HTP-3 links DSB formation with homolog pairing and crossing over during C. elegans meiosis

The conserved ATPase PCH-2 controls the number and distribution of crossovers by antagonizing crossover formation in C. elegans

Meiotic crossover recombination is essential for both accurate chromosome segregation and the generation of new haplotypes for natural selection to act upon. While the conserved role of the ATPase, PCH-2, during meiotic prophase has been enigmatic, a universal phenotype that is observed when pch-2 or its orthologs are mutated is a change in the number and distribution of meiotic crossovers. Here, we show that PCH-2 controls the number and distribution of crossovers by antagonizing crossover formation. This antagonism produces different effects at different stages of meiotic prophase: early in meiotic prophase, PCH-2 prevents double strand breaks from becoming crossovers, limiting crossovers at sites of initial DSB formation and homolog interactions. Later in meiotic prophase, PCH-2 winnows the number of crossover-eligible intermediates, contributing to the reinforcement of crossover-eligible intermediates, designation of crossovers and ultimately, crossover assurance. We also demonstrate that PCH-2 accomplishes this regulation through the meiotic HORMAD, HIM-3. Our data strongly support a model in which PCH-2's conserved role is to remodel meiotic HORMADs throughout meiotic prophase to destabilize crossover-eligible precursors, coordinate meiotic recombination with synapsis, and contribute to the progressive implementation of meiotic recombination, guaranteeing crossover control.

Canonical and noncanonical roles of Hop1 are crucial for meiotic prophase in the fungus Sordaria macrospora

We show here that in the fungus Sordaria macrospora, the meiosis-specific HORMA-domain protein Hop1 is not essential for the basic early events of chromosome axis development, recombination initiation, or recombination-mediated homolog coalignment/pairing. In striking contrast, Hop1 plays a critical role at the leptotene/zygotene transition which is defined by transition from pairing to synaptonemal complex (SC) formation. During this transition, Hop1 is required for maintenance of normal axis structure, formation of SC from telomere to telomere, and development of recombination foci. These hop1Δ mutant defects are DSB dependent and require Sme4/Zip1-mediated progression of the interhomolog interaction program, potentially via a pre-SC role. The same phenotype occurs not only in hop1Δ but also in absence of the cohesin Rec8 and in spo76-1, a non-null mutant of cohesin-associated Spo76/Pds5. Thus, Hop1 and cohesins collaborate at this crucial step of meiotic prophase. In addition, analysis of 4 non-null mutants that lack this transition defect reveals that Hop1 also plays important roles in modulation of axis length, homolog-axis juxtaposition, interlock resolution, and spreading of the crossover interference signal. Finally, unexpected variations in crossover density point to the existence of effects that both enhance and limit crossover formation. Links to previously described roles of the protein in other organisms are discussed.

Principles of chromosome organization for meiotic recombination

In meiotic cells, chromosomes are organized as chromatin loop arrays anchored to a protein axis. This organization is essential to regulate meiotic recombination, from DNA double-strand break (DSB) formation to their repair. In mammals, it is unknown how chromatin loops are organized along the genome and how proteins participating in DSB formation are tethered to the chromosome axes. Here, we identify three categories of axis-associated genomic sites: PRDM9 binding sites, where DSBs form; binding sites of the insulator protein CTCF; and H3K4me3-enriched sites. We demonstrate that PRDM9 promotes the recruitment of MEI4 and IHO1, two proteins essential for DSB formation. In turn, IHO1 anchors DSB sites to the axis components HORMAD1 and SYCP3. We discovered that IHO1, HORMAD1, and SYCP3 are associated at the DSB ends during DSB repair. Our results highlight how interactions of proteins with specific genomic elements shape the meiotic chromosome organization for recombination.

Divergence and conservation of the meiotic recombination machinery

Sexually reproducing eukaryotes use recombination between homologous chromosomes to promote chromosome segregation during meiosis. Meiotic recombination is almost universally conserved in its broad strokes, but specific molecular details often differ considerably between taxa, and the proteins that constitute the recombination machinery show substantial sequence variability. The extent of this variation is becoming increasingly clear because of recent increases in genomic resources and advances in protein structure prediction. We discuss the tension between functional conservation and rapid evolutionary change with a focus on the proteins that are required for the formation and repair of meiotic DNA double-strand breaks. We highlight phylogenetic relationships on different time scales and propose that this remarkable evolutionary plasticity is a fundamental property of meiotic recombination that shapes our understanding of molecular mechanisms in reproductive biology.

The dynamic recruitment of LAB proteins senses meiotic chromosome axis differentiation in C. elegans

During meiosis, cohesin and meiosis-specific proteins organize chromatin into an axis-loop architecture, coordinating homologous synapsis, recombination, and ordered chromosome segregation. However, how the meiotic chromosome axis is assembled and differentiated with meiotic progression remains elusive. Here, we explore the dynamic recruitment of two long arms of the bivalent proteins, LAB-1 and LAB-2, in Caenorhabditis elegans. LAB proteins directly interact with the axis core HORMA complexes and weak interactions contribute to their recruitment. LAB proteins phase separate in vitro, and this capacity is promoted by HORMA complexes. During early prophase, synapsis oppositely regulates the axis enrichment of LAB proteins. After the pachytene exit, LAB proteins switch from a reciprocal localization pattern to a colocalization pattern, and the normal dynamic pattern of LAB proteins is altered in meiotic mutants. We propose that LAB recruitment senses axis differentiation, and phase separation of meiotic structures helps subdomain establishment and accurate segregation of the chromosomes.

