HAL-2 promotes homologous pairing during Caenorhabditis elegans meiosis by antagonizing inhibitory effects of synaptonemal complex precursors

Homologous chromosome pairing: The linchpin of accurate segregation in meiosis

Meiosis is a specialized cell division that occurs in sexually reproducing organisms, generating haploid gametes containing half the chromosome number through two rounds of cell division. Homologous chromosomes pair and prepare for their proper segregation in subsequent divisions. How homologous chromosomes recognize each other and achieve pairing is an important question. Early studies showed that in most organisms, homologous pairing relies on homologous recombination. However, pairing mechanisms differ across species. Evidence indicates that chromosomes are dynamic and move during early meiotic stages, facilitating pairing. Recent studies in various model organisms suggest conserved mechanisms and key regulators of homologous chromosome pairing. This review summarizes these findings and compare similarities and differences in homologous chromosome pairing mechanisms across species.

Sexual dimorphic regulation of recombination by the synaptonemal complex in C. elegans

In sexually reproducing organisms, germ cells faithfully transmit the genome to the next generation by forming haploid gametes, such as eggs and sperm. Although most meiotic proteins are conserved between eggs and sperm, many aspects of meiosis are sexually dimorphic, including the regulation of recombination. The synaptonemal complex (SC), a large ladder-like structure that forms between homologous chromosomes, is essential for regulating meiotic chromosome organization and promoting recombination. To assess whether sex-specific differences in the SC underpin sexually dimorphic aspects of meiosis, we examined Caenorhabditis elegans SC central region proteins (known as SYP proteins) in oogenesis and spermatogenesis and uncovered sex-specific roles for the SYPs in regulating meiotic recombination. We find that SC composition, specifically SYP-2, SYP-3, SYP-5, and SYP-6, is regulated by sex-specific mechanisms throughout meiotic prophase I. During pachytene, both oocytes and spermatocytes differentially regulate the stability of SYP-2 and SYP-3 within an assembled SC. Further, we uncover that the relative amount of SYP-2 and SYP-3 within the SC is independently regulated in both a sex-specific and a recombination-dependent manner. Specifically, we find that SYP-2 regulates the early steps of recombination in both sexes, while SYP-3 controls the timing and positioning of crossover recombination events across the genomic landscape in only oocytes. Finally, we find that SYP-2 and SYP-3 dosage can influence the composition of the other SYPs in the SC via sex-specific mechanisms during pachytene. Taken together, we demonstrate dosage-dependent regulation of individual SC components with sex-specific functions in recombination. These sexual dimorphic features of the SC provide insights into how spermatogenesis and oogenesis adapted similar chromosome structures to differentially regulate and execute recombination.

Synaptonemal Complex in Human Biology and Disease

The synaptonemal complex (SC) is a meiosis-specific multiprotein complex that forms between homologous chromosomes during prophase of meiosis I. Upon assembly, the SC mediates the synapses of the homologous chromosomes, leading to the formation of bivalents, and physically supports the formation of programmed double-strand breaks (DSBs) and their subsequent repair and maturation into crossovers (COs), which are essential for genome haploidization. Defects in the assembly of the SC or in the function of the associated meiotic recombination machinery can lead to meiotic arrest and human infertility. The majority of proteins and complexes involved in these processes are exclusively expressed during meiosis or harbor meiosis-specific subunits, although some have dual functions in somatic DNA repair and meiosis. Consistent with their functions, aberrant expression and malfunctioning of these genes have been associated with cancer development. In this review, we focus on the significance of the SC and their meiotic-associated proteins in human fertility, as well as how human genetic variants encoding for these proteins affect the meiotic process and contribute to infertility and cancer development.

Recruitment of Polo-like kinase couples synapsis to meiotic progression via inactivation of CHK-2

Meiotic chromosome segregation relies on synapsis and crossover (CO) recombination between homologous chromosomes. These processes require multiple steps that are coordinated by the meiotic cell cycle and monitored by surveillance mechanisms. In diverse species, failures in chromosome synapsis can trigger a cell cycle delay and/or lead to apoptosis. How this key step in 'homolog engagement' is sensed and transduced by meiotic cells is unknown. Here we report that in C. elegans, recruitment of the Polo-like kinase PLK-2 to the synaptonemal complex triggers phosphorylation and inactivation of CHK-2, an early meiotic kinase required for pairing, synapsis, and double-strand break (DSB) induction. Inactivation of CHK-2 terminates DSB formation and enables CO designation and cell cycle progression. These findings illuminate how meiotic cells ensure CO formation and accurate chromosome segregation.

Targeting Polo-like kinase in space and time during C. elegans meiosis

A central player in meiotic chromosome dynamics is the conserved Polo-like kinase (PLK) family. PLKs are dynamically localized to distinct structures during meiotic prophase and phosphorylate a diverse group of substrates to control homolog pairing, synapsis, and meiotic recombination. In a recent study, we uncovered the mechanisms that control the targeting of a meiosis-specific PLK-2 in C. elegans. In early meiotic prophase, PLK-2 localizes to special chromosome regions known as pairing centers and drives homolog pairing and synapsis. PLK-2 then relocates to the synaptonemal complex (SC) after crossover designation and mediates chromosome remodeling required for homolog separation. What controls this intricate targeting of PLK-2 in space and time? We discuss recent findings and remaining questions for the future.

