The many facets of SC function during C. elegans meiosis

Single-nucleus resolution mapping of the adult C. elegans and its application to elucidate inter- and trans-generational response to alcohol

Single-cell transcriptomic platforms provide an opportunity to map an organism's response to environmental cues with high resolution. Here, we applied single-nucleus RNA sequencing (snRNA-seq) to establish the tissue and cell type-resolved transcriptome of the adult C. elegans and characterize the inter- and trans-generational transcriptional impact of ethanol. We profiled the transcriptome of 41,749 nuclei resolving into 31 clusters, representing a diverse array of adult cell types including syncytial tissues. Following exposure to human-relevant doses of alcohol, several germline, striated muscle, and neuronal clusters were identified as being the most transcriptionally impacted at the F1 and F3 generations. The effect on germline clusters was confirmed by phenotypic enrichment analysis as well as by functional validation, which revealed a remarkable inter- and trans-generational increase in germline apoptosis, aneuploidy, and embryonic lethality. Together, snRNA-seq represents a valuable approach for the detailed examination of an adult organism's response to environmental exposures.

Loss, Gain, and Retention: Mechanisms Driving Late Prophase I Chromosome Remodeling for Accurate Meiotic Chromosome Segregation

To generate gametes, sexually reproducing organisms need to achieve a reduction in ploidy, via meiosis. Several mechanisms are set in place to ensure proper reductional chromosome segregation at the first meiotic division (MI), including chromosome remodeling during late prophase I. Chromosome remodeling after crossover formation involves changes in chromosome condensation and restructuring, resulting in a compact bivalent, with sister kinetochores oriented to opposite poles, whose structure is crucial for localized loss of cohesion and accurate chromosome segregation. Here, we review the general processes involved in late prophase I chromosome remodeling, their regulation, and the strategies devised by different organisms to produce bivalents with configurations that promote accurate segregation.

Exposure to phthalates: germline dysfunction and aneuploidy

Epidemiological studies continue to reveal the enduring impact of exposures to environmental chemicals on human physiology, including our reproductive health. Phthalates, a well characterized class of endocrine disrupting chemicals and commonly utilized plasticizers, are among one of the many toxicants ubiquitously present in our environment. Phthalate exposure has been linked to increases in the rate of human aneuploidy, a phenomenon that is detected in 0.3% of livebirths resulting in genetic disorders including trisomy 21, approximately 4% of stillbirths, and over 35% of miscarriages. Here we review recent epidemiological and experimental studies that have examined the role that phthalates play in germline dysfunction, including increases in apoptosis, oxidative stress, DNA damage, and impaired genomic integrity, resulting in aneuploidy. We will further discuss subject variability, as it relates to diet and polymorphisms, and the sexual dimorphic effects of phthalate exposure, as it relates to sex-specific targets. Lastly, we discuss some of the conserved effects of phthalate exposure across humans, mammalian models and nonmammalian model organisms, highlighting the importance of using model organisms to our advantage for chemical risk assessment and unveiling potential mechanisms that underlie phthalate-induced reproductive health issues across species.

Systematic analysis of long intergenic non-coding RNAs in C. elegans germline uncovers roles in somatic growth

Long intergenic non-coding RNAs (lincRNAs) are transcripts longer than 200 nucleotides that are transcribed from non-coding loci yet undergo biosynthesis similar to coding mRNAs. The disproportional number of lincRNAs expressed in testes suggests that lincRNAs are important during gametogenesis, but experimental evidence has implicated very few lincRNAs in this process. We took advantage of the relatively limited number of lincRNAs in the genome of the nematode Caenorhabditis elegans to systematically analyse the functions of lincRNAs during meiosis. We deleted six lincRNA genes that are highly and dynamically expressed in the C. elegans gonad and tested the effects on central meiotic processes. Surprisingly, whereas the lincRNA deletions did not strongly impact fertility, germline apoptosis, crossovers, or synapsis, linc-4 was required for somatic growth. Slower growth was observed in linc-4-deletion mutants and in worms depleted of linc-4 using RNAi, indicating that linc-4 transcripts are required for this post-embryonic process. Unexpectedly, analysis of worms depleted of linc-4 in soma versus germline showed that the somatic role stems from linc-4 expression in germline cells. This unique feature suggests that some lincRNAs, like some small non-coding RNAs, are required for germ-soma interactions.

Slx5p-Slx8p Promotes Accurate Chromosome Segregation by Mediating the Degradation of Synaptonemal Complex Components during Meiosis

Meiosis increases genetic diversity, yet the genome complement needs to be stable to ensure offspring viability. Both small ubiquitin-like modifier (SUMO) and ubiquitin have been reported to participate in meiotic regulation, yet functions of the SUMO-ubiquitination crosstalk in meiosis remain unclear. Here, it is reported that a SUMO-targeted ubiquitin ligase, Slx8p, promotes accurate chromosome segregation during meiosis, since the deletion of SLX8 leads to increased aneuploidy due to a defect in synaptonemal complex (SC) component degradation. Both the RING domain and SUMO interacting motifs of Slx8p are essential for meiotic progression and maintaining spore viability, and the expression of tetraubiquitin fused with SUMO partially rescues meiotic defects in the SLX8-deletion strain. Furthermore, Slx5p-Slx8p can directly add ubiquitin to SUMOylated Zip1p and Ecm11p, and forced degradation of Ecm11p partially rescues the sporulation defects of the SLX8 deletion strain. These findings provide a mechanism for SC disassembly and reveal that the crosstalk between SUMOylation and ubiquitination facilitates accurate chromosome segregation by promoting SC component degradation during meiosis.

Antioxidant CoQ10 Restores Fertility by Rescuing Bisphenol A-Induced Oxidative DNA Damage in the Caenorhabditis elegans Germline

Endocrine-disrupting chemicals are ubiquitously present in our environment, but the mechanisms by which they adversely affect human reproductive health and strategies to circumvent their effects remain largely unknown. Here, we show in Caenorhabditis elegans that supplementation with the antioxidant Coenzyme Q10 (CoQ10) rescues the reprotoxicity induced by the widely used plasticizer and endocrine disruptor bisphenol A (BPA), in part by neutralizing DNA damage resulting from oxidative stress. CoQ10 significantly reduces BPA-induced elevated levels of germ cell apoptosis, phosphorylated checkpoint kinase 1 (CHK-1), double-strand breaks (DSBs), and chromosome defects in diakinesis oocytes. BPA-induced oxidative stress, mitochondrial dysfunction, and increased gene expression of antioxidant enzymes in the germline are counteracted by CoQ10. Finally, CoQ10 treatment also reduced the levels of aneuploid embryos and BPA-induced defects observed in early embryonic divisions. We propose that CoQ10 may counteract BPA-induced reprotoxicity through the scavenging of reactive oxygen species and free radicals, and that this natural antioxidant could constitute a low-risk and low-cost strategy to attenuate the impact on fertility by BPA.

