: Competing interests statement:The authors declare no competing interest.

Meiotic progression requires coordinated assembly and disassembly of protein complexes involved in chromosome synapsis and meiotic recombination. Mouse TRIP13 and its ortholog Pch2 are instrumental in remodeling HORMA domain proteins. HORMAD proteins are associated with unsynapsed chromosome axes but depleted from the synaptonemal complex (SC) of synapsed homologs. Here we report that TRIP13 localizes to the synapsed SC in early pachytene spermatocytes and to telomeres throughout meiotic prophase I. Loss of TRIP13 leads to meiotic arrest and thus sterility in both sexes. Trip13 -null meiocytes exhibit abnormal persistence of HORMAD1 and HOMRAD2 on synapsed SC and chromosome asynapsis that preferentially affects XY and centromeric ends. These major phenotypes are consistent with reported phenotypes of Trip13 hypomorph alleles. Trip13 heterozygous mice exhibit meiotic defects that are less severe than the Trip13 -null mice, showing that TRIP13 is a dosage-sensitive regulator of meiosis. Localization of TRIP13 to the synapsed SC is independent of SC axial element proteins such as REC8 and SYCP2/SYCP3. Terminal FLAG-tagged TRIP13 proteins are functional and recapitulate the localization of native TRIP13 to SC and telomeres. Therefore, the evolutionarily conserved localization of TRIP13/Pch2 to the synapsed chromosomes provides an explanation for dissociation of HORMA domain proteins upon synapsis in diverse organisms., Competing Interests: JC, NL, FY, IC, YG, HL, PW No competing interests declared, (© 2023, Chotiner et al.)

Chromosome inversions are of unique importance in the evolution of genomes and species because when heterozygous with a standard arrangement chromosome, they suppress meiotic crossovers within the inversion. In Drosophila species, heterozygous inversions also cause the interchromosomal effect, whereby the presence of a heterozygous inversion induces a dramatic increase in crossover frequencies in the remainder of the genome within a single meiosis. To date, the interchromosomal effect has been studied exclusively in species that also have high frequencies of inversions in wild populations. We took advantage of a recently developed approach for generating inversions in Drosophila simulans, a species that does not have inversions in wild populations, to ask if there is an interchromosomal effect. We used the existing chromosome 3R balancer and generated a new chromosome 2L balancer to assay for the interchromosomal effect genetically and cytologically. We found no evidence of an interchromosomal effect in D. simulans. To gain insights into the underlying mechanistic reasons, we qualitatively analyzed the relationship between meiotic double-stranded break (DSB) formation and synaptonemal complex (SC) assembly. We found that the SC is assembled prior to DSB formation as in D. melanogaster; however, we show that the SC is assembled prior to localization of the oocyte determination factor Orb, whereas in D. melanogaster, SC formation does not begin until the Orb is localized. Together, our data show no evidence that heterozygous inversions in D. simulans induce an interchromosomal effect and that there are differences in the developmental programming of the early stages of meiosis., Competing Interests: Conflicts of interest The author(s) declare no conflict of interest., (© The Author(s) 2024. Published by Oxford University Press on behalf of The Genetics Society of America.)

Interactions between parental chromosomes during the formation of gametes can lead to entanglements, entrapments and interlocks between unrelated chromosomes. If unresolved, these topological constraints can lead to misregulation of exchanges between chromosomes and to chromosome mis-segregation. Interestingly, these configurations are largely resolved by the time parental chromosomes are aligned during pachytene. In this Review, we highlight the inevitability of topologically complex configurations and discuss possible mechanisms to resolve them. We focus on the dynamic nature of a conserved chromosomal interface - the synaptonemal complex - and the chromosome movements that accompany meiosis as potential mechanisms to resolve topological constraints. We highlight the advantages of the nematode Caenorhabditis elegans for understanding biophysical features of the chromosome axis and synaptonemal complex that could contribute to mechanisms underlying interlock resolution. In addition, we highlight advantages of using the zebrafish, Danio rerio, as a model to understand how entanglements and interlocks are avoided and resolved., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2024. Published by The Company of Biologists Ltd.)

Cohesin is a multi-subunit protein that plays a pivotal role in holding sister chromatids together during cell division. Sister chromatid cohesion 3 (SCC3), constituents of cohesin complex, is highly conserved from yeast to mammals. Since the deletion of individual cohesin subunit always causes lethality, it is difficult to dissect its biological function in both mitosis and meiosis. Here, we obtained scc3 weak mutants using CRISPR-Cas9 system to explore its function during rice mitosis and meiosis. The scc3 weak mutants displayed obvious vegetative defects and complete sterility, underscoring the essential roles of SCC3 in both mitosis and meiosis. SCC3 is localized on chromatin from interphase to prometaphase in mitosis. However, in meiosis, SCC3 acts as an axial element during early prophase I and subsequently situates onto centromeric regions following the disassembly of the synaptonemal complex. The loading of SCC3 onto meiotic chromosomes depends on REC8. scc3 shows severe defects in homologous pairing and synapsis. Consequently, SCC3 functions as an axial element that is essential for maintaining homologous chromosome pairing and synapsis during meiosis., Competing Interests: YZ, LR, TZ, HY, YM, HL, LC, BW, YS, YL, DT, ZC No competing interests declared, (© 2024, Zhao, Ren, Zhao et al.)

Formation of the synaptonemal complex (SC) is a prerequisite for proper recombination and chromosomal segregation during meiotic prophase I. One mechanism that ensures SC formation is chromosomal movement, which is driven by the force derived from cytoskeletal motors. Here, we report the phenotype of medaka mutants lacking the telomere repeat binding bouquet formation protein 1 (TERB1), which, in combination with the SUN/KASH protein, mediates chromosomal movement by connecting telomeres and cytoskeletal motors. Mutations in the terb1 gene exhibit defects in SC formation in medaka. Although SC formation was initiated, as seen by the punctate lateral elements and fragmented transverse filaments, it was not completed in the terb1 mutant meiocytes. The mutant phenotype further revealed that the introduction of double strand breaks was independent of synapsis completion. In association with these phenotypes, meiocytes in both the ovaries and testes exhibited an aberrant arrangement of homologous chromosomes. Interestingly, although oogenesis halted at the zygotene-like stage in terb1 mutant, testes continued to produce sperm-like cells with aberrant DNA content. This indicates that the mechanism of meiotic checkpoint is sexually different in medaka, similar to the mammalian checkpoint in which oogenesis proceeds while spermatogenesis is arrested. Moreover, our results suggest that spermatogenesis is mechanistically dissociable from meiosis.