Automated and customizable quantitative image analysis of whole Caenorhabditis elegans germlines

Arranged in a spatial-temporal gradient for germ cell development, the adult germline of Caenorhabditis elegans is an excellent system for understanding the generation, differentiation, function, and maintenance of germ cells. Imaging whole C. elegans germlines along the distal-proximal axis enables powerful cytological analyses of germ cell nuclei as they progress from the pre-meiotic tip through all the stages of meiotic prophase I. To enable high-content image analysis of whole C. elegans gonads, we developed a custom algorithm and pipelines to function with image processing software that enables: (1) quantification of cytological features at single nucleus resolution from immunofluorescence images; and (2) assessment of these individual nuclei based on their position within the germline. We show the capability of our quantitative image analysis approach by analyzing multiple cytological features of meiotic nuclei in whole C. elegans germlines. First, we quantify double-strand DNA breaks (DSBs) per nucleus by analyzing DNA-associated foci of the recombinase RAD-51 at single-nucleus resolution in the context of whole germline progression. Second, we quantify the DSBs that are licensed for crossover repair by analyzing foci of MSH-5 and COSA-1 when they associate with the synaptonemal complex during meiotic prophase progression. Finally, we quantify P-granule composition across the whole germline by analyzing the colocalization of PGL-1 and ZNFX-1 foci. Our image analysis pipeline is an adaptable and useful method for researchers spanning multiple fields using the C. elegans germline as a model system.

Spatial and temporal control of targeting Polo-like kinase during meiotic prophase

Polo-like kinases (PLKs) play widely conserved roles in orchestrating meiotic chromosome dynamics. However, how PLKs are targeted to distinct subcellular localizations during meiotic progression remains poorly understood. Here, we demonstrate that the cyclin-dependent kinase CDK-1 primes the recruitment of PLK-2 to the synaptonemal complex (SC) through phosphorylation of SYP-1 in C. elegans. SYP-1 phosphorylation by CDK-1 occurs just before meiotic onset. However, PLK-2 docking to the SC is prevented by the nucleoplasmic HAL-2/3 complex until crossover designation, which constrains PLK-2 to special chromosomal regions known as pairing centers to ensure proper homologue pairing and synapsis. PLK-2 is targeted to crossover sites primed by CDK-1 and spreads along the SC by reinforcing SYP-1 phosphorylation on one side of each crossover only when threshold levels of crossovers are generated. Thus, the integration of chromosome-autonomous signaling and a nucleus-wide crossover-counting mechanism partitions holocentric chromosomes relative to the crossover site, which ultimately defines the pattern of chromosome segregation during meiosis I.

Synaptonemal Complex dimerization regulates chromosome alignment and crossover patterning in meiosis

During sexual reproduction the parental homologous chromosomes find each other (pair) and align along their lengths by integrating local sequence homology with large-scale contiguity, thereby allowing for precise exchange of genetic information. The Synaptonemal Complex (SC) is a conserved zipper-like structure that assembles between the homologous chromosomes, bringing them together and regulating exchanges between them. However, the molecular mechanisms by which the SC carries out these functions remain poorly understood. Here we isolated and characterized two mutations in the dimerization interface in the middle of the SC zipper in C. elegans. The mutations perturb both chromosome alignment and the regulation of genetic exchanges. Underlying the chromosome-scale phenotypes are distinct alterations to the way SC subunits interact with one another. We propose a model whereby the SC brings homologous chromosomes together through two activities: obligate zipping that prevents assembly on unpaired chromosomes; and a tendency to extend pairing interactions along the entire length of the chromosomes.

Mixing and Matching Chromosomes during Female Meiosis

Meiosis is a key event in the manufacturing of an oocyte. During this process, the oocyte creates a set of unique chromosomes by recombining paternal and maternal copies of homologous chromosomes, and by eliminating one set of chromosomes to become haploid. While meiosis is conserved among sexually reproducing eukaryotes, there is a bewildering diversity of strategies among species, and sometimes within sexes of the same species, to achieve proper segregation of chromosomes. Here, we review the very first steps of meiosis in females, when the maternal and paternal copies of each homologous chromosomes have to move, find each other and pair. We explore the similarities and differences observed in C. elegans, Drosophila, zebrafish and mouse females.