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

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

Assessing effects of germline exposure to environmental toxicants by high-throughput screening in C. elegans

Chemicals that are highly prevalent in our environment, such as phthalates and pesticides, have been linked to problems associated with reproductive health. However, rapid assessment of their impact on reproductive health and understanding how they cause such deleterious effects, remain challenging due to their fast-growing numbers and the limitations of various current toxicity assessment model systems. Here, we performed a high-throughput screen in C. elegans to identify chemicals inducing aneuploidy as a result of impaired germline function. We screened 46 chemicals that are widely present in our environment, but for which effects in the germline remain poorly understood. These included pesticides, phthalates, and chemicals used in hydraulic fracturing and crude oil processing. Of the 46 chemicals tested, 41% exhibited levels of aneuploidy higher than those detected for bisphenol A (BPA), an endocrine disruptor shown to affect meiosis, at concentrations correlating well with mammalian reproductive endpoints. We further examined three candidates eliciting aneuploidy: dibutyl phthalate (DBP), a likely endocrine disruptor and frequently used plasticizer, and the pesticides 2-(thiocyanomethylthio) benzothiazole (TCMTB) and permethrin. Exposure to these chemicals resulted in increased embryonic lethality, elevated DNA double-strand break (DSB) formation, activation of p53/CEP-1-dependent germ cell apoptosis, chromosomal abnormalities in oocytes at diakinesis, impaired chromosome segregation during early embryogenesis, and germline-specific alterations in gene expression. This study indicates that this high-throughput screening system is highly reliable for the identification of environmental chemicals inducing aneuploidy, and provides new insights into the impact of exposure to three widely used chemicals on meiosis and germline function.

The ever-increasing number of new chemicals introduced into our environment poses a significant problem for risk assessment. In addition, assessing the direct impact of toxicants on human meiosis remains challenging. We successfully utilized a high-throughput platform in the nematode C. elegans, a genetically tractable model organism which shares a high degree of gene conservation with humans, to identify chemicals that affect the germline leading to aneuploidy. We assessed chemicals that are highly prevalent in the environment in worms carrying a fluorescent reporter construct allowing for the identification of X chromosome nondisjunction combined with a mutation increasing cuticle permeability for analysis of low doses of exposure. Follow up analysis of three chemicals: DBP, permethrin and TCMTB, further validated the use of this strategy. Exposure to these chemicals resulted in elevated levels of DNA double-strand breaks, activation of a DNA damage checkpoint, chromosome morphology defects in late meiotic prophase I as well as impaired early embryogenesis and germline-specific changes in gene expression. Our results support the use of this high-throughput screening system to identify environmental chemicals inducing aneuploidy, and provide new insights into the effects of exposure to DBP, permethrin, and TCMTB on meiosis and germline function.

A Role in Apoptosis Regulation for the rad-51 Gene of Caenorhabditis elegans

The evolutionarily conserved RAD-51 protein is essential for homologous recombination in the germ line as well as homologous repair of DNA double-strand breaks in all eukaryotic cells. In the nematode Caenorhabditis elegans, the rad-51 gene is transcribed into messenger RNAs potentially coding three alternative protein isoforms. Null rad-51 alleles display embryonic lethality, severe defects in chromosome structure, and high levels of germ line apoptosis. To dissect its functions, we genetically modified the C. elegans rad-51 gene by clustered regularly interspaced short palindromic repeats/Cas9 genome-editing technology, obtaining a separation-of-function (sfi-) mutant allele that only disrupts the long-transcript isoform. This mutant shows no defects in an otherwise wild-type meiosis and is able to activate physiological germ cell death, which occurs at the late pachytene stage. However, although the mutant is competent in DNA damage checkpoint activation after exposure to ionizing radiation, it is defective for induction of DNA damage-induced apoptosis in meiotic germ cells. These results suggest that RAD-51 plays a novel role in germ line apoptosis independent of RAD-51-mediated strand invasion for homologous recombination.

Fast Functional Germline and Epigenetic Assays in the Nematode Caenorhabditis elegans

Germ cells are unique in their ability to transfer traits and genetic information from one generation to the next. The proper development and integrity of their genome are therefore of utmost importance for the health of organisms and survival of species. Many features of mammalian germ cells, including their long development span and difficulty of access, present challenges for their study in the context of toxicity assays. In light of these barriers, the model system Caenorhabditis elegans shows great potential given its ease of manipulation and genetic tractability which can be easily adapted for high-throughput analysis. In this chapter, we discuss the advantages of examining germ cell processes in C. elegans, and describe three functional germline assays for the examination of chemical impact on germline maintenance and function including assays probing germ cell differentiation, germline apoptosis, and germline epigenetic regulation.

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.

Polo-like kinase-dependent phosphorylation of the synaptonemal complex protein SYP-4 regulates double-strand break formation through a negative feedback loop

The synaptonemal complex (SC) is an ultrastructurally conserved proteinaceous structure that holds homologous chromosomes together and is required for the stabilization of pairing interactions and the completion of crossover (CO) formation between homologs during meiosis I. Here, we identify a novel role for a central region component of the SC, SYP-4, in negatively regulating formation of recombination-initiating double-strand breaks (DSBs) via a feedback loop triggered by crossover designation in C. elegans. We found that SYP-4 is phosphorylated dependent on Polo-like kinases PLK-1/2. SYP-4 phosphorylation depends on DSB formation and crossover designation, is required for stabilizing the SC in pachytene by switching the central region of the SC from a more dynamic to a less dynamic state, and negatively regulates DSB formation. We propose a model in which Polo-like kinases recognize crossover designation and phosphorylate SYP-4 thereby stabilizing the SC and making chromosomes less permissive for further DSB formation.

The MAP kinase pathway coordinates crossover designation with disassembly of synaptonemal complex proteins during meiosis

Asymmetric disassembly of the synaptonemal complex (SC) is crucial for proper meiotic chromosome segregation. However, the signaling mechanisms that directly regulate this process are poorly understood. Here we show that the mammalian Rho GEF homolog, ECT-2, functions through the conserved RAS/ERK MAP kinase signaling pathway in the C. elegans germline to regulate the disassembly of SC proteins. We find that SYP-2, a SC central region component, is a potential target for MPK-1-mediated phosphorylation and that constitutively phosphorylated SYP-2 impairs the disassembly of SC proteins from chromosomal domains referred to as the long arms of the bivalents. Inactivation of MAP kinase at late pachytene is critical for timely disassembly of the SC proteins from the long arms, and is dependent on the crossover (CO) promoting factors ZHP-3/RNF212/Zip3 and COSA-1/CNTD1. We propose that the conserved MAP kinase pathway coordinates CO designation with the disassembly of SC proteins to ensure accurate chromosome segregation.

DOI:http://dx.doi.org/10.7554/eLife.12039.001

Most plants and animals, including humans, have cells that contain two copies of every chromosome, with one set inherited from each parent. However, reproductive cells (such as eggs and sperm) contain just one copy of every chromosome so that when they fuse together at fertilization, the resulting cell will have the usual two copies of each chromosome.

Embryos that have incorrect numbers of chromosome copies either fail to survive or develop disorders such as Down syndrome. Therefore, it is important that when cells divide to form new reproductive cells, their chromosomes are correctly segregated.