The sugarcane (Saccharum spp.) genome is one of the most complex of all. Modern varieties are highly polyploid and aneuploid as a result of hybridization between Saccharum officinarum and S. spontaneum. Little research has been done on meiotic control in polyploid species, with the exception of the wheat Ph1 locus harboring the ZIP4 gene (TaZIP4-B2) which promotes pairing between homologous chromosomes while suppressing crossover between homeologs. In sugarcane, despite its interspecific origin, bivalent association is favored, and multivalents, if any, are resolved at the end of prophase I. Thus, our aim herein was to investigate the purported genetic control of meiosis in the parental species and in sugarcane itself. We investigated the ZIP4 gene and immunolocalized meiotic proteins, namely synaptonemal complex proteins Zyp1 and Asy1. The sugarcane ZIP4 gene is located on chromosome 2 and expressed more abundantly in flowers, a similar profile to that found for TaZIP4-B2. ZIP4 expression is higher in S. spontaneum a neoautopolyploid, with lower expression in S. officinarum, a stable octoploid species. The sugarcane Zip4 protein contains a TPR domain, essential for scaffolding. Its 3D structure was also predicted, and it was found to be very similar to that of TaZIP4-B2, reflecting their functional relatedness. Immunolocalization of the Asy1 and Zyp1 proteins revealed that S. officinarum completes synapsis. However, in S. spontaneum and SP80-3280 (a modern variety), no nuclei with complete synapsis were observed. Importantly, our results have implications for sugarcane cytogenetics, genetic mapping, and genomics., (© 2024 Society for Experimental Biology and John Wiley & Sons Ltd.)

The segregation of homologous chromosomes during meiosis typically requires tight end-to-end chromosome pairing. However, in Drosophila spermatogenesis, male flies segregate their chromosomes without classic synaptonemal complex formation and without recombination, instead compartmentalizing homologs into subnuclear domains known as chromosome territories (CTs). How homologs find each other in the nucleus and are separated into CTs has been one of the biggest riddles in chromosome biology. Here, we discuss our current understanding of pairing and CT formation in flies and review recent data on how homologs are linked and partitioned during meiosis in male flies., Competing Interests: The authors have declared that no competing interests exist., (Copyright: This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.)

Background: The synaptonemal complex (SC) is a protein axis formed along chromosomes during meiotic prophase to ensure proper pairing and crossing over. SC analysis has been widely used to study the chromosomes of mammals and less frequently of birds, reptiles, and fish. It is a promising method to investigate the evolution of fish genomes and chromosomes as a part of complex approach., Summary: Compared with conventional metaphase chromosomes, pachytene chromosomes are less condensed and exhibit pairing between homologous chromosomes. These features of SCs facilitate the study of the small chromosomes that are typical in fish. Moreover, it allows the study of heteromorphisms in sex chromosomes and supernumerary chromosomes. In addition, it enables the investigation of the pairing between orthologous chromosomes in hybrids, which is crucial for uncovering the causes of hybrid sterility and asexual reproduction, such as gynogenesis or hybridogenesis. However, the application of SC analysis to fish chromosomes is limited by the associated complications. First, in most fish, meiosis does not occur during every season and life stage. Second, different SC preparation methods are optimal for different fish species. Third, commercial antibodies targeting meiotic proteins have been primarily developed against mammalian antigens, and not all of them are suitable for fish chromosomes., Key Messages: In the present review, we provide an overview of the methods for preparing fish SCs and highlight important studies using SC analysis in fish. This study will be valuable for planning and designing research that applies SC analysis to fish cytogenetics and genomics., (© 2024 S. Karger AG, Basel.)

Successful chromosome segregation into gametes depends on tightly regulated interactions between the parental chromosomes. During meiosis, chromosomes are aligned end-to-end by an interface called the synaptonemal complex, which also regulates exchanges between them. However, despite the functional and ultrastructural conservation of this essential interface, how protein-protein interactions within the synaptonemal complex regulate chromosomal interactions remains poorly understood. Here, we describe a genetic interaction in the C. elegans synaptonemal complex, comprised of short segments of three proteins, SYP-1, SYP-3, and SYP-4. We identified the interaction through a saturated suppressor screen of a mutant that destabilizes the synaptonemal complex. The specificity and tight distribution of suppressors suggest a charge-based interface that promotes interactions between synaptonemal complex subunits and, in turn, allows intimate interactions between chromosomes. Our work highlights the power of genetic studies to illuminate the mechanisms that underlie meiotic chromosome interactions., Competing Interests: Competing interests statement:The authors declare no competing interest.

The synaptonemal complex (SC) is a proteinaceous structure that forms between homologous chromosomes during meiosis prophase. The SC is widely conserved across species, but its structure and roles during meiotic recombination are still debated. While the SC central region is made up of transverse filaments and central element proteins in mammals and fungi, few central element proteins have been identified in other species. Here we report the identification of two coiled-coil proteins, SCEP1 and SCEP2, that form a complex and localize at the centre of the Arabidopsis thaliana SC. In scep1 and scep2 mutants, chromosomes are aligned but not synapsed (the ZYP1 transverse filament protein is not loaded), crossovers are increased compared with the wild type, interference is lost and heterochiasmy is strongly reduced. We thus report the identification of two plant SC central elements, and homologues of these are found in all major angiosperm clades., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

In almost all sexually reproducing organisms, meiotic recombination and cell division require the synapsis of homologous chromosomes by a large proteinaceous structure, the synaptonemal complex (SC). While the SC's overall structure is highly conserved across eukaryotes, its constituent proteins diverge between phyla. Transverse filament protein, SYCP1, spans the width of the SC and undergoes amino-terminal head-to-head self-assembly in vitro through a motif that is unusually highly conserved across kingdoms of life. Here, we report creation of mouse mutants, Sycp1 L102E and Sycp1 L106E , that target SYCP1's head-to-head interface. L106E resulted in a complete loss of synapsis, while L102E had no apparent effect on synapsis, in agreement with their differential effects on the SYCP1 head-to-head interface in molecular dynamics simulations. In Sycp1 L106E mice, homologs aligned and recruited low levels of mutant SYCP1 and other SC proteins, but the absence of synapsis led to failure of crossover formation and meiotic arrest. We conclude that SYCP1's conserved head-to-head interface is essential for meiotic chromosome synapsis in vivo.

In meiosis, homologous chromosome synapsis is mediated by a supramolecular protein structure, the synaptonemal complex (SC), that assembles between homologous chromosome axes. The mammalian SC comprises at least eight largely coiled-coil proteins that interact and self-assemble to generate a long, zipper-like structure that holds homologous chromosomes in close proximity and promotes the formation of genetic crossovers and accurate meiotic chromosome segregation. In recent years, numerous mutations in human SC genes have been associated with different types of male and female infertility. Here, we integrate structural information on the human SC with mouse and human genetics to describe the molecular mechanisms by which SC mutations can result in human infertility. We outline certain themes in which different SC proteins are susceptible to different types of disease mutation and how genetic variants with seemingly minor effects on SC proteins may act as dominant-negative mutations in which the heterozygous state is pathogenic.