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.

Building the synaptonemal complex: Molecular interactions between the axis and the central region

The successful delivery of genetic material to gametes requires tightly regulated interactions between the parental chromosomes. Central to this regulation is a conserved chromosomal interface called the synaptonemal complex (SC), which brings the parental chromosomes in close proximity along their length. While many of its components are known, the interfaces that mediate the assembly of the SC remain a mystery. Here, we survey findings from different model systems while focusing on insight gained in the nematode C. elegans. We synthesize our current understanding of the structure, dynamics, and biophysical properties of the SC and propose mechanisms for SC assembly.

The conserved AAA ATPase PCH-2 distributes its regulation of meiotic prophase events through multiple meiotic HORMADs in C. elegans

During meiotic prophase, the essential events of homolog pairing, synapsis, and recombination are coordinated with meiotic progression to promote fidelity and prevent aneuploidy. The conserved AAA+ ATPase PCH-2 coordinates these events to guarantee crossover assurance and accurate chromosome segregation. How PCH-2 accomplishes this coordination is poorly understood. Here, we provide evidence that PCH-2 decelerates pairing, synapsis and recombination in C. elegans by remodeling meiotic HORMADs. We propose that PCH-2 converts the closed versions of these proteins, which drive these meiotic prophase events, to unbuckled conformations, destabilizing interhomolog interactions and delaying meiotic progression. Further, we find that PCH-2 distributes this regulation among three essential meiotic HORMADs in C. elegans: PCH-2 acts through HTP-3 to regulate pairing and synapsis, HIM-3 to promote crossover assurance, and HTP-1 to control meiotic progression. In addition to identifying a molecular mechanism for how PCH-2 regulates interhomolog interactions, our results provide a possible explanation for the expansion of the meiotic HORMAD family as a conserved evolutionary feature of meiosis. Taken together, our work demonstrates that PCH-2's remodeling of meiotic HORMADs has functional consequences for the rate and fidelity of homolog pairing, synapsis, recombination and meiotic progression, ensuring accurate meiotic chromosome segregation.

ZYP1-mediated recruitment of PCH2 to the synaptonemal complex remodels the chromosome axis leading to crossover restriction

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

PARG and BRCA1-BARD1 cooperative function regulates DNA repair pathway choice during gametogenesis

Meiotic chromosome segregation relies on programmed DNA double-strand break induction. These are in turn repaired by homologous recombination, generating physical attachments between the parental chromosomes called crossovers. A subset of breaks yields recombinant outcomes, while crossover-independent mechanisms repair the majority of lesions. The balance between different repair pathways is crucial to ensure genome integrity. We show that Caenorhabditis elegans BRC-1/BRCA1-BRD-1/BARD1 and PARG-1/PARG form a complex in vivo, essential for accurate DNA repair in the germline. Simultaneous depletion of BRC-1 and PARG-1 causes synthetic lethality due to reduced crossover formation and impaired break repair, evidenced by hindered RPA-1 removal and presence of aberrant chromatin bodies in diakinesis nuclei, whose formation depends on spo-11 function. These factors undergo a similar yet independent loading in developing oocytes, consistent with operating in different pathways. Abrogation of KU- or Theta-mediated end joining elicits opposite effects in brc-1; parg-1 doubles, suggesting a profound impact in influencing DNA repair pathway choice by BRC-1-PARG-1. Importantly, lack of PARG-1 catalytic activity suppresses untimely accumulation of RAD-51 foci in brc-1 mutants but is only partially required for fertility. Our data show that BRC-1/BRD-1-PARG-1 joint function is essential for genome integrity in meiotic cells by regulating multiple DNA repair pathways.

ATM/ATR kinases link the synaptonemal complex and DNA double-strand break repair pathway choice

DNA double-strand breaks (DSBs) are deleterious lesions, which must be repaired precisely to maintain genomic stability. During meiosis, programmed DSBs are repaired via homologous recombination (HR) while repair using the nonhomologous end joining (NHEJ) pathway is inhibited, thereby ensuring crossover formation and accurate chromosome segregation.1,2 How DSB repair pathway choice is implemented during meiosis is unknown. In C. elegans, meiotic DSB repair takes place in the context of the fully formed, highly dynamic zipper-like structure present between homologous chromosomes called the synaptonemal complex (SC).3,4,5,6,7,8,9 The SC consists of a pair of lateral elements bridged by a central region composed of the SYP proteins in C. elegans. How the structural components of the SC are regulated to maintain the architectural integrity of the assembled SC around DSB repair sites remained unclear. Here, we show that SYP-4, a central region component of the SC, is phosphorylated at Serine 447 in a manner dependent on DSBs and the ATM/ATR DNA damage response kinases. We show that this SYP-4 phosphorylation is critical for preserving the SC structure following exogenous (γ-IR-induced) DSB formation and for promoting normal DSB repair progression and crossover patterning following SPO-11-dependent and exogenous DSBs. We propose a model in which ATM/ATR-dependent phosphorylation of SYP-4 at the S447 site plays important roles both in maintaining the architectural integrity of the SC following DSB formation and in warding off repair via the NHEJ repair pathway, thereby preventing aneuploidy.