To end up with one copy of each chromosome, reproductive cells undergo a form of cell division called meiosis. During meiosis, pairs of chromosomes are held together by a zipper-like structure called the synaptonemal complex. While held together like this, each chromosome in the pair exchanges DNA with the other by forming junctions called crossovers. Once DNA exchange is completed, the synaptonemal complex disappears from certain regions of the chromosome.

Using a range of genetic, biochemical and cell biological approaches, Nadarajan et al. have now investigated how crossover formation and the disassembly of the synaptonemal complex are coordinated in the reproductive cells of a roundworm called Caenorhabditis elegans. This revealed that a signaling pathway called the MAP kinase pathway regulates the removal of synaptonemal complex proteins from particular sites between the paired chromosomes. Turning off this pathway’s activity is required for the timely disassembly of this complex, and depends on proteins that are involved in crossover formation.

This regulatory mechanism likely ensures that the synaptonemal complex starts to disassemble only after the physical attachments between the paired chromosomes are “locked in”, thus ensuring that reproductive cells receive the correct number of chromosomes. Given that the MAP kinase pathway regulates cell processes in many different organisms, a future challenge is to determine whether this pathway regulates the synaptonemal complex in other species as well.

DOI:http://dx.doi.org/10.7554/eLife.12039.002

NatB domain-containing CRA-1 antagonizes hydrolase ACER-1 linking acetyl-CoA metabolism to the initiation of recombination during C. elegans meiosis

The formation of DNA double-strand breaks (DSBs) must take place during meiosis to ensure the formation of crossovers, which are required for accurate chromosome segregation, therefore avoiding aneuploidy. However, DSB formation must be tightly regulated to maintain genomic integrity. How this regulation operates in the context of different chromatin architectures and accessibility, and how it is linked to metabolic pathways, is not understood. We show here that global histone acetylation levels undergo changes throughout meiotic progression. Moreover, perturbations to global histone acetylation levels are accompanied by changes in the frequency of DSB formation in C. elegans. We provide evidence that the regulation of histone acetylation requires CRA-1, a NatB domain-containing protein homologous to human NAA25, which controls the levels of acetyl-Coenzyme A (acetyl-CoA) by antagonizing ACER-1, a previously unknown and conserved acetyl-CoA hydrolase. CRA-1 is in turn negatively regulated by XND-1, an AT-hook containing protein. We propose that this newly defined protein network links acetyl-CoA metabolism to meiotic DSB formation via modulation of global histone acetylation.

Achieving accurate chromosome segregation is a critical outcome for any cell division process. Programmed DNA double-strand break formation is a central mechanism set in place to promote faithful chromosome segregation during meiosis. A subset of these DSBs is repaired as crossovers via reciprocal exchange of genetic information between homologous chromosomes resulting in physical attachments (chiasmata) between homologs, which ensure proper chromosome alignment at the metaphase plate at meiosis I, and also promote genetic diversity. How this regulation operates in the context of different chromatin architectures and accessibility, and how it is linked to metabolic pathways, is not understood. In this study, we found that CRA-1, a NatB domain-containing protein, promotes histone acetylation by maintaining the levels of acetyl-Coenzyme A (acetyl-CoA) through antagonizing ACER-1, a previously unknown and conserved acetyl-CoA hydrolase. CRA-1 is in turn negatively regulated by XND-1, an AT-hook containing protein. We leveraged this discovery to find a connection between the levels of acetyl-CoA, histone acetylation and DSB formation. We identified a novel protein network that links the regulation of DSB formation to the modulation of global levels of histone acetylation, and revealed a link between metabolism and the regulation of DSB formation.

Reproductive toxicity and meiotic dysfunction following exposure to the pesticides Maneb, Diazinon and Fenarimol

The comprehensive identification and mechanistic analysis of reproductive toxicants constitutes one of the major hurdles in the toxicological assessment of chemicals originating from the large number of chemicals to be tested and the difficulty in examining germ cells at various stages of their development. We previously described the development of an assay in the roundworm Caenorhabditis elegans that allows the detection of chemicals bearing aneugenic activity and that could be used for the detection of germline toxicity. We present here new evidence for the reproductive toxicity of three pesticides identified in our germline toxicity assay: Maneb, Diazinon and Fenarimol. We show that all three pesticides cause an acute germline nuclear loss in exposed nematodes in a dose-dependent fashion. The loss of germline nuclei coincides with the meiotic stage of pachytene during Prophase I and is dependent on the germline apoptotic machinery suggesting activation of a meiotic checkpoint. Further investigation revealed a profound dysregulation of the meiotic program revealed by (1) an alteration of the kinetics of double strand repair, (2) the disruption of the process of chromosome morphogenesis at the end of Prophase I and (3) the reorganization of the meiotic differentiation gradient inherent to the C. elegans germline following exposure to Maneb and Diazinon. These defects correlate with a significant increase in embryonic lethality and a corresponding decrease in the number of progeny. These results therefore provide strong evidence for the reproductive toxicity of Maneb, Diazinon and Fenarimol rooted in the alteration of early steps of germ cell differentiation.

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.

The meiotic checkpoint network: step-by-step through meiotic prophase

The generation of haploid gametes by meiosis is a highly conserved process for sexually reproducing organisms that, in almost all cases, involves the extensive breakage of chromosomes. These chromosome breaks occur during meiotic prophase and are essential for meiotic recombination as well as the subsequent segregation of homologous chromosomes. However, their formation and repair must be carefully monitored and choreographed with nuclear dynamics and the cell division program to avoid the creation of aberrant chromosomes and defective gametes. It is becoming increasingly clear that an intricate checkpoint-signaling network related to the canonical DNA damage response is deeply interwoven with the meiotic program and preserves order during meiotic prophase. This meiotic checkpoint network (MCN) creates a wide range of dependent relationships controlling chromosome movement, chromosome pairing, chromatin structure, and double-strand break (DSB) repair. In this review, we summarize our current understanding of the MCN. We discuss commonalities and differences in different experimental systems, with a particular emphasis on the emerging design principles that control and limit cross talk between signals to ultimately ensure the faithful inheritance of chromosomes by the next generation.

Conservation and variability of synaptonemal complex proteins in phylogenesis of eukaryotes

The problems of the origin and evolution of meiosis include the enigmatic variability of the synaptonemal complexes (SCs) which, being morphology similar, consist of different proteins in different eukaryotic phyla. Using bioinformatics methods, we monitored all available eukaryotic proteomes to find proteins similar to known SC proteins of model organisms. We found proteins similar to SC lateral element (LE) proteins and possessing the HORMA domain in the majority of the eukaryotic taxa and assume them the most ancient among all SC proteins. Vertebrate LE proteins SYCP2, SYCP3, and SC65 proved to have related proteins in many invertebrate taxa. Proteins of SC central space are most evolutionarily variable. It means that different protein-protein interactions can exist to connect LEs. Proteins similar to the known SC proteins were not found in Euglenophyta, Chrysophyta, Charophyta, Xanthophyta, Dinoflagellata, and primitive Coelomata. We conclude that different proteins whose common feature is the presence of domains with a certain conformation are involved in the formation of the SC in different eukaryotic phyla. This permits a targeted search for orthologs of the SC proteins using phylogenetic trees. Here we consider example of phylogenetic trees for protozoans, fungi, algae, mosses, and flowering plants.