The successful delivery of genetic material to gametes requires tightly regulated interactions between the parental chromosomes. Central to this regulation is a conserved chromosomal interface called the synaptonemal complex (SC), which brings the parental chromosomes in close proximity along their length. While many of its components are known, the interfaces that mediate the assembly of the SC remain a mystery. Here, we survey findings from different model systems while focusing on insight gained in the nematode C. elegans. We synthesize our current understanding of the structure, dynamics, and biophysical properties of the SC and propose mechanisms for SC assembly., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Gordon, Rog. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The shuffling of genetic material facilitated by meiotic crossovers is a critical driver of genetic variation. Therefore, the number and positions of crossover events must be carefully controlled. In Arabidopsis, an obligate crossover and repression of nearby crossovers on each chromosome pair are abolished in mutants that lack the synaptonemal complex (SC), a conserved protein scaffold. We use mathematical modelling and quantitative super-resolution microscopy to explore and mechanistically explain meiotic crossover pattering in Arabidopsis lines with full, incomplete, or abolished synapsis. For zyp1 mutants, which lack an SC, we develop a coarsening model in which crossover precursors globally compete for a limited pool of the pro-crossover factor HEI10, with dynamic HEI10 exchange mediated through the nucleoplasm. We demonstrate that this model is capable of quantitatively reproducing and predicting zyp1 experimental crossover patterning and HEI10 foci intensity data. Additionally, we find that a model combining both SC- and nucleoplasm-mediated coarsening can explain crossover patterning in wild-type Arabidopsis and in pch2 mutants, which display partial synapsis. Together, our results reveal that regulation of crossover patterning in wild-type Arabidopsis and SC-defective mutants likely acts through the same underlying coarsening mechanism, differing only in the spatial compartments through which the pro-crossover factor diffuses., Competing Interests: JF, CM, MH No competing interests declared, (© 2023, Fozard et al.)

During meiosis, programmed double-strand DNA breaks are repaired to form exchanges between the parental chromosomes called crossovers. Chromosomes lacking a crossover fail to segregate accurately into the gametes, leading to aneuploidy. In addition to engaging the homolog, crossover formation requires the promotion of exchanges, rather than non-exchanges, as repair products. However, the mechanism underlying this meiosis-specific preference is not fully understood. Here, we study the regulation of meiotic sister chromatid exchanges in Caenorhabditis elegans by direct visualization. We find that a conserved chromosomal interface that promotes exchanges between the parental chromosomes, the synaptonemal complex, can also promote exchanges between the sister chromatids. In both cases, exchanges depend on the recruitment of the same set of pro-exchange factors to repair sites. Surprisingly, although the synaptonemal complex usually assembles between the two DNA molecules undergoing an exchange, its activity does not rely on a specific chromosome conformation. This suggests that the synaptonemal complex regulates exchanges-both crossovers and sister exchanges-by establishing a nuclear domain conducive to nearby recruitment of exchange-promoting factors., (© 2023 Almanzar et al.)

Chromosome axis-associated HORMA domain proteins (HORMADs), e.g. ASY1 in Arabidopsis, are crucial for meiotic recombination. ASY1, as other HORMADs, is assembled on the axis at early meiosis and depleted when homologous chromosomes synapse. Puzzlingly, both processes are catalyzed by AAA+ ATPase PCH2 together with its cofactor COMET. Here, we show that the ASY1 remodeling complex is temporally and spatially differently assembled. While PCH2 and COMET appear to directly interact in the cytoplasm in early meiosis, PCH2 is recruited by the transverse filament protein ZYP1 and brought to the ASY1-bound COMET assuring the timely removal of ASY1 during chromosome synapsis. Since we found that the PCH2 homolog TRIP13 also binds to the ZYP1 homolog SYCP1 in mouse, we postulate that this mechanism is conserved among eukaryotes. Deleting the PCH2 binding site of ZYP1 led to a failure of ASY1 removal. Interestingly, the placement of one obligatory crossover per homologous chromosome pair, compromised by ZYP1 depletion, is largely restored in this separation-of-function zyp1 allele suggesting that crossover assurance is promoted by synapsis. In contrast, this zyp1 allele, similar to the zyp1 null mutant, showed elevated type I crossover numbers indicating that PCH2-mediated eviction of ASY1 from the axis restricts crossover formation., (© The Author(s) 2022. Published by Oxford University Press on behalf of Nucleic Acids Research.)

DNA double-strand breaks (DSBs) are deleterious lesions, which must be repaired precisely to maintain genomic stability. During meiosis, programmed DSBs are repaired via homologous recombination (HR) while repair using the nonhomologous end joining (NHEJ) pathway is inhibited, thereby ensuring crossover formation and accurate chromosome segregation. 1 , 2 How DSB repair pathway choice is implemented during meiosis is unknown. In C. elegans, meiotic DSB repair takes place in the context of the fully formed, highly dynamic zipper-like structure present between homologous chromosomes called the synaptonemal complex (SC). 3 , 4 , 5 , 6 , 7 , 8 , 9 The SC consists of a pair of lateral elements bridged by a central region composed of the SYP proteins in C. elegans. How the structural components of the SC are regulated to maintain the architectural integrity of the assembled SC around DSB repair sites remained unclear. Here, we show that SYP-4, a central region component of the SC, is phosphorylated at Serine 447 in a manner dependent on DSBs and the ATM/ATR DNA damage response kinases. We show that this SYP-4 phosphorylation is critical for preserving the SC structure following exogenous (γ-IR-induced) DSB formation and for promoting normal DSB repair progression and crossover patterning following SPO-11-dependent and exogenous DSBs. We propose a model in which ATM/ATR-dependent phosphorylation of SYP-4 at the S447 site plays important roles both in maintaining the architectural integrity of the SC following DSB formation and in warding off repair via the NHEJ repair pathway, thereby preventing aneuploidy., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 Elsevier Inc. All rights reserved.)

The synaptonemal complex (SC) is a proteinaceous scaffold that is assembled between paired homologous chromosomes during the onset of meiosis. Timely expression of SC coding genes is essential for SC assembly and successful meiosis. However, SC components have an intrinsic tendency to self-organize into abnormal repetitive structures, which are not assembled between the paired homologs and whose formation is potentially deleterious for meiosis and gametogenesis. This creates an interesting conundrum, where SC genes need to be robustly expressed during meiosis, but their expression must be carefully regulated to prevent the formation of anomalous SC structures. In this manuscript, we show that the Polycomb group protein Sfmbt, the Drosophila ortholog of human MBTD1 and L3MBTL2, is required to avoid excessive expression of SC genes during prophase I. Although SC assembly is normal after Sfmbt depletion, SC disassembly is abnormal with the formation of multiple synaptonemal complexes (polycomplexes) within the oocyte. Overexpression of the SC gene corona and depletion of other Polycomb group proteins are similarly associated with polycomplex formation during SC disassembly. These polycomplexes are highly dynamic and have a well-defined periodic structure. Further confirming the importance of Sfmbt, germ line depletion of this protein is associated with significant metaphase I defects and a reduction in female fertility. Since transcription of SC genes mostly occurs during early prophase I, our results suggest a role of Sfmbt and other Polycomb group proteins in downregulating the expression of these and other early prophase I genes during later stages of meiosis.