SCRE serves as a unique synaptonemal complex fastener and is essential for progression of meiosis prophase I in mice

Meiosis is a specialized cell division for producing haploid gametes from diploid germ cells. During meiosis, synaptonemal complex (SC) mediates the alignment of homologs and plays essential roles in homologous recombination and therefore in promoting accurate chromosome segregation. In this study, we have identified a novel protein SCRE (synaptonemal complex reinforcing element) as a key molecule in maintaining the integrity of SC during meiosis prophase I in mice. Deletion of Scre (synaptonemal complex reinforcing element) caused germ cell death in both male and female mice, resulting in infertility. Our mechanistic studies showed that the synapses and SCs in Scre knockout mice were unstable due to the lack of the SC reinforcing function of SCRE, which is sparsely localized as discrete foci along the central elements in normal synaptic homologous chromosomes. The lack of Scre leads to meiosis collapse at the late zygotene stage. We further showed that SCRE interacts with synaptonemal complex protein 1 (SYCP1) and synaptonemal complex central element 3 (SYCE3). We conclude that the function of SCRE is to reinforce the integrity of the central elements, thereby stabilizing the SC and ensuring meiotic cell cycle progression. Our study identified SCRE as a novel SC fastener protein that is distinct from other known SC proteins.

Spatial Regulation of Polo-Like Kinase Activity During Caenorhabditis elegans Meiosis by the Nucleoplasmic HAL-2/HAL-3 Complex

Proper partitioning of homologous chromosomes during meiosis relies on the coordinated execution of multiple interconnected events: Homologs must locate, recognize, and align with their correct pairing partners. Further, homolog pairing must be coupled to assembly of the synaptonemal complex (SC), a meiosis-specific tripartite structure that maintains stable associations between the axes of aligned homologs and regulates formation of crossovers between their DNA molecules to create linkages that enable their segregation. Here, we identify HAL-3 (Homolog Alignment 3) as an important player in coordinating these key events during Caenorhabditis elegans meiosis. HAL-3, and the previously identified HAL-2, are interacting and interdependent components of a protein complex that localizes to the nucleoplasm of germ cells. hal-3 (or hal-2) mutants exhibit multiple meiotic prophase defects including failure to establish homolog pairing, inappropriate loading of SC subunits onto unpaired chromosome axes, and premature loss of synapsis checkpoint protein PCH-2. Further, loss of hal function results in misregulation of the subcellular localization and activity of Polo-like kinases (PLK-1 and PLK-2), which dynamically localize to different defined subnuclear sites during wild-type prophase progression to regulate distinct cellular events. Moreover, loss of PLK-2 activity partially restores tripartite SC structure in a hal mutant background, suggesting that the defect in pairwise SC assembly in hal mutants reflects inappropriate PLK activity. Together, our data support a model in which the nucleoplasmic HAL-2/HAL-3 protein complex constrains both localization and activity of meiotic Polo-like kinases, thereby preventing premature interaction with stage-inappropriate targets.

Condensin I protects meiotic cohesin from WAPL-1 mediated removal

Condensin complexes are key determinants of higher-order chromatin structure and are required for mitotic and meiotic chromosome compaction and segregation. We identified a new role for condensin in the maintenance of sister chromatid cohesion during C. elegans meiosis. Using conventional and stimulated emission depletion (STED) microscopy we show that levels of chromosomally-bound cohesin were significantly reduced in dpy-28 mutants, which lack a subunit of condensin I. SYP-1, a component of the synaptonemal complex central region, was also diminished, but no decrease in the axial element protein HTP-3 was observed. Surprisingly, the two key meiotic cohesin complexes of C. elegans were both depleted from meiotic chromosomes following the loss of condensin I, and disrupting condensin I in cohesin mutants increased the frequency of detached sister chromatids. During mitosis and meiosis in many organisms, establishment of cohesion is antagonized by cohesin removal by Wapl, and we found that condensin I binds to C. elegans WAPL-1 and counteracts WAPL-1-dependent cohesin removal. Our data suggest that condensin I opposes WAPL-1 to promote stable binding of cohesin to meiotic chromosomes, thereby ensuring linkages between sister chromatids in early meiosis.

Zipping and Unzipping: Protein Modifications Regulating Synaptonemal Complex Dynamics

The proteinaceous zipper-like structure known as the synaptonemal complex (SC), which forms between pairs of homologous chromosomes during meiosis from yeast to humans, plays important roles in promoting interhomolog crossover formation, regulating cessation of DNA double-strand break (DSB) formation following crossover designation, and ensuring accurate meiotic chromosome segregation. Recent studies are starting to reveal critical roles for different protein modifications in regulating SC dynamics. Protein SUMOylation, N-terminal acetylation, and phosphorylation have been shown to be essential for the regulated assembly and disassembly of the SC. Moreover, phosphorylation of specific SC components has been found to link changes in SC dynamics with meiotic recombination. This review highlights the latest findings on how protein modifications regulate SC dynamics and functions.

Moving and stopping: Regulation of chromosome movement to promote meiotic chromosome pairing and synapsis

Meiosis is a specialized cellular division occurring in organisms capable of sexual reproduction that leads to the formation of gametes containing half of the original chromosome number. During the earliest stage of meiosis, prophase I, pairing of homologous chromosomes is achieved in preparation for their proper distribution in the coming divisions. An important question is how do homologous chromosomes find each other and establish pairing interactions. Early studies demonstrated that chromosomes are dynamic in nature and move during this early stage of meiosis. More recently, there have been several studies across different models showing the conserved nature and importance of this chromosome movement, as well as the key components involved in chromosome movement. This review will cover these major findings and also introduce unexamined areas of regulation in meiotic prophase I chromosome movement.