Meiotic double-strand breaks uncover and protect against mitotic errors in the C. elegans germline

In sexually reproducing multicellular organisms, genetic information is propagated via the germline, the specialized tissue that generates haploid gametes. The C. elegans germline generates gametes in an assembly line-like process-mitotic divisions under the control of the stem cell niche produce nuclei that, upon leaving the niche, enter into meiosis and progress through meiotic prophase [1]. Here, we characterize the effects of perturbing cell division in the mitotic region of the C. elegans germline. We show that mitotic errors result in a spindle checkpoint-dependent cell-cycle delay, but defective nuclei are eventually formed and enter meiosis. These defective nuclei are eliminated by programmed cell death during meiotic prophase. The cell death-based removal of defective nuclei does not require the spindle checkpoint but instead depends on the DNA damage checkpoint. Removal of nuclei resulting from errors in mitosis also requires Spo11, the enzyme that creates double-strand breaks to initiate meiotic recombination. Consistent with this, double-strand breaks are increased in number and persist longer in germlines with mitotic defects. These findings reveal that the process of initiating meiotic recombination inherently selects against nuclei with abnormal chromosomal content generated by mitotic errors, thereby ensuring the genomic integrity of gametes.

Pro-crossover factors regulate damage-dependent apoptosis in the Caenorhabditis elegans germ line

During meiosis, DNA double-strand breaks (DSBs) are physiologically induced to start the recombination process and promote the formation of interhomologue crossovers (COs), which are required to ensure faithful chromosome segregation into the gametes. The timely repair of DSBs is an essential part of the meiotic programme, as accumulation of unprocessed DSBs during the pachytene stage of meiotic prophase triggers a DNA damage checkpoint response that induces apoptosis of damaged cells. We show that CO-promoting factors MSH-4, MSH-5, and ZHP-3, but not COSA-1, are required for the apoptotic response of the meiotic DNA damage checkpoint. Lack of MSH-4 or MSH-5 suppresses the apoptotic response observed in some DNA repair-defective mutants such as fcd-2 and brc-1 (orthologues of FANCD2 and BRCA1), irrespectively of the amount of DSBs present in pachytene nuclei. Although ionizing radiation fails to induce apoptosis in msh-4/5-mutant backgrounds, it induces transcriptional activation of the apoptosis-activator egl-1, which is controlled by the Caenorhabditis elegans p53 orthologue CEP-1. This finding suggests that MSH-4/5 involvement in the apoptotic response occurs downstream or independently of damage sensing and checkpoint activation. This study establishes a role for pro-CO factors MSH-4/5 and ZHP-3 in the execution of apoptosis at late meiotic prophase following the accumulation of exogenous or endogenous DNA damage.

A C. elegans screening platform for the rapid assessment of chemical disruption of germline function

BACKGROUND

Despite the developmental impact of chromosome segregation errors, we lack the tools to assess environmental effects on the integrity of the germline in animals.

OBJECTIVES

We developed an assay in Caenorhabditis elegans that fluorescently marks aneuploid embryos after chemical exposure.

METHODS

We qualified the predictive value of the assay against chemotherapeutic agents as well as environmental compounds from the ToxCast Phase I library by comparing results from the C. elegans assay with the comprehensive mammalian in vivo end point data from the ToxRef database.

RESULTS

The assay was highly predictive of mammalian reproductive toxicities, with a 69% maximum balanced accuracy. We confirmed the effect of select compounds on germline integrity by monitoring germline apoptosis and meiotic progression.

CONCLUSIONS

This C. elegans assay provides a comprehensive strategy for assessing environmental effects on germline function.

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.

Meiotic development in Caenorhabditis elegans

Caenorhabditis elegans has become a powerful experimental organism with which to study meiotic processes that promote the accurate segregation of chromosomes during the generation of haploid gametes. Haploid reproductive cells are produced through one round of chromosome replication followed by two -successive cell divisions. Characteristic meiotic chromosome structure and dynamics are largely conserved in C. elegans. Chromosomes adopt a meiosis-specific structure by loading cohesin proteins, assembling axial elements, and acquiring chromatin marks. Homologous chromosomes pair and form physical connections though synapsis and recombination. Synaptonemal complex and crossover formation allow for the homologs to stably associate prior to remodeling that facilitates their segregation. This chapter will cover conserved meiotic processes as well as highlight aspects of meiosis that are unique to C. elegans.

LAB-1 targets PP1 and restricts Aurora B kinase upon entrance into meiosis to promote sister chromatid cohesion

At the onset of the first meiotic division, the protein LAB-1 recruits the PP1 phosphatase to cohesion complexes, preventing Aurora B kinase from targeting cohesins for degradation prematurely and thereby ensuring proper progression of meiotic events in C. elegans.

Successful execution of the meiotic program depends on the timely establishment and removal of sister chromatid cohesion. LAB-1 has been proposed to act in the latter by preventing the premature removal of the meiosis-specific cohesin REC-8 at metaphase I in C. elegans, yet the mechanism and scope of LAB-1 function remained unknown. Here we identify an unexpected earlier role for LAB-1 in promoting the establishment of sister chromatid cohesion in prophase I. LAB-1 and REC-8 are both required for the chromosomal association of the cohesin complex subunit SMC-3. Depletion of lab-1 results in partial loss of sister chromatid cohesion in rec-8 and coh-4 coh-3 mutants and further enhanced chromatid dissociation in worms where all three kleisins are mutated. Moreover, lab-1 depletion results in increased Aurora B kinase (AIR-2) signals in early prophase I nuclei, coupled with a parallel decrease in signals for the PP1 homolog, GSP-2. Finally, LAB-1 directly interacts with GSP-1 and GSP-2. We propose that LAB-1 targets the PP1 homologs to the chromatin at the onset of meiosis I, thereby antagonizing AIR-2 and cooperating with the cohesin complex to promote sister chromatid association and normal progression of the meiotic program.

A critical step for achieving successful cell division is the regulation of how the cohesin complexes that bind sister chromatids are initially deposited, then maintained, and finally removed to allow the chromatids to separate into daughter cells. This is particularly challenging during meiosis, when the sister chromatids must remain partially connected to each other through the first division. In organisms that have a single focal centromere on each chromosome, such as mammals and flies, cohesin is protected through the first meiotic division by the protein Shugoshin, which binds the PP2A phosphatase. PP2A counteracts phosphorylation by the Aurora B kinase; if certain cohesins are phosphorylated by Aurora B they become targeted for removal, which allows the chromatids to separate. In the nematode C. elegans, the chromosomes lack a localized centromere and the predicted Shugoshin homolog is not required for protection of cohesins; instead, this function is executed in metaphase of the first meiotic division by the protein LAB-1. But it is not completely understood what leads to the deposition of cohesin prior to entry into meiosis and to its maintenance throughout early meiosis I. In this study, we show that LAB-1 is also required for the loading and maintenance of the cohesin complex. LAB-1 ensures that the chromatids are not separated prematurely, and thus enables the proper progression of events through prophase I of meiosis. We propose that LAB-1 may act at the onset of meiosis in a manner akin to Shugoshin, by recruiting the PP1 phosphatase to counteract Aurora B kinase, thereby ensuring sister chromatid cohesion.