Meiotic dysfunction accelerates somatic aging in Caenorhabditis elegans

An expanding body of evidence, from studies in model organisms to human clinical data, reveals that reproductive health influences organismal aging. However, the impact of germline integrity on somatic aging is poorly understood. Moreover, assessing the causal relationship of such an impact is challenging to address in human and vertebrate models. Here, we demonstrate that disruption of meiosis, a germline restricted process, shortened lifespan, impaired individual aspects of healthspan, and accelerated somatic aging in Caenorhabditis elegans. Young meiotic mutants exhibited transcriptional profiles that showed remarkable overlap with the transcriptomes of old worms and shared similarities with transcriptomes of aging human tissues as well. We found that meiosis dysfunction caused increased expression of functionally relevant longevity determinants whose inactivation enhanced the lifespan of normal animals. Further, meiotic mutants manifested destabilized protein homeostasis and enhanced proteasomal activity partially rescued the associated lifespan defects. Our study demonstrates a role for meiotic integrity in controlling somatic aging and reveals proteostasis control as a potential mechanism through which germline status impacts overall organismal health.

TOP-2 is differentially required for the proper maintenance of the cohesin subunit REC-8 on meiotic chromosomes in Caenorhabditis elegans spermatogenesis and oogenesis

During meiotic prophase I, accurate segregation of homologous chromosomes requires the establishment of chromosomes with a meiosis-specific architecture. The sister chromatid cohesin complex and the enzyme Topoisomerase II (TOP-2) are important components of meiotic chromosome architecture, but the relationship of these proteins in the context of meiotic chromosome segregation is poorly defined. Here, we analyzed the role of TOP-2 in the timely release of the sister chromatid cohesin subunit REC-8 during spermatogenesis and oogenesis of Caenorhabditis elegans. We show that there is a different requirement for TOP-2 in meiosis of spermatogenesis and oogenesis. The loss-of-function mutation top-2(it7) results in premature REC-8 removal in spermatogenesis, but not oogenesis. This correlates with a failure to maintain the HORMA-domain proteins HTP-1 and HTP-2 (HTP-1/2) on chromosome axes at diakinesis and mislocalization of the downstream components that control REC-8 release including Aurora B kinase. In oogenesis, top-2(it7) causes a delay in the localization of Aurora B to oocyte chromosomes but can be rescued through premature activation of the maturation promoting factor via knockdown of the inhibitor kinase WEE-1.3. The delay in Aurora B localization is associated with an increase in the length of diakinesis bivalents and wee-1.3 RNAi mediated rescue of Aurora B localization in top-2(it7) is associated with a decrease in diakinesis bivalent length. Our results imply that the sex-specific effects of TOP-2 on REC-8 release are due to differences in the temporal regulation of meiosis and chromosome structure in late prophase I in spermatogenesis and oogenesis.

Continuous double-strand break induction and their differential processing sustain chiasma formation during Caenorhabditis elegans meiosis

Faithful chromosome segregation into gametes depends on Spo11-induced DNA double-strand breaks (DSBs). These yield single-stranded 3' tails upon resection to promote crossovers (COs). While early Mre11-dependent end resection is the predominant pathway in most organisms, Exo1 or Dna2/BLM can also contribute to the efficient processing of meiotic DSBs. Although its enzymatic activity has been thoroughly dissected, the temporal dynamics underlying Spo11 activity have remained mostly elusive. We show that, in Caenorhabditis elegans, SPO-11-mediated DSB induction takes place throughout early meiotic prophase I until mid-late pachynema. We find that late DSBs are essential for CO formation and are preferentially processed by EXO-1 and DNA-2 in a redundant fashion. Further, EXO-1-DNA-2-mediated resection ensures completion of conservative DSB repair and discourages activation of KU-dependent end joining. Taken together, our data unveil important temporal aspects of DSB induction and identify previously unknown functional implications for EXO-1-DNA-2-mediated resection activity in C. elegans.

PCH-2 and meiotic HORMADs: A module for evolutionary innovation in meiosis?

Sexual reproduction and the specialized cell division it relies upon, meiosis, are biological processes that present an incredible degree of both evolutionary conservation and divergence. One clear example of this paradox is the role of the evolutionarily ancient PCH-2/HORMAD module during meiosis. On one hand, the complex, and sometimes disparate, meiotic defects observed when PCH-2 and/or the meiotic HORMADS are mutated in different model systems have prevented a straightforward characterization of their conserved functions. On the other hand, these functional variations demonstrate the impressive molecular rewiring that accompanies evolution of the meiotic processes these factors are involved in. While the defects observed in pch-2 mutants appear to vary in different systems, in this review, I argue that PCH-2 has a conserved meiotic function: to coordinate meiotic recombination with synapsis to ensure an appropriate number and distribution of crossovers. Further, given the dramatic variation in how the events of recombination and synapsis are themselves regulated in different model systems, the mechanistic differences in PCH-2 and meiotic HORMAD function make biological sense when viewed as species-specific elaborations layered onto this fundamental, conserved role.