Time-Course Analysis of Early Meiotic Prophase Events Informs Mechanisms of Homolog Pairing and Synapsis in Caenorhabditis elegans

Segregation of homologous chromosomes during meiosis depends on their ability to reorganize within the nucleus, discriminate among potential partners, and stabilize pairwise associations through assembly of the synaptonemal complex (SC). Here we report a high-resolution time-course analysis of these key early events during Caenorhabditis elegans meiosis. Labeled nucleotides are incorporated specifically into the X chromosomes during the last 2 hr of S phase, a property we exploit to identify a highly synchronous cohort of nuclei. By tracking X-labeled nuclei through early meiotic prophase, we define the sequence and duration of chromosome movement, nuclear reorganization, pairing at pairing centers (PCs), and SC assembly. Appearance of ZYG-12 foci (marking attachment of PCs to the nuclear envelope) and onset of active mobilization occur within an hour after S-phase completion. Movement occurs for nearly 2 hr before stable pairing is observed at PCs, and autosome movement continues for ∼4 hr thereafter. Chromosomes are tightly clustered during a 2-3 hr postpairing window, during which the bulk of SC assembly occurs; however, initiation of SC assembly can precede evident chromosome clustering. SC assembly on autosomes begins immediately after PC pairing is detected and is completed within ∼3.5 hr. For the X chromosomes, PC pairing is contemporaneous with autosomal pairing, but autosomes complete synapsis earlier (on average) than X chromosomes, implying that X chromosomes have a delay in onset and/or a slower rate of SC assembly. Additional evidence suggests that transient association among chromosomes sharing the same PC protein may contribute to partner discrimination.

Regulating the construction and demolition of the synaptonemal complex

The synaptonemal complex (SC) is a meiosis-specific scaffold that links homologous chromosomes from end to end during meiotic prophase and is required for the formation of meiotic crossovers. Assembly of SC components is regulated by a combination of associated nonstructural proteins and post-translational modifications, such as SUMOylation, which together coordinate the timing between homologous chromosome pairing, double-strand-break formation and recombination. In addition, transcriptional and translational control mechanisms ensure the timely disassembly of the SC after crossover resolution and before chromosome segregation at anaphase I.

Meiosis

Sexual reproduction requires the production of haploid gametes (sperm and egg) with only one copy of each chromosome; fertilization then restores the diploid chromosome content in the next generation. This reduction in genetic content is accomplished during a specialized cell division called meiosis, in which two rounds of chromosome segregation follow a single round of DNA replication. In preparation for the first meiotic division, homologous chromosomes pair and synapse, creating a context that promotes formation of crossover recombination events. These crossovers, in conjunction with sister chromatid cohesion, serve to connect the two homologs and facilitate their segregation to opposite poles during the first meiotic division. During the second meiotic division, which is similar to mitosis, sister chromatids separate; the resultant products are haploid cells that become gametes. In Caenorhabditis elegans (and most other eukaryotes) homologous pairing and recombination are required for proper chromosome inheritance during meiosis; accordingly, the events of meiosis are tightly coordinated to ensure the proper execution of these events. In this chapter, we review the seminal events of meiosis: pairing of homologous chromosomes, the changes in chromosome structure that chromosomes undergo during meiosis, the events of meiotic recombination, the differentiation of homologous chromosome pairs into structures optimized for proper chromosome segregation at Meiosis I, and the ultimate segregation of chromosomes during the meiotic divisions. We also review the regulatory processes that ensure the coordinated execution of these meiotic events during prophase I.

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.

The synaptonemal complex has liquid crystalline properties and spatially regulates meiotic recombination factors

The synaptonemal complex (SC) is a polymer that spans ~100 nm between paired homologous chromosomes during meiosis. Its striated, periodic appearance in electron micrographs led to the idea that transverse filaments within this structure ‘crosslink’ the axes of homologous chromosomes, stabilizing their pairing. SC proteins can also form polycomplexes, three-dimensional lattices that recapitulate the periodic structure of SCs but do not associate with chromosomes. Here we provide evidence that SCs and polycomplexes contain mobile subunits and that their assembly is promoted by weak hydrophobic interactions, indicative of a liquid crystalline phase. We further show that in the absence of recombination intermediates, polycomplexes recapitulate the dynamic localization of pro-crossover factors during meiotic progression, revealing how the SC might act as a conduit to regulate chromosome-wide crossover distribution. Properties unique to liquid crystals likely enable long-range signal transduction along meiotic chromosomes and underlie the rapid evolution of SC proteins.

DOI:http://dx.doi.org/10.7554/eLife.21455.001

The genetic information in cells is encoded within long molecules of DNA called chromosomes. In most human cells, the two copies of each chromosome – the one inherited from our mother and the one from our father – are physically separated and behave independently. However, in the reproductive cells that give rise to eggs or sperm, each chromosome must pair with its partner. Pairing first occurs at one or more positions along each chromosome. This triggers a protein-based polymer called the “synaptonemal complex” to assemble between the paired chromosomes, and then spread along the interface between the partners until they are fully lined up side-by-side. Chromosomes in reproductive cells must pair in this particular way to exchange genetic information and generate new combinations of traits.