The mammalian synaptonemal complex: protein components, assembly and role in meiotic recombination

The synaptonemal complex (SC) is a proteinaceous structure of chromosome bivalents whose assembly is indispensable for the successful progression of the first meiotic division of sexually reproducing organisms. In this mini-review we will focus on recent progress dealing with the composition and assembly of the mammalian SC. These advances mainly resulted from the systematic use of knockout mice for all known mammalian SC proteins as well as from protein polymerization studies performed in heterologous systems.

MRG-1 is required for genomic integrity in Caenorhabditis elegans germ cells

During meiotic cell division, proper chromosome synapsis and accurate repair of DNA double strand breaks (DSBs) are required to maintain genomic integrity, loss of which leads to apoptosis or meiotic defects. The mechanisms underlying meiotic chromosome synapsis, DSB repair and apoptosis are not fully understood. Here, we report that the chromodomain-containing protein MRG-1 is an important factor for genomic integrity in meiosis in Caenorhabditis elegans. Loss of mrg-1 function resulted in a significant increase in germ cell apoptosis that was partially inhibited by mutations affecting DNA damage checkpoint genes. Consistently, mrg-1 mutant germ lines exhibited SPO-11-generated DSBs and elevated exogenous DNA damage-induced chromosome fragmentation at diakinesis. In addition, the excessive apoptosis in mrg-1 mutants was partially suppressed by loss of the synapsis checkpoint gene pch-2, and a significant number of meiotic nuclei accumulated at the leptotene/zygotene stages with an elevated level of H3K9me2 on the chromatin, which was similarly observed in mutants deficient in the synaptonemal complex, suggesting that the proper progression of chromosome synapsis is likely impaired in the absence of mrg-1. Altogether, these findings suggest that MRG-1 is critical for genomic integrity by promoting meiotic DSB repair and synapsis progression in meiosis.

Polo kinases establish links between meiotic chromosomes and cytoskeletal forces essential for homolog pairing

During meiosis, chromosomes must find and align with their homologous partners. SUN and KASH-domain protein pairs play a conserved role by establishing transient linkages between chromosome ends and cytoskeletal forces across the intact nuclear envelope (NE). In C. elegans, a pairing center (PC) on each chromosome mediates homolog pairing and linkage to the microtubule network. We report that the polo kinases PLK-1 and PLK-2 are targeted to the PC by ZIM/HIM-8-pairing proteins. Loss of plk-2 inhibits chromosome pairing and licenses synapsis between nonhomologous chromosomes, indicating that PLK-2 is required for PC-mediated interhomolog interactions. plk-2 is also required for meiosis-specific phosphorylation of SUN-1 and establishment of dynamic SUN/KASH (SUN-1/ZYG-12) modules that promote homolog pairing. Our results provide key insights into the regulation of homolog pairing and reveal that targeting of polo-like kinases to the NE by meiotic chromosomes establishes the conserved linkages to cytoskeletal forces needed for homology assessment.

Pairing centers recruit a Polo-like kinase to orchestrate meiotic chromosome dynamics in C. elegans

Faithful segregation of homologous chromosomes during meiosis requires pairing, synapsis, and crossing-over. In C. elegans, homolog pairing and synapsis depend on pairing centers (PCs), special regions near one end of each chromosome that interact with the nuclear envelope (NE) and cytoplasmic microtubules. Here, we report that PCs are required for nuclear reorganization at the onset of meiosis. We demonstrate that PCs recruit the Polo-like kinase PLK-2 to induce NE remodeling, chromosome pairing, and synapsis. Recruitment of PLK-2 is also required to mediate a cell cycle delay and selective apoptosis of nuclei containing unsynapsed chromosomes, establishing a molecular link between these two quality control mechanisms. This work reveals unexpected functions for the conserved family of Polo-like kinases, and advances our understanding of how meiotic processes are properly coordinated to ensure transmission of genetic information from parents to progeny.

A new thermosensitive smc-3 allele reveals involvement of cohesin in homologous recombination in C. elegans

The cohesin complex is required for the cohesion of sister chromatids and for correct segregation during mitosis and meiosis. Crossover recombination, together with cohesion, is essential for the disjunction of homologous chromosomes during the first meiotic division. Cohesin has been implicated in facilitating recombinational repair of DNA lesions via the sister chromatid. Here, we made use of a new temperature-sensitive mutation in the Caenorhabditis elegans SMC-3 protein to study the role of cohesin in the repair of DNA double-strand breaks (DSBs) and hence in meiotic crossing over. We report that attenuation of cohesin was associated with extensive SPO-11-dependent chromosome fragmentation, which is representative of unrepaired DSBs. We also found that attenuated cohesin likely increased the number of DSBs and eliminated the need of MRE-11 and RAD-50 for DSB formation in C. elegans, which suggests a role for the MRN complex in making cohesin-loaded chromatin susceptible to meiotic DSBs. Notably, in spite of largely intact sister chromatid cohesion, backup DSB repair via the sister chromatid was mostly impaired. We also found that weakened cohesins affected mitotic repair of DSBs by homologous recombination, whereas NHEJ repair was not affected. Our data suggest that recombinational DNA repair makes higher demands on cohesins than does chromosome segregation.

An extracellular matrix protein prevents cytokinesis failure and aneuploidy in the C. elegans germline

Interactions between extracellular matrix (ECM) proteins and their transmembrane receptors mediate cytoskeletal reorganization and corresponding changes in cell shape during cell migration, adhesion, differentiation and polarization. Cytokinesis is the final step in cell division as cells employ a contractile ring composed of actin and myosin to partition one cell into two. Cells undergo dramatic changes in cell shape during the division process, creating new membrane and forming an extracellular invagination called the cleavage furrow. However, existing models of cytokinesis include no role for the ECM. In a recent paper, we demonstrate that depletion of a large secreted protein, hemicentin, results in membrane destabilization, cleavage furrow retraction and cytokinesis failure in C. elegans germ cells and in pre-implantation mouse embryos. Here, we demonstrate that cytokinesis failure produces tetraploid intermediate cells with multipolar spindles, providing a potential explanation for the large number of aneuploid progeny observed among C. elegans hemicentin mutant hermaphrodites.