Recombinases RAD51 and its meiosis-specific paralog DMC1 accumulate on single-stranded DNA (ssDNA) of programmed DNA double strand breaks (DSBs) in meiosis. Here we used three-color dSTORM microscopy, and a mouse model with severe defects in meiotic DSB formation and synapsis (Hormad1-/-) to obtain more insight in the recombinase accumulation patterns in relation to repair progression. First, we used the known reduction in meiotic DSB frequency in Hormad1-/- spermatocytes to be able to conclude that the RAD51/DMC1 nanofoci that preferentially localize at distances of ~300 nm form within a single DSB site, whereas a second preferred distance of ~900 nm, observed only in wild type, represents inter-DSB distance. Next, we asked whether the proposed role of HORMAD1 in repair inhibition affects the RAD51/DMC1 accumulation patterns. We observed that the two most frequent recombinase configurations (1 DMC1 and 1 RAD51 nanofocus (D1R1), and D2R1) display coupled frequency dynamics over time in wild type, but were constant in the Hormad1-/- model, indicating that the lifetime of these intermediates was altered. Recombinase nanofoci were also smaller in Hormad1-/- spermatocytes, consistent with changes in ssDNA length or protein accumulation. Furthermore, we established that upon synapsis, recombinase nanofoci localized closer to the synaptonemal complex (SYCP3), in both wild type and Hormad1-/- spermatocytes. Finally, the data also revealed a hitherto unknown function of HORMAD1 in inhibiting coil formation in the synaptonemal complex. SPO11 plays a similar but weaker role in coiling and SYCP1 had the opposite effect. Using this large super-resolution dataset, we propose models with the D1R1 configuration representing one DSB end containing recombinases, and the other end bound by other ssDNA binding proteins, or both ends loaded by the two recombinases, but in below-resolution proximity. This may then often evolve into D2R1, then D1R2, and finally back to D1R1, when DNA synthesis has commenced., Competing Interests: The authors have declared that no competing interests exist.

Background: An impeccable female meiotic prophase is critical for producing a high-quality oocyte and, ultimately, a healthy newborn. SYCP3 is a key component of the synaptonemal complex regulating meiotic homologous recombination. However, what regulates SYCP3 stability is unknown., Methods: Fertility assays, follicle counting, meiotic prophase stage (leptotene, zygotene, pachytene and diplotene) analysis and live imaging were employed to examine how FBXW24 knockout (KO) affect female fertility, follicle reserve, oocyte quality, meiotic prophase progression of female germ cells, and meiosis of oocytes. Western blot and immunostaining were used to examined the levels & signals (intensity, foci) of SYCP3 and multiple key DSB indicators & repair proteins (γH2AX, RPA2, p-CHK2, RAD51, MLH1, HORMAD1, TRIP13) after FBXW24 KO. Co-IP and immuno-EM were used to examined the interaction between FBXW24 and SYCP3; Mass spec was used to characterize the ubiquitination sites in SYCP3; In vivo & in vitro ubiquitination assays were utilized to determine the key sites in SYCP3 & FBXW24 for ubiquitination., Results: Fbxw24-knockout (KO) female mice were infertile due to massive oocyte death upon meiosis entry. Fbxw24-KO oocytes were defective due to elevated DNA double-strand breaks (DSBs) and inseparable homologous chromosomes. Fbxw24-KO germ cells showed increased SYCP3 levels, delayed prophase progression, increased DSBs, and decreased crossover foci. Next, we found that FBXW24 directly binds and ubiquitinates SYCP3 to regulate its stability. In addition, several key residues important for SYCP3 ubiquitination and FBXW24 ubiquitinating activity were characterized., Conclusions: We proposed that FBXW24 regulates the timely degradation of SYCP3 to ensure normal crossover and DSB repair during pachytene. FBXW24-KO delayed SYCP3 degradation and DSB repair from pachytene until metaphase II (MII), ultimately causing failure in oocyte maturation, oocyte death, and infertility., (© 2022 The Authors. Clinical and Translational Medicine published by John Wiley & Sons Australia, Ltd on behalf of Shanghai Institute of Clinical Bioinformatics.)

Ubiquitin is highly conserved in most eukaryotes and involved in diverse physiological processes, including cell division, protein quality control, and protein degradation mediated by the ubiquitin-proteasome system after heat shock, glucose-starvation, and oxidative stress. However, the role of the ubiquitin gene UBI4, which contains five consecutive head-to-tail ubiquitin repeats, in meiosis has not been investigated. In this study, we show that the Saccharomyces cerevisiae polyubiquitin precursor gene, UBI4, is required to promote synaptonemal complex (SC) formation and suppress excess double-strand break formation. Moreover, the proportion of Zip1 polycomplexes, which indicate abnormal SC formation, in cells with a mutation in UBI4 (i.e., ubi4Δ cells) is higher than that of wild-type cells, implying that the UBI4 plays an important role in the early meiotic prophase I. Interestingly, although ubi4Δ cells rarely form full-length SCs in the pachytene stage of prophase I, the Zip3 foci are still seen, as in wild-type cells. Moreover, ubi4Δ cells proficiently form crossover and noncrossover products with a slight delay compared to wild-type cells, suggesting that UBI4 is dispensable in SC-coupled recombination. Our findings demonstrate that UBI4 exhibits dual functions that are associated with both positive and negative roles in SC formation and recombination during meiosis., (© 2022. Author(s).)

Meiosis is a hallmark event in germ cell development that accompanies sequential events executed by numerous molecules. Therefore, characterization of these factors is one of the best strategies to clarify the mechanism of meiosis. Here, we report tripartite motif-containing 41 (TRIM41), a ubiquitin ligase E3, as an essential factor for proper meiotic progression and fertility in male mice. Trim41 knockout (KO) spermatocytes exhibited synaptonemal complex protein 3 (SYCP3) overloading, especially on the X chromosome. Furthermore, mutant mice lacking the RING domain of TRIM41, required for the ubiquitin ligase E3 activity, phenocopied Trim41 KO mice. We then examined the behavior of mutant TRIM41 (ΔRING-TRIM41) and found that ΔRING-TRIM41 accumulated on the chromosome axes with overloaded SYCP3. This result suggested that TRIM41 exerts its function on the chromosome axes. Our study revealed that Trim41 is essential for preventing SYCP3 overloading, suggesting a TRIM41-mediated mechanism for regulating chromosome axis protein dynamics during male meiotic progression., Competing Interests: The authors declare that they have no conflict of interest.

Crossover formation is essential for proper segregation of homologous chromosomes during meiosis. Here, we show that Caenorhabditis elegans cyclin-dependent kinase 2 (CDK-2) partners with cyclin-like protein COSA-1 to promote crossover formation by promoting conversion of meiotic double-strand breaks into crossover–specific recombination intermediates. Further, we identify MutSγ component MSH-5 as a CDK-2 phosphorylation target. MSH-5 has a disordered C-terminal tail that contains 13 potential CDK phosphosites and is required to concentrate crossover–promoting proteins at recombination sites. Phosphorylation of the MSH-5 tail appears dispensable in a wild-type background, but when MutSγ activity is partially compromised, crossover formation and retention of COSA-1 at recombination sites are exquisitely sensitive to phosphosite loss. Our data support a model in which robustness of crossover designation reflects a positive feedback mechanism involving CDK-2–mediated phosphorylation and scaffold-like properties of the MSH5 C-terminal tail, features that combine to promote full recruitment and activity of crossover–promoting complexes.