Phosphoregulation of DSB-1 mediates control of meiotic double-strand break activity

In the first meiotic cell division, proper segregation of chromosomes in most organisms depends on chiasmata, exchanges of continuity between homologous chromosomes that originate from the repair of programmed double-strand breaks (DSBs) catalyzed by the Spo11 endonuclease. Since DSBs can lead to irreparable damage in germ cells, while chromosomes lacking DSBs also lack chiasmata, the number of DSBs must be carefully regulated to be neither too high nor too low. Here, we show that in Caenorhabditis elegans, meiotic DSB levels are controlled by the phosphoregulation of DSB-1, a homolog of the yeast Spo11 cofactor Rec114, by the opposing activities of PP4^PPH-4.1 phosphatase and ATR^ATL-1 kinase. Increased DSB-1 phosphorylation in pph-4.1 mutants correlates with reduction in DSB formation, while prevention of DSB-1 phosphorylation drastically increases the number of meiotic DSBs both in pph-4.1 mutants and in the wild-type background. C. elegans and its close relatives also possess a diverged paralog of DSB-1, called DSB-2, and loss of dsb-2 is known to reduce DSB formation in oocytes with increasing age. We show that the proportion of the phosphorylated, and thus inactivated, form of DSB-1 increases with age and upon loss of DSB-2, while non-phosphorylatable DSB-1 rescues the age-dependent decrease in DSBs in dsb-2 mutants. These results suggest that DSB-2 evolved in part to compensate for the inactivation of DSB-1 through phosphorylation, to maintain levels of DSBs in older animals. Our work shows that PP4^PPH-4.1, ATR^ATL-1, and DSB-2 act in concert with DSB-1 to promote optimal DSB levels throughout the reproductive lifespan.

Meiotic chromosome organization and its role in recombination and cancer

Chromosomes adopt specific conformations to regulate various cellular processes. A well-documented chromosome configuration is the highly compacted chromosome structure during metaphase. More regional chromatin conformations have also been reported, including topologically associated domains encompassing mega-bases of DNA and local chromatin loops formed by kilo-bases of DNA. In this review, we discuss the changes in chromatin conformation taking place between somatic and meiotic cells, with a special focus on the establishment of a proteinaceous structure, called the chromosome axis, at the beginning of meiosis. The chromosome axis is essential to support key meiotic processes such as chromosome pairing, homologous recombination, and balanced chromosome segregation to transition from a diploid to a haploid stage. We review the role of the chromosome axis in meiotic chromatin organization and provide a detailed description of its protein composition. We also review the conserved and distinct roles between species of axis proteins in meiotic recombination, which is a major factor contributing to the creation of genetic diversity and genome evolution. Finally, we discuss situations where the chromosome axis is deregulated and evaluate the effects on genome integrity and the consequences from protein deregulation in meiocytes exposed to heat stress, and aberrant expression of genes encoding axis proteins in mammalian somatic cells associated with certain types of cancers.

Phosphorylation of HORMA-domain protein HTP-3 at Serine 285 is dispensable for crossover formation

Generation of functional gametes is accomplished through a multilayered and finely orchestrated succession of events during meiotic progression. In the Caenorhabditis elegans germline, the HORMA-domain-containing protein HTP-3 plays pivotal roles for the establishment of chromosome axes and the efficient induction of programmed DNA double-strand breaks, both of which are crucial for crossover formation. Double-strand breaks allow for accurate chromosome segregation during the first meiotic division and therefore are an essential requirement for the production of healthy gametes. Phosphorylation-dependent regulation of HORMAD protein plays important roles in controlling meiotic chromosome behavior. Here, we document a phospho-site in HTP-3 at Serine 285 that is constitutively phosphorylated during meiotic prophase I. pHTP-3S285 localization overlaps with panHTP-3 except in nuclei undergoing physiological apoptosis, in which pHTP-3 is absent. Surprisingly, we observed that phosphorylation of HTP-3 at S285 is independent of the canonical kinases that control meiotic progression in nematodes. During meiosis, the htp-3(S285A) mutant displays accelerated RAD-51 turnover, but no other meiotic abnormalities. Altogether, these data indicate that the Ser285 phosphorylation is independent of canonical meiotic protein kinases and does not regulate HTP-3-dependent meiotic processes. We propose a model wherein phosphorylation of HTP-3 occurs through noncanonical or redundant meiotic kinases and/or is likely redundant with additional phospho-sites for function in vivo.

Functions and Regulation of Meiotic HORMA-Domain Proteins

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

Ubiquitin-Proteasome System-Regulated Protein Degradation in Spermatogenesis

Spermatogenesis is a prolonged and highly ordered physiological process that produces haploid male germ cells through more than 40 steps and experiences dramatic morphological and cellular transformations. The ubiquitin proteasome system (UPS) plays central roles in the precise control of protein homeostasis to ensure the effectiveness of certain protein groups at a given stage and the inactivation of them after this stage. Many UPS components have been demonstrated to regulate the progression of spermatogenesis at different levels. Especially in recent years, novel testis-specific proteasome isoforms have been identified to be essential and unique for spermatogenesis. In this review, we set out to discuss our current knowledge in functions of diverse USP components in mammalian spermatogenesis through: (1) the composition of proteasome isoforms at each stage of spermatogenesis; (2) the specificity of each proteasome isoform and the associated degradation events; (3) the E3 ubiquitin ligases mediating protein ubiquitination in male germ cells; and (4) the deubiquitinases involved in spermatogenesis and male fertility. Exploring the functions of UPS machineries in spermatogenesis provides a global picture of the proteome dynamics during male germ cell production and shed light on the etiology and pathogenesis of human male infertility.