A novel mouse synaptonemal complex protein is essential for loading of central element proteins, recombination, and fertility

The synaptonemal complex (SC) is a proteinaceous, meiosis-specific structure that is highly conserved in evolution. During meiosis, the SC mediates synapsis of homologous chromosomes. It is essential for proper recombination and segregation of homologous chromosomes, and therefore for genome haploidization. Mutations in human SC genes can cause infertility. In order to gain a better understanding of the process of SC assembly in a model system that would be relevant for humans, we are investigating meiosis in mice. Here, we report on a newly identified component of the murine SC, which we named SYCE3. SYCE3 is strongly conserved among mammals and localizes to the central element (CE) of the SC. By generating a Syce3 knockout mouse, we found that SYCE3 is required for fertility in both sexes. Loss of SYCE3 blocks synapsis initiation and results in meiotic arrest. In the absence of SYCE3, initiation of meiotic recombination appears to be normal, but its progression is severely impaired resulting in complete absence of MLH1 foci, which are presumed markers of crossovers in wild-type meiocytes. In the process of SC assembly, SYCE3 is required downstream of transverse filament protein SYCP1, but upstream of the other previously described CE–specific proteins. We conclude that SYCE3 enables chromosome loading of the other CE–specific proteins, which in turn would promote synapsis between homologous chromosomes.

Meiosis is a special type of cell division that takes place in the germ line of sexually reproducing diploid organisms. Major events during meiosis are the pairing, recombination, and segregation of homologous chromosomes. As a consequence, daughter cells are haploid and genetically diverse. Therefore, meiosis is of utmost importance for the life of sexually reproducing species as it maintains the species-specific chromosome number and generates genetic diversity within a species. Proper segregation of homologous chromosomes during meiosis requires homolog pairs to be physically linked. The synaptonemal complex (SC), a meiosis-specific structure conserved in evolution, is essential for this process. Defective assembly of the SC has deleterious effects on germ cells and can cause infertility in mice and humans. Here, we report on a newly identified protein component of the mammalian SC that we have named SYCE3. SYCE3 is strongly conserved among mammals. Using the mouse as a model system, we demonstrate that loss of SYCE3 leads to infertility in both sexes. Infertility is caused by disruption of meiosis due to the inability of Syce3^−/− mice to assemble the central element of SCs. Our findings provide new insights into the complexity of SC assembly and its relevance to mammalian fertility.

DNA damage during meiosis induces chromatin remodeling and synaptonemal complex disassembly

DNA damage to the germline genome must be accurately repaired to ensure transmission of intact genetic information to following generations. Meiosis presents challenges to the DNA damage response (DDR) because it universally requires changes to chromosome structure that can affect DNA repair outcomes. We report the existence of a meiotic DDR at chromosome axes that results in chromatin remodeling, synaptonemal complex disassembly, and axis separation in response to irradiation at late pachytene stages in C. elegans. The axis component HTP-3 is required for germline acquisition of H2AacK5, an axis-specific chromatin mark that is DNA damage responsive. Irradiated wild-types show reduction of H2AacK5 and axis separation that are dependent on the acetyltransferase MYS-1/TIP60. Restoration of H2AacK5 levels requires ATM-1 kinase and correlates with resynapsis. We propose that the meiotic DDR involves early chromatin remodeling at chromosome axes to dismantle structures promoting interhomolog recombination and facilitate efficient nonhomolog-based repair before pachytene exit.

Leptotene/zygotene chromosome movement via the SUN/KASH protein bridge in Caenorhabditis elegans

The Caenorhabditis elegans inner nuclear envelope protein matefin/SUN-1 plays a conserved, pivotal role in the process of genome haploidization. CHK-2-dependent phosphorylation of SUN-1 regulates homologous chromosome pairing and interhomolog recombination in Caenorhabditis elegans. Using time-lapse microscopy, we characterized the movement of matefin/SUN-1::GFP aggregates (the equivalent of chromosomal attachment plaques) and showed that the dynamics of matefin/SUN-1 aggregates remained unchanged throughout leptonene/zygotene, despite the progression of pairing. Movement of SUN-1 aggregates correlated with chromatin polarization. We also analyzed the requirements for the formation of movement-competent matefin/SUN-1 aggregates in the context of chromosome structure and found that chromosome axes were required to produce wild-type numbers of attachment plaques. Abrogation of synapsis led to a deceleration of SUN-1 aggregate movement. Analysis of matefin/SUN-1 in a double-strand break deficient mutant revealed that repair intermediates influenced matefin/SUN-1 aggregate dynamics. Investigation of movement in meiotic regulator mutants substantiated that proper orchestration of the meiotic program and effective repair of DNA double-strand breaks were necessary for the wild-type behavior of matefin/SUN-1 aggregates.

Bisphenol A impairs the double-strand break repair machinery in the germline and causes chromosome abnormalities

Bisphenol A (BPA) is a highly prevalent constituent of plastics that has been associated with diabetes, cardiovascular disease, and an increased risk of miscarriages in humans. In mice, BPA exposure disrupts the process of meiosis; however, analysis of the affected molecular pathways is lagging and has been particularly challenging. Here we show that exposure of the nematode Caenorhabditis elegans to BPA, at internal concentrations consistent with mammalian models, causes increased sterility and embryonic lethality. BPA exposure results in impaired chromosome synapsis and disruption of meiotic double-strand break repair (DSBR) progression. BPA carries an anti-estrogenic activity in the germline and results in germline-specific down-regulation of DSBR genes, thereby impairing maintenance of genomic integrity during meiosis. C. elegans therefore constitutes a model of remarkable relevance to mammals with which to assess how our chemical landscape affects germ cells and meiosis.

TGF-β and insulin signaling regulate reproductive aging via oocyte and germline quality maintenance

Reproductive cessation is perhaps the earliest aging phenotype that humans experience. Similarly, reproduction of Caenorhabditis elegans ceases in mid-adulthood. Although somatic aging has been studied in both worms and humans, mechanisms regulating reproductive aging are not yet understood. Here, we show that TGF-β Sma/Mab and Insulin/IGF-1 signaling regulate C. elegans reproductive aging by modulating multiple aspects of the reproductive process, including embryo integrity, oocyte fertilizability, chromosome segregation fidelity, DNA damage resistance, and oocyte and germline morphology. TGF-β activity regulates reproductive span and germline/oocyte quality noncell-autonomously and is temporally and transcriptionally separable from its regulation of growth. Chromosome segregation, cell cycle, and DNA damage response genes are upregulated in TGF-β mutant oocytes, decline in aged mammalian oocytes, and are critical for oocyte quality maintenance. Our data suggest that C. elegans and humans share many aspects of reproductive aging, including the correlation between reproductive aging and declining oocyte quality and mechanisms determining oocyte quality.