During meiosis, homologous chromosomes must recognize, pair, and recombine with one another to ensure the formation of inter-homologue crossover events, which, together with sister chromatid cohesion, promote correct chromosome orientation on the first meiotic spindle. Crossover formation requires the assembly of axial elements, proteinaceous structures that assemble along the length of each chromosome during early meiosis, as well as checkpoint mechanisms that control meiotic progression by monitoring pairing and recombination intermediates. A conserved family of proteins defined by the presence of a HORMA (HOp1, Rev7, MAd2) domain, referred to as HORMADs, associate with axial elements to control key events of meiotic prophase. The highly conserved HORMA domain comprises a flexible safety belt sequence, enabling it to adopt at least two of the following protein conformations: one closed, where the safety belt encircles a small peptide motif present within an interacting protein, causing its topological entrapment, and the other open, where the safety belt is reorganized and no interactor is trapped. Although functional studies in multiple organisms have revealed that HORMADs are crucial regulators of meiosis, the mechanisms by which HORMADs implement key meiotic events remain poorly understood. In this review, we summarize protein complexes formed by HORMADs, discuss their roles during meiosis in different organisms, draw comparisons to better characterize non-meiotic HORMADs (MAD2 and REV7), and highlight possible areas for future research.

About 10% of reproductive-aged couples suffer from infertility. However, the genetic causes of human infertility cases are largely unknown. Meiosis produces haploid gametes for fertilization and errors in meiosis are associated with human infertility in both males and females. Successful meiosis relies on the assembly of the synaptonemal complex (SC) between paired homologous chromosomes during the meiotic prophase. The SC is ultrastructurally and functionally conserved, promoting inter-homologous recombination and crossover formation, thus critical for accurate meiotic chromosome segregation. With whole-genome/exome sequencing and mouse models, a list of mutations in SC coding genes has been linked to human infertility. Here we summarize those findings. We also analyzed SC gene variants present in the general population and presented complex interaction networks associated with SC components. Whether a combination of genetic variations and environmental factors causes human infertility demands further investigations., Competing Interests: Competing Interests: The authors have declared that no competing interest exists., (© The author(s).)

A large subset of meiotic recombination intermediates form within the physical context of synaptonemal complex (SC), but the functional relationship between SC structure and homologous recombination remains obscure. Our prior analysis of strains deficient for SC central element proteins demonstrated that tripartite SC is dispensable for interhomolog recombination in Saccharomyces cerevisiae. Here, we report that while dispensable for recombination per se, SC proteins promote efficient mismatch repair at interhomolog recombination sites. Failure to repair mismatches within heteroduplex-containing meiotic recombination intermediates leads to genotypically sectored colonies (postmeiotic segregation events). We discovered increased postmeiotic segregation at THR1 in cells lacking Ecm11 or Gmc2, or in the SC-deficient but recombination-proficient zip1[Δ21-163] mutant. High-throughput sequencing of octad meiotic products furthermore revealed a genome-wide increase in recombination events with unrepaired mismatches in ecm11 mutants relative to wildtype. Meiotic cells missing Ecm11 display longer gene conversion tracts, but tract length alone does not account for the higher frequency of unrepaired mismatches. Interestingly, the per-nucleotide mismatch frequency is elevated in ecm11 when analyzing all gene conversion tracts, but is similar between wildtype and ecm11 if considering only those events with unrepaired mismatches. Thus, in both wildtype and ecm11 strains a subset of recombination events is susceptible to a similar degree of inefficient mismatch repair, but in ecm11 mutants a larger fraction of events fall into this inefficient repair category. Finally, we observe elevated postmeiotic segregation at THR1 in mutants with a dual deficiency in MutSγ crossover recombination and SC assembly, but not in the mlh3 mutant, which lacks MutSγ crossovers but has abundant SC. We propose that SC structure promotes efficient mismatch repair of joint molecule recombination intermediates, and that absence of SC is the molecular basis for elevated postmeiotic segregation in both MutSγ crossover-proficient (ecm11, gmc2) and MutSγ crossover-deficient (msh4, zip3) strains., (© The Author(s) 2021. Published by Oxford University Press on behalf of Genetics Society of America. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

The dynein motor complex is thought to aid in homolog pairing in many organisms by moving chromosomes within the nuclear periphery to promote and test homologous interactions. This precedes synaptonemal complex (SC) formation during homolog synapsis, which stabilizes homolog proximity during recombination. We observed that depletion of the dynein light chain (DLC-1) in Caenorhabditis elegans irreversibly prevents synapsis, causing an increase in off-chromatin formation of SC protein foci with increasing temperature. This requirement for DLC-1 is independent of its function in dynein motors, as SYP protein foci do not form with depletion of other dynein motor components. In contrast to normal SC-related structures, foci formed with DLC-1 depletion are resistant to dissolution with 1,6-hexanediol, similar to aggregates of SC proteins formed in high growth temperatures. Dynein light chains have been shown to act as hub proteins that interact with other proteins through a conserved binding motif. We identified a similar DLC-1 binding motif in the C. elegans SC protein SYP-2, and mutation of the putative motif causes meiosis defects that are exacerbated by elevated temperatures. We propose that DLC-1 acts as a pre-synapsis chaperone-like factor for SYP proteins to help regulate their self-association prior to the signals for SC assembly, a role that is revealed by its increased essentiality at elevated temperatures., (© The Author(s) 2021. Published by Oxford University Press on behalf of Genetics Society of America. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Meiotic recombination is triggered by programmed double-strand breaks (DSBs), a subset of these being repaired as crossovers, promoted by eight evolutionarily conserved proteins, named ZMM. Crossover formation is functionally linked to synaptonemal complex (SC) assembly between homologous chromosomes, but the underlying mechanism is unknown. Here we show that Ecm11, a SC central element protein, localizes on both DSB sites and sites that attach chromatin loops to the chromosome axis, which are the starting points of SC formation, in a way that strictly requires the ZMM protein Zip4. Furthermore, Zip4 directly interacts with Ecm11, and point mutants that specifically abolish this interaction lose Ecm11 binding to chromosomes and exhibit defective SC assembly. This can be partially rescued by artificially tethering interaction-defective Ecm11 to Zip4. Mechanistically, this direct connection ensuring SC assembly from CO sites could be a way for the meiotic cell to shut down further DSB formation once enough recombination sites have been selected for crossovers, thereby preventing excess crossovers. Finally, the mammalian ortholog of Zip4, TEX11, also interacts with the SC central element TEX12, suggesting a general mechanism., (© 2022 Pyatnitskaya et al.; Published by Cold Spring Harbor Laboratory Press.)

Meiosis produces the haploid gametes required by all sexually reproducing organisms, occurring in specific temperature ranges in different organisms. However, how meiotic thermotolerance is regulated remains largely unknown. Using the model organism Caenorhabditis elegans, here, we identified the synaptonemal complex (SC) protein SYP-5 as a critical regulator of meiotic thermotolerance. syp-5-null mutants maintained a high percentage of viable progeny at 20°C but produced significantly fewer viable progeny at 25°C, a permissive temperature in wild-type worms. Cytological analysis of meiotic events in the mutants revealed that while SC assembly and disassembly, as well as DNA double-strand break repair kinetics, were not affected by the elevated temperature, crossover designation, and bivalent formation were significantly affected. More severe homolog segregation errors were also observed at elevated temperature. A temperature switching assay revealed that late meiotic prophase events were not temperature-sensitive and that meiotic defects during pachytene stage were responsible for the reduced viability of syp-5 mutants at the elevated temperature. Moreover, SC polycomplex formation and hexanediol sensitivity analysis suggested that SYP-5 was required for the normal properties of the SC, and charge-interacting elements in SC components were involved in regulating meiotic thermotolerance. Together, these findings provide a novel molecular mechanism for meiotic thermotolerance regulation., (© The Author(s) (2021). Published by Oxford University Press on behalf of Journal of Molecular Cell Biology, CEMCS, CAS.)