Meiotic chromosome organization and crossover patterns

Meiosis is the foundation of sexual reproduction, and crossover recombination is one hallmark of meiosis. Crossovers establish the physical connections between homolog chromosomes (homologs) for their proper segregation and exchange DNA between homologs to promote genetic diversity in gametes and thus progenies. Aberrant crossover patterns, e.g. absence of the obligatory crossover, are the leading cause of infertility, miscarriage, and congenital disease. Therefore, crossover patterns have to be tightly controlled. During meiosis, loop/axis organized chromosomes provide the structural basis and regulatory machinery for crossover patterning. Accumulating evidence shows that chromosome axis length regulates not only the numbers but also the positions of crossovers. In addition, recent studies suggest that alterations in axis length and the resultant alterations in crossover frequency may contribute to evolutionary adaptation. Here, current advances regarding these issues are reviewed, the possible mechanisms for axis length regulating crossover frequency are discussed, and important issues that need further investigations are suggested.

Release of CHK-2 from PPM-1.D anchorage schedules meiotic entry

Transition from the stem/progenitor cell fate to meiosis is mediated by several redundant posttranscriptional regulatory pathways in Caenorhabditis elegans. Interfering with all three branches causes tumorous germ lines. SCFPROM-1 comprises one branch and mediates a scheduled degradation step at entry into meiosis. prom-1 mutants show defects in the timely initiation of meiotic prophase I events, resulting in high rates of embryonic lethality. Here, we identify the phosphatase PPM-1.D/Wip1 as crucial substrate for PROM-1. We report that PPM-1.D antagonizes CHK-2 kinase, a key regulator for meiotic prophase initiation, including DNA double-strand breaks, chromosome pairing, and synaptonemal complex formation. We propose that PPM-1.D controls the amount of active CHK-2 via both catalytic and noncatalytic activities; notably, noncatalytic regulation seems to be crucial at meiotic entry. PPM-1.D sequesters CHK-2 at the nuclear periphery, and programmed SCFPROM-1-mediated degradation of PPM-1.D liberates the kinase and promotes meiotic entry.

Evolutionary Dynamics and Molecular Mechanisms of HORMA Domain Protein Signaling

Controlled assembly and disassembly of multi-protein complexes is central to cellular signaling. Proteins of the widespread and functionally diverse HORMA family nucleate assembly of signaling complexes by binding short peptide motifs through a distinctive safety-belt mechanism. HORMA proteins are now understood as key signaling proteins across kingdoms, serving as infection sensors in a bacterial immune system and playing central roles in eukaryotic cell cycle, genome stability, sexual reproduction, and cellular homeostasis pathways. Here, we describe how HORMA proteins' unique ability to adopt multiple conformational states underlies their functions in these diverse contexts. We also outline how a dedicated AAA+ ATPase regulator, Pch2/TRIP13, manipulates HORMA proteins' conformational states to activate or inactivate signaling in different cellular contexts. The emergence of Pch2/TRIP13 as a lynchpin for HORMA protein action in multiple genome-maintenance pathways accounts for its frequent misregulation in human cancers and highlights TRIP13 as a novel therapeutic target. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A motor independent requirement for dynein light chain in Caenorhabditis elegans meiotic synapsis

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

SYP-5 regulates meiotic thermotolerance in Caenorhabditis elegans

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

Heterologous synapsis in C. elegans is regulated by meiotic double-strand breaks and crossovers

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

The organization, regulation, and biological functions of the synaptonemal complex

The synaptonemal complex (SC) is a meiosis-specific proteinaceous macromolecular structure that assembles between paired homologous chromosomes during meiosis in various eukaryotes. The SC has a highly conserved ultrastructure and plays critical roles in controlling multiple steps in meiotic recombination and crossover formation, ensuring accurate meiotic chromosome segregation. Recent studies in different organisms, facilitated by advances in super-resolution microscopy, have provided insights into the macromolecular structure of the SC, including the internal organization of the meiotic chromosome axis and SC central region, the regulatory pathways that control SC assembly and dynamics, and the biological functions exerted by the SC and its substructures. This review summarizes recent discoveries about how the SC is organized and regulated that help to explain the biological functions associated with this meiosis-specific structure.

Caenorhabditis elegans DSB-3 reveals conservation and divergence among protein complexes promoting meiotic double-strand breaks

Meiotic recombination plays dual roles in the evolution and stable inheritance of genomes: Recombination promotes genetic diversity by reassorting variants, and it establishes temporary connections between pairs of homologous chromosomes that ensure their future segregation. Meiotic recombination is initiated by generation of double-strand DNA breaks (DSBs) by the conserved topoisomerase-like protein Spo11. Despite strong conservation of Spo11 across eukaryotic kingdoms, auxiliary complexes that interact with Spo11 complexes to promote DSB formation are poorly conserved. Here, we identify DSB-3 as a DSB-promoting protein in the nematode Caenorhabditis elegans Mutants lacking DSB-3 are proficient for homolog pairing and synapsis but fail to form crossovers. Lack of crossovers in dsb-3 mutants reflects a requirement for DSB-3 in meiotic DSB formation. DSB-3 concentrates in meiotic nuclei with timing similar to DSB-1 and DSB-2 (predicted homologs of yeast/mammalian Rec114/REC114), and DSB-1, DSB-2, and DSB-3 are interdependent for this localization. Bioinformatics analysis and interactions among the DSB proteins support the identity of DSB-3 as a homolog of MEI4 in conserved DSB-promoting complexes. This identification is reinforced by colocalization of pairwise combinations of DSB-1, DSB-2, and DSB-3 foci in structured illumination microscopy images of spread nuclei. However, unlike yeast Rec114, DSB-1 can interact directly with SPO-11, and in contrast to mouse REC114 and MEI4, DSB-1, DSB-2, and DSB-3 are not concentrated predominantly at meiotic chromosome axes. We speculate that variations in the meiotic program that have coevolved with distinct reproductive strategies in diverse organisms may contribute to and/or enable diversification of essential components of the meiotic machinery.