A yeast two-hybrid screen for SYP-3 interactors identifies SYP-4, a component required for synaptonemal complex assembly and chiasma formation in Caenorhabditis elegans meiosis

The proper assembly of the synaptonemal complex (SC) between homologs is critical to ensure accurate meiotic chromosome segregation. The SC is a meiotic tripartite structure present from yeast to humans, comprised of proteins assembled along the axes of the chromosomes and central region (CR) proteins that bridge the two chromosome axes. Here we identify SYP-4 as a novel structural component of the SC in Caenorhabditis elegans. SYP-4 interacts in a yeast two-hybrid assay with SYP-3, one of components of the CR of the SC, and is localized at the interface between homologs during meiosis. SYP-4 is essential for the localization of SYP-1, SYP-2, and SYP-3 CR proteins onto chromosomes, thereby playing a crucial role in the stabilization of pairing interactions between homologous chromosomes. In the absence of SYP-4, the levels of recombination intermediates, as indicated by RAD-51 foci, are elevated in mid-prophase nuclei, and crossover recombination events are significantly reduced. The lack of chiasmata observed in syp-4 mutants supports the elevated levels of chromosome nondisjunction manifested in high embryonic lethality. Altogether our findings place SYP-4 as a central player in SC formation and broaden our understanding of the structure of the SC and its assembly.

Meiosis is a two-part cell division program that ensures the formation of haploid gametes (e.g. eggs and sperm), which can then reconstitute a species' ploidy through fertilization. A critical step towards accomplishing this task is the accurate segregation of homologous chromosomes away from each other during meiosis I. This requires the formation of at least one obligatory crossover event (genetic exchange) between each pair of homologous chromosomes. In most organisms, the formation of all crossover events greatly relies on the synaptonemal complex (SC). This “zipper-like” structure holds the pairs of homologous chromosomes together during meiotic prophase I, and crossover recombination is completed in the context of the fully formed SCs. Here, we identify SYP-4 as a novel structural component of the SC in the nematode C. elegans. In its absence, SCs fail to form, resulting in a lack of crossover formation and increased errors in chromosome segregation. SYP-4 interacts in a yeast two-hybrid assay with SYP-3, one of the SC proteins, and its localization onto chromosomes is interdependent with SYP-1, SYP-2, and SYP-3 proteins. SYP-4 therefore plays a critical role during C. elegans meiosis in generating the ultrastructurally conserved SC that is ubiquitously present from yeast to humans.

Ipl1/Aurora B kinase coordinates synaptonemal complex disassembly with cell cycle progression and crossover formation in budding yeast meiosis

Several protein kinases collaborate to orchestrate and integrate cellular and chromosomal events at the G2/M transition in both mitotic and meiotic cells. During the G2/M transition in meiosis, this includes the completion of crossover recombination, spindle formation, and synaptonemal complex (SC) breakdown. We identified Ipl1/Aurora B kinase as the main regulator of SC disassembly. Mutants lacking Ipl1 or its kinase activity assemble SCs with normal timing, but fail to dissociate the central element component Zip1, as well as its binding partner, Smt3/SUMO, from chromosomes in a timely fashion. Moreover, lack of Ipl1 activity causes delayed SC disassembly in a cdc5 as well as a CDC5-inducible ndt80 mutant. Crossover levels in the ipl1 mutant are similar to those observed in wild type, indicating that full SC disassembly is not a prerequisite for joint molecule resolution and subsequent crossover formation. Moreover, expression of meiosis I and meiosis II-specific B-type cyclins occur normally in ipl1 mutants, despite delayed formation of anaphase I spindles. These observations suggest that Ipl1 coordinates changes to meiotic chromosome structure with resolution of crossovers and cell cycle progression at the end of meiotic prophase.

Spermatogenesis-specific features of the meiotic program in Caenorhabditis elegans

In most sexually reproducing organisms, the fundamental process of meiosis is implemented concurrently with two differentiation programs that occur at different rates and generate distinct cell types, sperm and oocytes. However, little is known about how the meiotic program is influenced by such contrasting developmental programs. Here we present a detailed timeline of late meiotic prophase during spermatogenesis in Caenorhabditis elegans using cytological and molecular landmarks to interrelate changes in chromosome dynamics with germ cell cellularization, spindle formation, and cell cycle transitions. This analysis expands our understanding C. elegans spermatogenesis, as it identifies multiple spermatogenesis-specific features of the meiotic program and provides a framework for comparative studies. Post-pachytene chromatin of spermatocytes is distinct from that of oocytes in both composition and morphology. Strikingly, C. elegans spermatogenesis includes a previously undescribed karyosome stage, a common but poorly understood feature of meiosis in many organisms. We find that karyosome formation, in which chromosomes form a constricted mass within an intact nuclear envelope, follows desynapsis, involves a global down-regulation of transcription, and may support the sequential activation of multiple kinases that prepare spermatocytes for meiotic divisions. In spermatocytes, the presence of centrioles alters both the relative timing of meiotic spindle assembly and its ultimate structure. These microtubule differences are accompanied by differences in kinetochores, which connect microtubules to chromosomes. The sperm-specific features of meiosis revealed here illuminate how the underlying molecular machinery required for meiosis is differentially regulated in each sex.

Three-dimensional structured illumination microscopy and its application to chromosome structure

This review discusses the exploration of chromosome structure with a recently developed high-resolution microscopy technique, three-dimensional structured illumination microscopy (3dSIM). 3dSIM surpasses the diffraction limit of conventional widefield optical microscopy, increasing the level of detail in images by a factor of 2, while retaining the sample preparation methods, ease of use and flexibility of conventional microscopy. Special attention will be given to the ways in which imaging beyond the diffraction limit can shed light on the structural organization of meiotic chromosomes.

CRA-1 uncovers a double-strand break-dependent pathway promoting the assembly of central region proteins on chromosome axes during C. elegans meiosis

The synaptonemal complex (SC), a tripartite proteinaceous structure that forms between homologous chromosomes during meiosis, is crucial for faithful chromosome segregation. Here we identify CRA-1, a novel and conserved protein that is required for the assembly of the central region of the SC during C. elegans meiosis. In the absence of CRA-1, central region components fail to extensively localize onto chromosomes at early prophase and instead mostly surround the chromatin at this stage. Later in prophase, central region proteins polymerize along chromosome axes, but for the most part fail to connect the axes of paired homologous chromosomes. This defect results in an inability to stabilize homologous pairing interactions, altered double-strand break (DSB) repair progression, and a lack of chiasmata. Surprisingly, DSB formation and repair are required to promote the polymerization of the central region components along meiotic chromosome axes in cra-1 mutants. In the absence of both CRA-1 and any one of the C. elegans homologs of SPO11, MRE11, RAD51, or MSH5, the polymerization observed along chromosome axes is perturbed, resulting in the formation of aggregates of the SC central region proteins. While radiation-induced DSBs rescue this polymerization in cra-1; spo-11 mutants, they fail to do so in cra-1; mre-11, cra-1; rad-51, and cra-1; msh-5 mutants. Taken together, our studies place CRA-1 as a key component in promoting the assembly of a tripartite SC structure. Moreover, they reveal a scenario in which DSB formation and repair can drive the polymerization of SC components along chromosome axes in C. elegans.