Alignment of the parental chromosomes during meiotic prophase is key to the formation of genetic exchanges, or crossovers, and consequently to the successful production of gametes. In almost all studied organisms, alignment involves synapsis: the assembly of a conserved inter-chromosomal interface called the synaptonemal complex (SC). While the SC usually synapses homologous sequences, it can assemble between heterologous sequences. However, little is known about the regulation of heterologous synapsis. Here, we study the dynamics of heterologous synapsis in the nematode C. elegans. We characterize two experimental scenarios: SC assembly onto a folded-back chromosome that cannot pair with its homologous partner; and synapsis of pseudo-homologs, a fusion chromosome partnering with an unfused chromosome half its size. We observed elevated levels of heterologous synapsis when the number of meiotic double-strand breaks or crossovers were reduced, indicating that the promiscuity of synapsis is regulated by break formation or repair. In addition, our data suggests the existence of both chromosome-specific and nucleus-wide regulation on heterologous synapsis., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The specialized two-stage meiotic cell division program halves a cell's chromosome complement in preparation for sexual reproduction. This reduction in ploidy requires that in meiotic prophase, each pair of homologous chromosomes (homologs) identify one another and form physical links through DNA recombination. Here, we review recent advances in understanding the complex morphological changes that chromosomes undergo during meiotic prophase to promote homolog identification and crossing over. We focus on the structural maintenance of chromosomes (SMC) family cohesin complexes and the meiotic chromosome axis, which together organize chromosomes and promote recombination. We then discuss the architecture and dynamics of the conserved synaptonemal complex (SC), which assembles between homologs and mediates local and global feedback to ensure high fidelity in meiotic recombination. Finally, we discuss exciting new advances, including mechanisms for boosting recombination on particular chromosomes or chromosomal domains and the implications of a new liquid crystal model for SC assembly and structure.

Synaptonemal complex (SC) assembly and homologous recombination, the most critical events during prophase I, are the prerequisite for faithful meiotic chromosome segregation. However, the underlying regulatory mechanism remains largely unknown. Here, we reveal that a functional RING finger E3 ubiquitin ligase, DESYNAPSIS1 (DSNP1), plays significant roles in SC assembly and homologous recombination during rice meiosis. In the dsnp1 mutant, homologous synapsis is discontinuous and aberrant SC-like polycomplexes occur independent of coaligned homologous chromosomes. Accompanying the decreased foci of HEI10, ZIP4, and MER3 on meiotic chromosomes, the number of crossovers (COs) decreases dramatically in dsnp1 meiocytes. Furthermore, the absence of central elements largely restores the localization of non-ZEP1 ZMM proteins and the number of COs in the dsnp1 background. Collectively, DSNP1 stabilizes the canonical tripartite SC structure along paired homologous chromosomes and further promotes the formation of COs., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

The synaptonemal complex (SC) is a meiosis-specific proteinaceous macromolecular structure that assembles between paired homologous chromosomes during meiosis in various eukaryotes. The SC has a highly conserved ultrastructure and plays critical roles in controlling multiple steps in meiotic recombination and crossover formation, ensuring accurate meiotic chromosome segregation. Recent studies in different organisms, facilitated by advances in super-resolution microscopy, have provided insights into the macromolecular structure of the SC, including the internal organization of the meiotic chromosome axis and SC central region, the regulatory pathways that control SC assembly and dynamics, and the biological functions exerted by the SC and its substructures. This review summarizes recent discoveries about how the SC is organized and regulated that help to explain the biological functions associated with this meiosis-specific structure., Competing Interests: None

Meiotic recombination is integrated into and regulated by meiotic chromosomes, which is organized as loop/axis architecture. However, the regulation of chromosome organization is poorly understood. Here, we show Esa1, the NuA4 complex catalytic subunit, is constitutively expressed and localizes on chromatin loops during meiosis. Esa1 plays multiple roles including homolog synapsis, sporulation efficiency, spore viability, and chromosome segregation in meiosis. Detailed analyses show the meiosis-specific depletion of Esa1 results in decreased chromosome axis length independent of another axis length regulator Pds5, which further leads to a decreased number of Mer2 foci, and consequently a decreased number of DNA double-strand breaks, recombination intermediates, and crossover frequency. However, Esa1 depletion does not impair the occurrence of the obligatory crossover required for faithful chromosome segregation, or the strength of crossover interference. Further investigations demonstrate Esa1 regulates chromosome axis length via acetylating the N-terminal tail of histone H4 but not altering transcription program. Therefore, we firstly show a non-chromosome axis component, Esa1, acetylates histone H4 on chromatin loops to regulate chromosome axis length and consequently recombination frequency but does not affect the basic meiotic recombination process. Additionally, Esa1 depletion downregulates middle induced meiotic genes, which probably causing defects in sporulation and chromosome segregation., (© The Author(s) 2021. Published by Oxford University Press on behalf of Nucleic Acids Research.)

Whole-chromosome fusions play a major role in the karyotypic evolution of reptiles. It has been suggested that certain chromosomes tend to fuse with sex chromosomes more frequently than others. However, the comparative genomic synteny data are too scarce to draw strong conclusions. We obtained and sequenced chromosome-specific DNA pools of Sceloporus malachiticus , an iguanian species which has experienced many chromosome fusions. We found that four of seven lineage-specific fusions involved sex chromosomes, and that certain syntenic blocks which constitute the sex chromosomes, such as the homologues of the Anolis carolinensis chromosomes 11 and 16, are repeatedly involved in sex chromosome formation in different squamate species. To test the hypothesis that the karyotypic shift could be associated with changes in recombination patterns, we performed a synaptonemal complex analysis in this species and in Sceloporus variabilis (2 n = 34). It revealed that the sex chromosomes in S. malachiticus had two distal pseudoautosomal regions and a medial differentiated region. We found that multiple fusions little affected the recombination rate in S. malachiticus . Our data confirm more frequent involvement of certain chromosomes in sex chromosome formation, but do not reveal a connection between the gonosome-autosome fusions and the evolution of recombination rate. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.

Meiosis is a complex process involving the expression and interaction of numerous genes in a series of highly orchestrated molecular events. Fam9b localized in Xp22.3 has been found to be expressed in testes. However, FAM9B expression, localization, and its role in meiosis have not been previously reported. In this study, FAM9B expression was evaluated in the human testes and ovaries by RT-PCR, qPCR, and western blotting. FAM9B was found in the nuclei of primary spermatocytes in testes and specifically localized in the synaptonemal complex (SC) region of spermatocytes. FAM9B was also evident in the follicle cell nuclei and diffusely dispersed in the granular cell cytoplasm. FAM9B was partly co-localized with SYCP3, which is essential for both formation and maintenance of lateral SC elements. In addition, FAM9B had a similar distribution pattern and co-localization as γH2AX, which is a novel biomarker for DNA double-strand breaks during meiosis. All results indicate that FAM9B is a novel meiosis-associated protein that is co-localized with SYCP3 and γH2AX and may play an important role in SC formation and DNA recombination during meiosis. These findings offer a new perspective for understanding the molecular mechanisms involved in meiosis of human gametogenesis., Competing Interests: The authors have declared that no competing interests exist.