The synaptonemal complex was first observed over 60 years ago, but it remains enigmatic. Though its structure is highly ordered and looks very similar in different organisms from yeast to humans, little is known about how this polymer forms or what it does between chromosomes. Some evidence has suggested that the synaptonemal complex helps to regulate how much information can be transferred between each pair of chromosomes, but not all studies have supported this conclusion.

Several lines of evidence suggest that the synaptonemal complex might be fundamentally different from other protein-based polymers, such as those that form filamentous skeletal structures within cells, namely actin filaments and microtubules. Now, Rog et al. have tested the idea that the synaptonemal complex might actually have liquid-like properties, despite its highly ordered appearance.

The experiments showed that the proteins that make up the synaptonemal complex in yeast, worms and fruit flies are weakly bound to each other and can move around within the assembled structure. These are considered to be defining properties that distinguish liquids from solid materials. Together with its regular, repetitive organization, these findings indicate that the synaptonemal complex behaves like a liquid crystal. This intriguing class of materials has properties between those of conventional liquids and those of solid crystals, and is particularly sensitive to environmental conditions.

Rog et al. believe that this discovery helps to explain how signals are transmitted along the length of chromosomes to regulate the transfer of genetic information. In support of this idea, further experiments showed that proteins that are required for this recombination process were also found within the synaptonemal complex. As reproductive cells transition from one stage of their development to the next, these proteins abruptly move to a new location, indicating that a switch-like signal rapidly spreads throughout the synaptonemal complex.

Together the findings suggest that the liquid crystal-like properties of the synaptonemal complex allow signals to be transmitted along the interface between pairs of chromosomes. The next challenges are to understand what triggers these signals and to explore whether they are based upon physical or chemical changes within the synaptonemal complex. Further research is also needed to uncover how this information is propagated along the length of a chromosome.

DOI:http://dx.doi.org/10.7554/eLife.21455.002

Cohesin-interacting protein WAPL-1 regulates meiotic chromosome structure and cohesion by antagonizing specific cohesin complexes

Wapl induces cohesin dissociation from DNA throughout the mitotic cell cycle, modulating sister chromatid cohesion and higher-order chromatin structure. Cohesin complexes containing meiosis-specific kleisin subunits govern most aspects of meiotic chromosome function, but whether Wapl regulates these complexes remains unknown. We show that during C. elegans oogenesis WAPL-1 antagonizes binding of cohesin containing COH-3/4 kleisins, but not REC-8, demonstrating that sensitivity to WAPL-1 is dictated by kleisin identity. By restricting the amount of chromosome-associated COH-3/4 cohesin, WAPL-1 controls chromosome structure throughout meiotic prophase. In the absence of REC-8, WAPL-1 inhibits COH-3/4-mediated cohesion, which requires crossover-fated events formed during meiotic recombination. Thus, WAPL-1 promotes functional specialization of meiotic cohesin: WAPL-1-sensitive COH-3/4 complexes modulate higher-order chromosome structure, while WAPL-1-refractory REC-8 complexes provide stable cohesion. Surprisingly, a WAPL-1-independent mechanism removes cohesin before metaphase I. Our studies provide insight into how meiosis-specific cohesin complexes are regulated to ensure formation of euploid gametes.

Manipulation of Karyotype in Caenorhabditis elegans Reveals Multiple Inputs Driving Pairwise Chromosome Synapsis During Meiosis

Meiotic chromosome segregation requires pairwise association between homologs, stabilized by the synaptonemal complex (SC). Here, we investigate factors contributing to pairwise synapsis by investigating meiosis in polyploid worms. We devised a strategy, based on transient inhibition of cohesin function, to generate polyploid derivatives of virtually any Caenorhabditis elegans strain. We exploited this strategy to investigate the contribution of recombination to pairwise synapsis in tetraploid and triploid worms. In otherwise wild-type polyploids, chromosomes first sort into homolog groups, then multipartner interactions mature into exclusive pairwise associations. Pairwise synapsis associations still form in recombination-deficient tetraploids, confirming a propensity for synapsis to occur in a strictly pairwise manner. However, the transition from multipartner to pairwise association was perturbed in recombination-deficient triploids, implying a role for recombination in promoting this transition when three partners compete for synapsis. To evaluate the basis of synapsis partner preference, we generated polyploid worms heterozygous for normal sequence and rearranged chromosomes sharing the same pairing center (PC). Tetraploid worms had no detectable preference for identical partners, indicating that PC-adjacent homology drives partner choice in this context. In contrast, triploid worms exhibited a clear preference for identical partners, indicating that homology outside the PC region can influence partner choice. Together, our findings, suggest a two-phase model for C. elegans synapsis: an early phase, in which initial synapsis interactions are driven primarily by recombination-independent assessment of homology near PCs and by a propensity for pairwise SC assembly, and a later phase in which mature synaptic interactions are promoted by recombination.