Oocyte aging is controlled by mitogen-activated protein kinase signaling

Oogenesis is one of the first processes to fail during aging. In women, most oocytes cannot successfully complete meiotic divisions already during the fourth decade of life. Studies of the nematode Caenorhabditis elegans have uncovered conserved genetic pathways that control lifespan, but our knowledge regarding reproductive aging in worms and humans is limited. Specifically, little is known about germline internal signals that dictate the oogonial biological clock. Here, we report a thorough characterization of the changes in the worm germline during aging. We found that shortly after ovulation halts, germline proliferation declines, while apoptosis continues, leading to a gradual reduction in germ cell numbers. In late aging stages, we observed that meiotic progression is disturbed and crossover designation and DNA double-strand break repair decrease. In addition, we detected a decline in the quality of mature oocytes during aging, as reflected by decreasing size and elongation of interhomolog distance, a phenotype also observed in human oocytes. Many of these altered processes were previously attributed to MAPK signaling variations in young worms. In support of this, we observed changes in activation dynamics of MPK-1 during aging. We therefore tested the hypothesis that MAPK controls oocyte quality in aged worms using both genetic and pharmacological tools. We found that in mutants with high levels of activated MPK-1, oocyte quality deteriorates more rapidly than in wild-type worms, whereas reduction of MPK-1 levels enhances quality. Thus, our data suggest that MAPK signaling controls germline aging and could be used to attenuate the rate of oogenesis quality decline.

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.

Meiotic sister chromatid exchanges are rare in C. elegans

Sexual reproduction shuffles the parental genomes to generate new genetic combinations. To achieve that, the genome is subjected to numerous double-strand breaks, the repair of which involves two crucial decisions: repair pathway and repair template.¹ Use of crossover pathways with the homologous chromosome as template exchanges genetic information and directs chromosome segregation. Crossover repair, however, can compromise the integrity of the repair template and is therefore tightly regulated. The extent to which crossover pathways are used during sister-directed repair is unclear because the identical sister chromatids are difficult to distinguish. Nonetheless, indirect assays have led to the suggestion that inter-sister crossovers, or sister chromatid exchanges (SCEs), are quite common.^2-11 Here we devised a technique to directly score physiological SCEs in the C. elegans germline using selective sister chromatid labeling with the thymidine analog 5-ethynyl-2'-deoxyuridine (EdU). Surprisingly, we find SCEs to be rare in meiosis, accounting for <2% of repair events. SCEs remain rare even when the homologous chromosome is unavailable, indicating that almost all sister-directed repair is channeled into noncrossover pathways. We identify two mechanisms that limit SCEs. First, SCEs are elevated in the absence of the RecQ helicase BLM^HIM-6. Second, the synaptonemal complex-a conserved interface that promotes crossover repair^12,13-promotes SCEs when localized between the sisters. Our data suggest that crossover pathways in C. elegans are only used to generate the single necessary link between the homologous chromosomes. Noncrossover pathways repair almost all other breaks, regardless of the repair template.

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.

Multivalent weak interactions between assembly units drive synaptonemal complex formation

The synaptonemal complex (SC) is an ordered but highly dynamic structure assembled between homologous chromosomes to control interhomologous crossover formation, ensuring accurate meiotic chromosome segregation. However, the mechanisms regulating SC assembly and dynamics remain unclear. Here, we identified two new SC components, SYP-5 and SYP-6, in Caenorhabditis elegans that have distinct expression patterns and form distinct SC assembly units with other SYPs through stable interactions. SYP-5 and SYP-6 exhibit diverse in vivo SC regulatory functions and distinct phase separation properties in cells. Charge-interacting elements (CIEs) are enriched in SC intrinsically disordered regions (IDRs), and IDR deletion or CIE removal confirmed a requirement for these elements in SC regulation. Our data support the theory that multivalent weak interactions between the SC units drive SC formation and that CIEs confer multivalency to the assembly units.

Meiosis interrupted: the genetics of female infertility via meiotic failure

Idiopathic or 'unexplained' infertility represents as many as 30% of infertility cases worldwide. Conception, implantation, and term delivery of developmentally healthy infants require chromosomally normal (euploid) eggs and sperm. The crux of euploid egg production is error-free meiosis. Pathologic genetic variants dysregulate meiotic processes that occur during prophase I, meiotic resumption, chromosome segregation, and in cell cycle regulation. This dysregulation can result in chromosomally abnormal (aneuploid) eggs. In turn, egg aneuploidy leads to a broad range of clinical infertility phenotypes, including primary ovarian insufficiency and early menopause, egg fertilization failure and embryonic developmental arrest, or recurrent pregnancy loss. Therefore, maternal genetic variants are emerging as infertility biomarkers, which could allow informed reproductive decision-making. Here, we select and deeply examine human genetic variants that likely cause dysregulation of critical meiotic processes in 14 female infertility-associated genes: SYCP3, SYCE1, TRIP13, PSMC3IP, DMC1, MCM8, MCM9, STAG3, PATL2, TUBB8, CEP120, AURKB, AURKC, andWEE2. We discuss the function of each gene in meiosis, explore genotype-phenotype relationships, and delineate the frequencies of infertility-associated variants.