Accurate meiotic chromosome segregation relies on homologous chromosome pairing, synapsis, and recombination. Although formation of the “zipper-like” structure known as the synaptonemal complex (SC) is critical for homologous chromosome synapsis in most sexually reproducing organisms, regulation of SC formation is still poorly understood. Previous studies revealed that whereas SC formation is dependent on the formation of double-strand breaks (DSBs) in yeast, plants, and mammals, it is DSB-independent in flies and nematodes. Here we introduce CRA-1, a novel and conserved protein required for the formation of the SC in the nematode C. elegans. In cra-1 mutants, SC central region components for the most part fail to link homologous chromosome axes and stabilize homologous pairing interactions. As a result, crossover recombination is impaired and there is increased chromosome nondisjunction. Analysis of cra-1 mutants also reveals that DSB formation and repair can promote the assembly of SC proteins along chromosome axes. Therefore, we propose that CRA-1 promotes a productive SC assembly, and demonstrate, in our analysis of cra-1 mutants, an unanticipated interconnection between the recruitment of central region components onto chromosome axes and the recombination pathway in C. elegans.

BRC-1 acts in the inter-sister pathway of meiotic double-strand break repair

The breast and ovarian cancer susceptibility protein BRCA1 is evolutionarily conserved and functions in DNA double-strand break (DSB) repair through homologous recombination, but its role in meiosis is poorly understood. By using genetic analysis, we investigated the role of the Caenorhabditis elegans BRCA1 orthologue (brc-1) during meiotic prophase. The null mutant in the brc-1 gene is viable, fertile and shows the wild-type complement of six bivalents in most diakinetic nuclei, which is indicative of successful crossover recombination. However, brc-1 mutants show an abnormal increase in apoptosis and RAD-51 foci at pachytene that are abolished by loss of spo-11 function, suggesting a defect in meiosis rather than during premeiotic DNA replication. In genetic backgrounds in which chiasma formation is abrogated, such as him-14/MSH4 and syp-2, loss of brc-1 leads to chromosome fragmentation suggesting that brc-1 is dispensable for crossing over but essential for DSB repair through inter-sister recombination.

Prelude to a division

Accurate segregation of chromosomes during meiosis requires physical links between homologs. These links are usually established through chromosome pairing, synapsis, and recombination, which occur during meiotic prophase. How chromosomes pair with their homologous partners is one of the outstanding mysteries of meiosis. Surprisingly, experimental evidence indicates that different organisms have found more than one way to accomplish this feat. Whereas some species depend on recombination machinery to achieve homologous pairing, others are able to pair and synapse their homologs in the absence of recombination. To ensure specific pairing between homologous chromosomes, both recombination-dependent and recombination-independent mechanisms must strike the proper balance between forces that promote chromosome interactions and activities that temper the promiscuity of those interactions. The initiation of synapsis is likely to be a tightly regulated step in a process that must be mechanically coupled to homolog pairing.

Gene and genon concept: coding versus regulation. A conceptual and information-theoretic analysis of genetic storage and expression in the light of modern molecular biology

We analyse here the definition of the gene in order to distinguish, on the basis of modern insight in molecular biology, what the gene is coding for, namely a specific polypeptide, and how its expression is realized and controlled. Before the coding role of the DNA was discovered, a gene was identified with a specific phenotypic trait, from Mendel through Morgan up to Benzer. Subsequently, however, molecular biologists ventured to define a gene at the level of the DNA sequence in terms of coding. As is becoming ever more evident, the relations between information stored at DNA level and functional products are very intricate, and the regulatory aspects are as important and essential as the information coding for products. This approach led, thus, to a conceptual hybrid that confused coding, regulation and functional aspects. In this essay, we develop a definition of the gene that once again starts from the functional aspect. A cellular function can be represented by a polypeptide or an RNA. In the case of the polypeptide, its biochemical identity is determined by the mRNA prior to translation, and that is where we locate the gene. The steps from specific, but possibly separated sequence fragments at DNA level to that final mRNA then can be analysed in terms of regulation. For that purpose, we coin the new term "genon". In that manner, we can clearly separate product and regulative information while keeping the fundamental relation between coding and function without the need to introduce a conceptual hybrid. In mRNA, the program regulating the expression of a gene is superimposed onto and added to the coding sequence in cis - we call it the genon. The complementary external control of a given mRNA by trans-acting factors is incorporated in its transgenon. A consequence of this definition is that, in eukaryotes, the gene is, in most cases, not yet present at DNA level. Rather, it is assembled by RNA processing, including differential splicing, from various pieces, as steered by the genon. It emerges finally as an uninterrupted nucleic acid sequence at mRNA level just prior to translation, in faithful correspondence with the amino acid sequence to be produced as a polypeptide. After translation, the genon has fulfilled its role and expires. The distinction between the protein coding information as materialised in the final polypeptide and the processing information represented by the genon allows us to set up a new information theoretic scheme. The standard sequence information determined by the genetic code expresses the relation between coding sequence and product. Backward analysis asks from which coding region in the DNA a given polypeptide originates. The (more interesting) forward analysis asks in how many polypeptides of how many different types a given DNA segment is expressed. This concerns the control of the expression process for which we have introduced the genon concept. Thus, the information theoretic analysis can capture the complementary aspects of coding and regulation, of gene and genon.

Disruption of topoisomerase II perturbs pairing in drosophila cell culture

Homolog pairing refers to the alignment and physical apposition of homologous chromosomal segments. Although commonly observed during meiosis, homolog pairing also occurs in nonmeiotic cells of several organisms, including humans and Drosophila. The mechanism underlying nonmeiotic pairing, however, remains largely unknown. Here, we explore the use of established Drosophila cell lines for the analysis of pairing in somatic cells. Using fluorescent in situ hybridization (FISH), we assayed pairing at nine regions scattered throughout the genome of Kc167 cells, observing high levels of homolog pairing at all six euchromatic regions assayed and variably lower levels in regions in or near centromeric heterochromatin. We have also observed extensive pairing in six additional cell lines representing different tissues of origin, different ploidies, and two different species, demonstrating homolog pairing in cell culture to be impervious to cell type or culture history. Furthermore, by sorting Kc167 cells into G1, S, and G2 subpopulations, we show that even progression through these stages of the cell cycle does not significantly change pairing levels. Finally, our data indicate that disrupting Drosophila topoisomerase II (Top2) gene function with RNAi and chemical inhibitors perturbs homolog pairing, suggesting Top2 to be a gene important for pairing.

SYP-3 restricts synaptonemal complex assembly to bridge paired chromosome axes during meiosis in Caenorhabditis elegans

Synaptonemal complex (SC) formation must be regulated to occur only between aligned pairs of homologous chromosomes, ultimately ensuring proper chromosome segregation in meiosis. Here we identify SYP-3, a coiled-coil protein that is required for assembly of the central region of the SC and for restricting its loading to occur only in an appropriate context, forming structures that bridge the axes of paired meiotic chromosomes in Caenorhabditis elegans. We find that inappropriate loading of central region proteins interferes with homolog pairing, likely by triggering a premature change in chromosome configuration during early prophase that terminates the search for homologs. As a result, syp-3 mutants lack chiasmata and exhibit increased chromosome mis-segregation. Altogether, our studies lead us to propose that SYP-3 regulates synapsis along chromosomes, contributing to meiotic progression in early prophase.