Pseudosynapsis and decreased stringency of meiotic repair pathway choice on the hemizygous sex chromosome of Caenorhabditis elegans males

During meiosis, accurate chromosome segregation relies on homology to mediate chromosome pairing, synapsis, and crossover recombination. Crossovers are dependent upon formation and repair of double-strand breaks (DSBs) by homologous recombination (HR). In males of many species, sex chromosomes are largely hemizygous, yet DSBs are induced along nonhomologous regions. Here we analyzed the genetic requirements for meiotic DSB repair on the completely hemizygous X chromosome of Caenorhabditis elegans males. Our data reveal that the kinetics of DSB formation, chromosome pairing, and synapsis are tightly linked in the male germ line. Moreover, DSB induction on the X is concomitant with a brief period of pseudosynapsis that may allow X sister chromatids to masquerade as homologs. Consistent with this, neither meiotic kleisins nor the SMC-5/6 complex are essential for DSB repair on the X. Furthermore, early processing of X DSBs is dependent on the CtIP/Sae2 homolog COM-1, suggesting that as with paired chromosomes, HR is the preferred pathway. In contrast, the X chromosome is refractory to feedback mechanisms that ensure crossover formation on autosomes. Surprisingly, neither RAD-54 nor BRC-2 are essential for DSB repair on the X, suggesting that unlike autosomes, the X is competent for repair in the absence of HR. When both RAD-54 and the structure-specific nuclease XPF-1 are abrogated, X DSBs persist, suggesting that single-strand annealing is engaged in the absence of HR. Our findings indicate that alteration in sister chromatid interactions and flexibility in DSB repair pathway choice accommodate hemizygosity on sex chromosomes.

The fidelity of synaptonemal complex assembly is regulated by a signaling mechanism that controls early meiotic progression

Proper chromosome segregation during meiosis requires the assembly of the synaptonemal complex (SC) between homologous chromosomes. However, the SC structure itself is indifferent to homology, and poorly understood mechanisms that depend on conserved HORMA-domain proteins prevent ectopic SC assembly. Although HORMA-domain proteins are thought to regulate SC assembly as intrinsic components of meiotic chromosomes, here we uncover a key role for nuclear soluble HORMA-domain protein HTP-1 in the quality control of SC assembly. We show that a mutant form of HTP-1 impaired in chromosome loading provides functionality of an HTP-1-dependent checkpoint that delays exit from homology search-competent stages until all homolog pairs are linked by the SC. Bypassing of this regulatory mechanism results in premature meiotic progression and licensing of homology-independent SC assembly. These findings identify nuclear soluble HTP-1 as a regulator of early meiotic progression, suggesting parallels with the mode of action of Mad2 in the spindle assembly checkpoint.

The CSN/COP9 signalosome regulates synaptonemal complex assembly during meiotic prophase I of Caenorhabditis elegans

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

Meiosis is a cellular division required for the formation of gametes, and therefore sexual reproduction. Accurate chromosome segregation is dependent on the formation of crossovers, the exchange of DNA between homologous chromosomes. A key process in the formation of crossovers is the assembly of the synaptonemal complex (SC) between homologs during prophase I. How functional SC structure forms is still not well understood. Here we identify CSN/COP9 signalosome complex as having a clear role in chromosome synapsis. In CSN/COP9 mutants, SC proteins aggregate and fail to properly assemble on homologous chromosomes. This leads to defects in homolog pairing, repair of meiotic DNA damage and crossover formation. The data in this paper suggest that the role of the CSN/COP9 signalosome is to prevent the aggregation of central region proteins during SC assembly.

Protein phosphatase 4 promotes chromosome pairing and synapsis, and contributes to maintaining crossover competence with increasing age

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

Meiosis creates gametes by distributing diploid genomes containing homologous chromosome pairs into daughter cells that receive only one of each chromosome. To segregate correctly at the first meiotic division, chromosomes must pair and synapse with their homologous partners, and undergo crossover recombination, which requires breaking and repairing the DNA strands of all chromosomes. How chromosomes recognize their partners, and how a cell controls the amount of DNA breakage and recombination that occurs, are open questions. In this study, we observed meiosis in the nematode Caenorhabditis elegans to examine the role of Protein Phosphatase 4 (PP4). We found that in the absence of PP4, chromosomes often paired and synapsed with non-homologous chromosomes, or synapsed with themselves by folding in half. Additionally, without PP4 activity, the number of DNA breaks and of crossover recombination events were both independently reduced. The latter two defects became even worse with increasing age, indicating that older animals require PP4 to a greater extent. These findings shed light on how protein phosphorylation controls meiotic events, and demonstrate unanticipated, important roles for PP4.