Phosphoregulation of HORMA domain protein HIM-3 promotes asymmetric synaptonemal complex disassembly in meiotic prophase in Caenorhabditis elegans

In the two cell divisions of meiosis, diploid genomes are reduced into complementary haploid sets through the discrete, two-step removal of chromosome cohesion, a task carried out in most eukaryotes by protecting cohesion at the centromere until the second division. In eukaryotes without defined centromeres, however, alternative strategies have been innovated. The best-understood of these is found in the nematode Caenorhabditis elegans: after the single off-center crossover divides the chromosome into two segments, or arms, several chromosome-associated proteins or post-translational modifications become specifically partitioned to either the shorter or longer arm, where they promote the correct timing of cohesion loss through as-yet unknown mechanisms. Here, we investigate the meiotic axis HORMA-domain protein HIM-3 and show that it becomes phosphorylated at its C-terminus, within the conserved “closure motif” region bound by the related HORMA-domain proteins HTP-1 and HTP-2. Binding of HTP-2 is abrogated by phosphorylation of the closure motif in in vitro assays, strongly suggesting that in vivo phosphorylation of HIM-3 likely modulates the hierarchical structure of the chromosome axis. Phosphorylation of HIM-3 only occurs on synapsed chromosomes, and similarly to other previously-described phosphorylated proteins of the synaptonemal complex, becomes restricted to the short arm after designation of crossover sites. Regulation of HIM-3 phosphorylation status is required for timely disassembly of synaptonemal complex central elements from the long arm, and is also required for proper timing of HTP-1 and HTP-2 dissociation from the short arm. Phosphorylation of HIM-3 thus plays a role in establishing the identity of short and long arms, thereby contributing to the robustness of the two-step chromosome segregation.

To segregate properly in meiosis, cohesion between replicated chromosomes must remain after the first meiotic cell division, so chromosomes can be held together until they finally separate in the second division. While the majority of organisms use centromeres to protect chromosome cohesion in the first division, the nematode worm C. elegans, which lacks single centromeres, instead protects cohesion only on a segment of the chromosome known as the “long arm”. The long arm (and its complement, the short arm) are known to accumulate specific proteins and protein modifications, but it is not known how the short and long arms are first distinguished, nor how their separate functions are carried out. We report here that the chromosome axis protein HIM-3 and its modification by phosphorylation is important for ensuring the robust establishment of short and long arm functions. We show that phosphorylated HIM-3 partitions to the short arms after crossover recombination sites are designated, and HIM-3 mutants that mimic constitutive phosphorylation delay the normal establishment of the two complementary arm domains. Our findings reveal another layer of regulation to an outstanding mystery in chromosome biology.

DOT-1.1-dependent H3K79 methylation promotes normal meiotic progression and meiotic checkpoint function in C. elegans

Epigenetic modifiers are emerging as important regulators of the genome. However, how they regulate specific processes during meiosis is not well understood. Methylation of H3K79 by the histone methyltransferase Dot1 has been shown to be involved in the maintenance of genomic stability in various organisms. In S. cerevisiae, Dot1 modulates the meiotic checkpoint response triggered by synapsis and/or recombination defects by promoting Hop1-dependent Mek1 activation and Hop1 distribution along unsynapsed meiotic chromosomes, at least in part, by regulating Pch2 localization. However, how this protein regulates meiosis in metazoans is unknown. Here, we describe the effects of H3K79me depletion via analysis of dot-1.1 or zfp-1 mutants during meiosis in Caenorhabditis elegans. We observed decreased fertility and increased embryonic lethality in dot-1.1 mutants suggesting meiotic dysfunction. We show that DOT-1.1 plays a role in the regulation of pairing, synapsis and recombination in the worm. Furthermore, we demonstrate that DOT-1.1 is an important regulator of mechanisms surveilling chromosome synapsis during meiosis. In sum, our results reveal that regulation of H3K79me plays an important role in coordinating events during meiosis in C. elegans.

Poly(ADP-ribose) glycohydrolase coordinates meiotic DNA double-strand break induction and repair independent of its catalytic activity

Poly(ADP-ribosyl)ation is a reversible post-translational modification synthetized by ADP-ribose transferases and removed by poly(ADP-ribose) glycohydrolase (PARG), which plays important roles in DNA damage repair. While well-studied in somatic tissues, much less is known about poly(ADP-ribosyl)ation in the germline, where DNA double-strand breaks are introduced by a regulated program and repaired by crossover recombination to establish a tether between homologous chromosomes. The interaction between the parental chromosomes is facilitated by meiotic specific adaptation of the chromosome axes and cohesins, and reinforced by the synaptonemal complex. Here, we uncover an unexpected role for PARG in coordinating the induction of meiotic DNA breaks and their homologous recombination-mediated repair in Caenorhabditis elegans. PARG-1/PARG interacts with both axial and central elements of the synaptonemal complex, REC-8/Rec8 and the MRN/X complex. PARG-1 shapes the recombination landscape and reinforces the tightly regulated control of crossover numbers without requiring its catalytic activity. We unravel roles in regulating meiosis, beyond its enzymatic activity in poly(ADP-ribose) catabolism.