In most organisms, the number and distribution of crossovers that occur during meiosis are tightly controlled. All chromosomes must receive at least one 'obligatory crossover' and crossovers are prevented from occurring near one another by 'crossover interference'. However, the mechanistic basis of this phenomenon of crossover interference has remained mostly mysterious. Using quantitative super-resolution cytogenetics and mathematical modelling, we investigate crossover positioning in the Arabidopsis thaliana wild-type, an over-expressor of the conserved E3 ligase HEI10, and a hei10 heterozygous line. We show that crossover positions can be explained by a predictive, diffusion-mediated coarsening model, in which large, approximately evenly-spaced HEI10 foci grow at the expense of smaller, closely-spaced clusters. We propose this coarsening process explains many aspects of Arabidopsis crossover positioning, including crossover interference. Consistent with this model, we also demonstrate that crossover positioning can be predictably modified in vivo simply by altering HEI10 dosage, with higher and lower dosage leading to weaker and stronger crossover interference, respectively. As HEI10 is a conserved member of the RING finger protein family that functions in the interference-sensitive pathway for crossover formation, we anticipate that similar mechanisms may regulate crossover positioning in diverse eukaryotes., (© 2021. The Author(s).)

The generally conserved AAA+ ATPase Pch2/TRIP13 is involved in diverse aspects of meiosis, such as prophase checkpoint function, DNA break regulation, and meiotic recombination. The controlled recruitment of Pch2 to meiotic chromosomes allows it to use its ATPase activity to influence HORMA protein-dependent signaling. Because of the connection between Pch2 chromosomal recruitment and its functional roles in meiosis, it is important to reveal the molecular details that govern Pch2 localization. Here, we review the current understanding of the different factors that control the recruitment of Pch2 to meiotic chromosomes, with a focus on research performed in budding yeast. During meiosis in this organism, Pch2 is enriched within the nucleolus, where it likely associates with the specialized chromatin of the ribosomal (r)DNA. Pch2 is also found on non-rDNA euchromatin, where its recruitment is contingent on Zip1, a component of the synaptonemal complex (SC) that assembles between homologous chromosomes. We discuss recent findings connecting the recruitment of Pch2 with its association with the Origin Recognition Complex (ORC) and reliance on RNA Polymerase II-dependent transcription. In total, we provide a comprehensive overview of the pathways that control the chromosomal association of an important meiotic regulator.

Chromosome segregation in eukaryotes is driven by the kinetochore, a macromolecular complex that connects centromeric DNA to microtubules of the spindle apparatus. Kinetochores in well-studied model eukaryotes consist of a core set of proteins that are broadly conserved among distant eukaryotic phyla. By contrast, unicellular flagellates of the class Kinetoplastida have a unique set of 36 kinetochore components. The evolutionary origin and history of these kinetochores remain unknown. Here, we report evidence of homology between axial element components of the synaptonemal complex and three kinetoplastid kinetochore proteins KKT16-18. The synaptonemal complex is a zipper-like structure that assembles between homologous chromosomes during meiosis to promote recombination. By using sensitive homology detection protocols, we identify divergent orthologues of KKT16-18 in most eukaryotic supergroups, including experimentally established chromosomal axis components, such as Red1 and Rec10 in budding and fission yeast, ASY3-4 in plants and SYCP2-3 in vertebrates. Furthermore, we found 12 recurrent duplications within this ancient eukaryotic SYCP 2-3 gene family, providing opportunities for new functional complexes to arise, including KKT16-18 in the kinetoplastid parasite Trypanosoma brucei . We propose the kinetoplastid kinetochore system evolved by repurposing meiotic components of the chromosome synapsis and homologous recombination machinery that were already present in early eukaryotes.

The synaptonemal complex is a tripartite proteinaceous ultrastructure that forms between homologous chromosomes during prophase I of meiosis in the majority of eukaryotes. It is characterized by the coordinated installation of transverse filament proteins between two lateral elements and is required for wild-type levels of crossing over and meiotic progression. We have generated null mutants of the duplicated Arabidopsis transverse filament genes zyp1a and zyp1b using a combination of T-DNA insertional mutants and targeted CRISPR/Cas mutagenesis. Cytological and genetic analysis of the zyp1 null mutants reveals loss of the obligate chiasma, an increase in recombination map length by 1.3- to 1.7-fold and a virtual absence of cross-over (CO) interference, determined by a significant increase in the number of double COs. At diplotene, the numbers of HEI10 foci, a marker for Class I interference-sensitive COs, are twofold greater in the zyp1 mutant compared to wild type. The increase in recombination in zyp1 does not appear to be due to the Class II interference-insensitive COs as chiasmata were reduced by ∼52% in msh5/zyp1 compared to msh5 These data suggest that ZYP1 limits the formation of closely spaced Class I COs in Arabidopsis Our data indicate that installation of ZYP1 occurs at ASY1-labeled axial bridges and that loss of the protein disrupts progressive coalignment of the chromosome axes., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Meiosis generates genetic variation through homologous recombination (HR) that is harnessed during breeding. HR occurs in the context of meiotic chromosome axes and the synaptonemal complex. To study the role of axis remodelling in crossover (CO) formation in a crop species, we characterized mutants of the axis-associated protein ASY1 and the axis-remodelling protein PCH2 in Brassica rapa. asy1 plants form meiotic chromosome axes that fail to synapse. CO formation is almost abolished, and residual chiasmata are proportionally enriched in terminal chromosome regions, particularly in the nucleolar organizing region (NOR)-carrying chromosome arm. pch2 plants show impaired ASY1 loading and remodelling, consequently achieving only partial synapsis, which leads to reduced CO formation and loss of the obligatory CO. PCH2-independent chiasmata are proportionally enriched towards distal chromosome regions. Similarly, in Arabidopsis pch2, COs are increased towards telomeric regions at the expense of (peri-) centromeric COs compared with the wild type. Taken together, in B. rapa, axis formation and remodelling are critical for meiotic fidelity including synapsis and CO formation, and in asy1 and pch2 CO distributions are altered. While asy1 plants are sterile, pch2 plants are semi-sterile and thus PCH2 could be an interesting target for breeding programmes., (© The Author(s) 2021. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Human infertility is a multifactorial disease that affects 8%-12% of reproductive-aged couples worldwide. However, the genetic causes of human infertility are still poorly understood. Synaptonemal complex (SC) is a conserved tripartite structure that holds homologous chromosomes together and plays an indispensable role in the meiotic progression. Here, we identified three homozygous mutations in the SC coding gene C14orf39/SIX6OS1 in infertile individuals from different ethnic populations by whole-exome sequencing (WES). These mutations include a frameshift mutation (c.204_205del [p.His68Glnfs ∗ 2]) from a consanguineous Pakistani family with two males suffering from non-obstructive azoospermia (NOA) and one female diagnosed with premature ovarian insufficiency (POI) as well as a nonsense mutation (c.958G>T [p.Glu320 ∗ ]) and a splicing mutation (c.1180-3C>G) in two unrelated Chinese men (individual P3907 and individual P6032, respectively) with meiotic arrest. Mutations in C14orf39 resulted in truncated proteins that retained SYCE1 binding but exhibited impaired polycomplex formation between C14ORF39 and SYCE1. Further cytological analyses of meiosis in germ cells revealed that the affected familial males with the C14orf39 frameshift mutation displayed complete asynapsis between homologous chromosomes, while the affected Chinese men carrying the nonsense or splicing mutation showed incomplete synapsis. The phenotypes of NOA and POI in affected individuals were well recapitulated by Six6os1 mutant mice carrying an analogous mutation. Collectively, our findings in humans and mice highlight the conserved role of C14ORF39/SIX6OS1 in SC assembly and indicate that the homozygous mutations in C14orf39/SIX6OS1 described here are responsible for infertility of these affected individuals, thus expanding our understanding of the genetic basis of human infertility., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