Evidence that masking of synapsis imperfections counterbalances quality control to promote efficient meiosis

Reduction in ploidy to generate haploid gametes during sexual reproduction is accomplished by the specialized cell division program of meiosis. Pairing between homologous chromosomes and assembly of the synaptonemal complex at their interface (synapsis) represent intermediate steps in the meiotic program that are essential to form crossover recombination-based linkages between homologs, which in turn enable segregation of the homologs to opposite poles at the meiosis I division. Here, we challenge the mechanisms of pairing and synapsis during C. elegans meiosis by disrupting the normal 1∶1 correspondence between homologs through karyotype manipulation. Using a combination of cytological tools, including S-phase labeling to specifically identify X chromosome territories in highly synchronous cohorts of nuclei and 3D rendering to visualize meiotic chromosome structures and organization, our analysis of trisomic (triplo-X) and polyploid meiosis provides insight into the principles governing pairing and synapsis and how the meiotic program is “wired” to maximize successful sexual reproduction. We show that chromosomes sort into homologous groups regardless of chromosome number, then preferentially achieve pairwise synapsis during a period of active chromosome mobilization. Further, comparisons of synapsis configurations in triplo-X germ cells that are proficient or defective for initiating recombination suggest a role for recombination in restricting chromosomal interactions to a pairwise state. Increased numbers of homologs prolong markers of the chromosome mobilization phase and/or boost germline apoptosis, consistent with triggering quality control mechanisms that promote resolution of synapsis problems and/or cull meiocytes containing synapsis defects. However, we also uncover evidence for the existence of mechanisms that “mask” defects, thus allowing resumption of prophase progression and survival of germ cells despite some asynapsis. We propose that coupling of saturable masking mechanisms with stringent quality controls maximizes meiotic success by making progression and survival dependent on achieving a level of synapsis sufficient for crossover formation without requiring perfect synapsis.

Diploid organisms must produce haploid gametes prior to sexual reproduction in order to maintain a constant number of chromosomes from one generation to the next. Ploidy reduction is accomplished during meiosis and requires crossover recombination-based linkages between homologous chromosomes. Here, we manipulate karyotype in C. elegans to probe the mechanisms that govern stable, pairwise, homologous associations essential for crossover formation. We find that chromosomes sort into homolog groups regardless of number prior to stabilizing interactions (“synapsing”) in a preferentially pairwise manner. Increased numbers of homologs delay meiotic progression and/or boost cell death, reflecting operation of quality control mechanisms that either buy time to correct synapsis problems or eliminate defective cells. Moreover, we found evidence for mechanisms that can “mask” synapsis imperfections, thus allowing resumption of meiotic progression and survival of germ cells when synapsis is “good enough”, albeit imperfect. This strategy would maximize meiotic success by making progression and survival contingent on achieving a level of synapsis sufficient for crossover formation without imposing an onerous and unnecessary requirement for perfect synapsis. We suggest that the regulatory logic of coupling saturable masking mechanisms with stringent quality controls may be employed widely to maximize efficiency of biological circuits.

High throughput sequencing reveals alterations in the recombination signatures with diminishing Spo11 activity

Spo11 is the topoisomerase-like enzyme responsible for the induction of the meiosis-specific double strand breaks (DSBs), which initiates the recombination events responsible for proper chromosome segregation. Nineteen PCR-induced alleles of SPO11 were identified and characterized genetically and cytologically. Recombination, spore viability and synaptonemal complex (SC) formation were decreased to varying extents in these mutants. Arrest by ndt80 restored these events in two severe hypomorphic mutants, suggesting that ndt80-arrested nuclei are capable of extended DSB activity. While crossing-over, spore viability and synaptonemal complex (SC) formation defects correlated, the extent of such defects was not predictive of the level of heteroallelic gene conversions (prototrophs) exhibited by each mutant. High throughput sequencing of tetrads from spo11 hypomorphs revealed that gene conversion tracts associated with COs are significantly longer and gene conversion tracts unassociated with COs are significantly shorter than in wild type. By modeling the extent of these tract changes, we could account for the discrepancy in genetic measurements of prototrophy and crossover association. These findings provide an explanation for the unexpectedly low prototroph levels exhibited by spo11 hypomorphs and have important implications for genetic studies that assume an unbiased recovery of prototrophs, such as measurements of CO homeostasis. Our genetic and physical data support previous observations of DSB-limited meioses, in which COs are disproportionally maintained over NCOs (CO homeostasis).

Most eukaryotes depend on the meiotic division to segregate each pair of chromosomes properly into their gametes. Chromosome segregation mistakes happening during meiosis are responsible for most miscarriages as well as many diseases such as Down's and Kleinfelter's syndromes in humans. Proper chromosome segregation during meiosis depends on efficient and regulated recombination events that link homologous chromosomes prior to the first meiotic division. These linkages are initiated at double-stranded breaks (DSBs) in chromosomal DNA by Spo11 and associated proteins. We isolated a valuable new set of SPO11 alleles in yeast with a wide range of Spo11 activity. Genetic analysis and high throughput sequencing of tetrads from these mutants has revealed unexpected features of meiotic recombination. First, Spo11 DSBs likely continue to form throughout a pachytene arrest in cells compromised for Spo11 activity. Second, the number of recombination initiation events in a given meiosis influences the repair outcome of those events. In addition, our results provide support for crossover homeostasis – a phenomenon in which crossovers are disproportionately maintained over other types of repair in the face of a decrease in DSBs.

Chromosome movement in meiosis I prophase of Caenorhabditis elegans

Rapid chromosome movement during prophase of the first meiotic division has been observed in many organisms. It is generally concomitant with formation of the “meiotic chromosome bouquet,” a special chromosome configuration in which one or both chromosome ends attach to the nuclear envelope and become concentrated within a limited area. The precise function of the chromosomal bouquet is still not fully understood. Chromosome mobility is implicated in homologous chromosome pairing, synaptonemal complex formation, recombination, and resolution of chromosome entanglements. The basic mechanistic module through which forces are exerted on chromosomes is widely conserved; however, phenotypic differences have been reported among various model organisms once movement is abrogated. Movements are transmitted to the chromosome ends by the nuclear membrane-bridging SUN/KASH complex and are dependent on cytoskeletal filaments and motor proteins located in the cytoplasm. Here we review the recent findings on chromosome mobility during meiosis in an animal model system: the Caenorhabditis elegans nematode.

Chromosome movements promoted by the mitochondrial protein SPD-3 are required for homology search during Caenorhabditis elegans meiosis

Pairing of homologous chromosomes during early meiosis is essential to prevent the formation of aneuploid gametes. Chromosome pairing includes a step of homology search followed by the stabilization of homolog interactions by the synaptonemal complex (SC). These events coincide with dramatic changes in nuclear organization and rapid chromosome movements that depend on cytoskeletal motors and are mediated by SUN-domain proteins on the nuclear envelope, but how chromosome mobility contributes to the pairing process remains poorly understood. We show that defects in the mitochondria-localizing protein SPD-3 cause a defect in homolog pairing without impairing nuclear reorganization or SC assembly, which results in promiscuous installation of the SC between non-homologous chromosomes. Preventing SC assembly in spd-3 mutants does not improve homolog pairing, demonstrating that SPD-3 is required for homology search at the start of meiosis. Pairing center regions localize to SUN-1 aggregates at meiosis onset in spd-3 mutants; and pairing-promoting proteins, including cytoskeletal motors and polo-like kinase 2, are normally recruited to the nuclear envelope. However, quantitative analysis of SUN-1 aggregate movement in spd-3 mutants demonstrates a clear reduction in mobility, although this defect is not as severe as that seen in sun-1(jf18) mutants, which also show a stronger pairing defect, suggesting a correlation between chromosome-end mobility and the efficiency of pairing. SUN-1 aggregate movement is also impaired following inhibition of mitochondrial respiration or dynein knockdown, suggesting that mitochondrial function is required for motor-driven SUN-1 movement. The reduced chromosome-end mobility of spd-3 mutants impairs coupling of SC assembly to homology recognition and causes a delay in meiotic progression mediated by HORMA-domain protein HTP-1. Our work reveals how chromosome mobility impacts the different early meiotic events that promote homolog pairing and suggests that efficient homology search at the onset of meiosis is largely dependent on motor-driven chromosome movement.

Sexually reproducing organisms carry two copies of each chromosome (homologs), which must be separated during gamete formation to prevent chromosome duplication in each generation. This chromosome halving is achieved during meiosis, a type of cell division in which the homologs recognize and pair with one another before they become intimately glued together by a structure called the synaptonemal complex (SC). Homolog pairing and SC assembly coincide with movement of chromosomes inside the nucleus, but how chromosome mobility impacts these events is not understood. We find that the mitochondrial protein SPD-3 is required to ensure normal levels of motor-driven chromosome movement and that, although pairing-promoting proteins are normally recruited at the start of meiosis in spd-3 mutants, reduced chromosome mobility impairs homolog pairing. In contrast, SC assembly is normally started, leading to the installation of SC between non-homologous chromosomes and demonstrating a failure in the coordination of pairing and SC assembly. Reduced movement also causes a controlled delay in exit from early meiotic stages characterized by chromosome clustering and active homology search. Our findings show how the different events that lead to the correct association of homologous chromosomes during early meiosis are affected by chromosome mobility.

Application of advanced fluorescence microscopy to the structure of meiotic chromosomes

Chromosomes undergoing meiosis are defined by a macromolecular protein assembly called the synaptonemal complex which holds homologs together and carries out important meiotic functions. By retaining the molecular specificity, multiplexing ability, and in situ imaging capabilities of fluorescence microscopy, but with vastly increased resolution, 3D-SIM and other superresolution techniques are poised to make significant discoveries about the structure and function of the synaptonemal complex. This review discusses recent developments in this field and poses questions approachable with current and future technology.

Chromosome pairing and synapsis during Caenorhabditis elegans meiosis

Meiosis is the specialized cell division cycle that produces haploid gametes to enable sexual reproduction. Reduction of chromosome number by half requires elaborate chromosome dynamics that occur in meiotic prophase to establish physical linkages between each pair of homologous chromosomes. Caenorhabditis elegans has emerged as an excellent model organism for molecular studies of meiosis, enabling investigators to combine the power of molecular genetics, cytology, and live analysis. Here we focus on recent studies that have shed light on how chromosomes find and identify their homologous partners, and the structural changes that accompany and mediate these interactions.