The meiotic TopoVI B subunit (MTopVIB) plays an essential role in double-strand break formation in mouse ( Mus musculus ), Arabidopsis ( Arabidopsis thaliana ), and rice ( Oryza sativa ), and recent work reveals that rice MTopVIB also plays an unexpected role in meiotic bipolar spindle assembly, highlighting multiple functions of MTopVIB during rice meiosis. In this work, we characterized the meiotic TopVIB in maize ( Zea mays ; ZmMTOPVIB ). The ZmmtopVIB mutant plants exhibited normal vegetative growth but male and female sterility. Meiotic double-strand break formation was abolished in mutant meiocytes. Despite normal assembly of axial elements, mutants showed severely affected synapsis and disrupted homologous pairing. Importantly, we showed that bipolar spindle assembly was also affected in ZmmtopVIB , resulting in triad and polyad formation. Overall, our results demonstrate that ZmMTOPVIB plays critical roles in double-strand break formation and homologous recombination. In addition, our results suggest that the function of MTOPVIB in bipolar spindle assembly is likely conserved across different monocots., (© 2020 American Society of Plant Biologists. All Rights Reserved.)

The number of DNA double-strand breaks (DSBs) initiating meiotic recombination is elevated in Saccharomyces cerevisiae mutants that are globally defective in forming crossovers and synaptonemal complex (SC), a protein scaffold juxtaposing homologous chromosomes. These mutants thus appear to lack a negative feedback loop that inhibits DSB formation when homologs engage one another. This feedback is predicted to be chromosome autonomous, but this has not been tested. Moreover, what chromosomal process is recognized as "homolog engagement" remains unclear. To address these questions, we evaluated effects of homolog engagement defects restricted to small portions of the genome using karyotypically abnormal yeast strains with a homeologous chromosome V pair, monosomic V, or trisomy XV. We found that homolog engagement-defective chromosomes incurred more DSBs, concomitant with prolonged retention of the DSB-promoting protein Rec114, while the rest of the genome remained unaffected. SC-deficient, crossover-proficient mutants ecm11 and gmc2 experienced increased DSB numbers diagnostic of homolog engagement defects. These findings support the hypothesis that SC formation provokes DSB protein dissociation, leading in turn to loss of a DSB competent state. Our findings show that DSB number is regulated in a chromosome-autonomous fashion and provide insight into how homeostatic DSB controls respond to aneuploidy during meiosis., (© 2020 Mu et al.; Published by Cold Spring Harbor Laboratory Press.)

Experiments performed in different genetic backgrounds occasionally exhibit failure in experimental reproducibility. This is a serious issue in Drosophila where there are no standard control stocks. Here, we illustrate the importance of controlling genetic background by showing that the timing of a major meiotic event, the breakdown of the synaptonemal complex (SC), varies in different genetic backgrounds. We assessed SC breakdown in three different control stocks and found that in one control stock, y w; sv spa-pol , the SC broke down earlier than in Oregon-R and w 1118 stocks. We further examined SC breakdown in these three control backgrounds with flies heterozygous for a null mutation in c(3)G, which encodes a key structural component of the SC. Flies heterozygous for c(3)G displayed differences in the timing of SC breakdown in different control backgrounds, providing evidence of a sensitizing effect of this mutation. These observations suggest that SC maintenance is associated with the dosage of c(3)G in some backgrounds. Lastly, chromosome segregation was not affected by premature SC breakdown in mid-prophase, consistent with previous findings that chromosome segregation is not dependent on full-length SC in mid-prophase. Thus, genetic background is an important variable to consider with respect to SC behavior during Drosophila meiosis.

Meiotic reductional division depends on the synaptonemal complex (SC), a supramolecular protein assembly that mediates homologous chromosomes synapsis and promotes crossover formation. The mammalian SC has eight structural components, including SYCE1, the only central element protein with known causative mutations in human infertility. We combine mouse genetics, cellular, and biochemical studies to reveal that SYCE1 undergoes multivalent interactions with SC component SIX6OS1. The N terminus of SIX6OS1 binds and disrupts SYCE1's core dimeric structure to form a 1:1 complex, while their downstream sequences provide a distinct second interface. These interfaces are separately disrupted by SYCE1 mutations associated with nonobstructive azoospermia and premature ovarian failure (POF), respectively. Mice harboring SYCE1's POF mutation and a targeted deletion within SIX6OS1's N terminus are infertile with failure of chromosome synapsis. We conclude that both SYCE1-SIX6OS1 binding interfaces are essential for SC assembly, thus explaining how SYCE1's reported clinical mutations give rise to human infertility., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY).)

Pch2 is an AAA+ protein that controls DNA break formation, recombination and checkpoint signaling during meiotic G2/prophase. Chromosomal association of Pch2 is linked to these processes, and several factors influence the association of Pch2 to euchromatin and the specialized chromatin of the ribosomal (r)DNA array of budding yeast. Here, we describe a comprehensive mapping of Pch2 localization across the budding yeast genome during meiotic G2/prophase. Within non-rDNA chromatin, Pch2 associates with a subset of actively RNA Polymerase II (RNAPII)-dependent transcribed genes. Chromatin immunoprecipitation (ChIP)- and microscopy-based analysis reveals that active transcription is required for chromosomal recruitment of Pch2. Similar to what was previously established for association of Pch2 with rDNA chromatin, we find that Orc1, a component of the Origin Recognition Complex (ORC), is required for the association of Pch2 to these euchromatic, transcribed regions, revealing a broad connection between chromosomal association of Pch2 and Orc1/ORC function. Ectopic mitotic expression is insufficient to drive recruitment of Pch2, despite the presence of active transcription and Orc1/ORC in mitotic cells. This suggests meiosis-specific 'licensing' of Pch2 recruitment to sites of transcription, and accordingly, we find that the synaptonemal complex (SC) component Zip1 is required for the recruitment of Pch2 to transcription-associated binding regions. Interestingly, Pch2 binding patterns are distinct from meiotic axis enrichment sites (as defined by Red1, Hop1, and Rec8). Inactivating RNAPII-dependent transcription/Orc1 does not lead to effects on the chromosomal abundance of Hop1, a known chromosomal client of Pch2, suggesting a complex relationship between SC formation, Pch2 recruitment and Hop1 chromosomal association. We thus report characteristics and dependencies for Pch2 recruitment to meiotic chromosomes, and reveal an unexpected link between Pch2, SC formation, chromatin and active transcription., Competing Interests: The authors have declared that no competing interests exist.