Poly(ADP-ribose) glycohydrolase (PARG) is involved in different cellular processes including DNA repair. Here the authors reveal a role for PARG in regulating meiotic DNA double strand break induction and repair in Caenorhabditis elegans.

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.

Homologous chromosome pairing in meiosis of higher eukaryotes-still an enigma?

Meiosis is the basis of the generative reproduction of eukaryotes. The crucial first step is homologous chromosome pairing. In higher eukaryotes, micrometer-scale chromosomes, micrometer distances apart, are brought together by nanometer DNA sequences, at least a factor of 1000 size difference. Models of homology search, homologue movement, and pairing at the DNA level in higher eukaryotes are primarily based on studies with yeast where the emphasis is on the induction and repair of DNA double-strand breaks (DSB). For such a model, the very large nuclei of most plants and animals present serious problems. Homology search without DSBs cannot be explained by models based on DSB repair. The movement of homologues to meet each other and make contact at the molecular level is not understood. These problems are discussed and the conclusion is that at present practically nothing is known of meiotic homologue pairing in higher eukaryotes up to the formation of the synaptonemal complex, and that new, necessarily speculative models must be developed. Arguments are given that RNA plays a central role in homology search and a tentative model involving RNA in homology search is presented. A role of actin in homologue movement is proposed. The primary role of DSBs in higher eukaryotes is concluded to not be in paring but in the preparation of Holliday junctions, ultimately leading to chromatid exchange.

Surveillance of cohesin-supported chromosome structure controls meiotic progression

Chromosome movements and programmed DNA double-strand breaks (DSBs) promote homologue pairing and initiate recombination at meiosis onset. Meiotic progression involves checkpoint-controlled termination of these events when all homologue pairs achieve synapsis and form crossover precursors. Exploiting the temporo-spatial organisation of the C. elegans germline and time-resolved methods of protein removal, we show that surveillance of the synaptonemal complex (SC) controls meiotic progression. In nuclei with fully synapsed homologues and crossover precursors, removing different meiosis-specific cohesin complexes, which are individually required for SC stability, or a SC central region component causes functional redeployment of the chromosome movement and DSB machinery, triggering whole-nucleus reorganisation. This apparent reversal of the meiotic programme requires CHK-2 kinase reactivation via signalling from chromosome axes containing HORMA proteins, but occurs in the absence of transcriptional changes. Our results uncover an unexpected plasticity of the meiotic programme and show how chromosome signalling orchestrates nuclear organisation and meiotic progression.

Alternative Synaptonemal Complex Structures: Too Much of a Good Thing?

The synaptonemal complex (SC), a highly conserved structure built between homologous meiotic chromosomes, is required for crossover formation and ensuring proper chromosome segregation. In many organisms, SC components can also form alternative structures, including repeating SC structures that are known as polycomplexes (PCs), and extensively modified SC structures that are maintained late in meiosis. PCs display differences in their ability to localize with lateral element proteins, recombination machinery, and DNA. They can be created by defects in post-translational modification, suggesting that these modifications have roles in preventing alternate SC structures. These SC-like structures provide insight into the rules for building and maintaining the SC by offering an 'in vivo laboratory' for models of SC assembly, structure, and disassembly. Here, we discuss what these structures can tell us about the rules for building the SC and the roles of the SC in meiotic processes.

Quantitative cytogenetics reveals molecular stoichiometry and longitudinal organization of meiotic chromosome axes and loops

During meiosis, chromosomes adopt a specialized organization involving assembly of a cohesin-based axis along their lengths, with DNA loops emanating from this axis. We applied novel, quantitative, and widely applicable cytogenetic strategies to elucidate the molecular bases of this organization using Caenorhabditis elegans. Analyses of wild-type (WT) chromosomes and de novo circular minichromosomes revealed that meiosis-specific HORMA-domain proteins assemble into cohorts in defined numbers and co-organize the axis together with 2 functionally distinct cohesin complexes (REC-8 and COH-3/4) in defined stoichiometry. We further found that REC-8 cohesins, which load during S phase and mediate sister-chromatid cohesion, usually occur as individual complexes, supporting a model wherein sister cohesion is mediated locally by a single cohesin ring. REC-8 complexes are interspersed in an alternating pattern with cohorts of axis-organizing COH-3/4 complexes (averaging 3 per cohort), which are insufficient to confer cohesion but can bind to individual chromatids, suggesting a mechanism to enable formation of asymmetric sister-chromatid loops. Indeed, immunofluorescence/fluorescence in situ hybridization (immuno-FISH) assays demonstrate frequent asymmetry in genomic content between the loops formed on sister chromatids. We discuss how features of chromosome axis/loop architecture inferred from our data can help to explain enigmatic, yet essential, aspects of the meiotic program.

PCH-2 collaborates with CMT-1 to proofread meiotic homolog interactions

The conserved ATPase, PCH-2/TRIP13, is required during both the spindle checkpoint and meiotic prophase. However, its specific role in regulating meiotic homolog pairing, synapsis and recombination has been enigmatic. Here, we report that this enzyme is required to proofread meiotic homolog interactions. We generated a mutant version of PCH-2 in C. elegans that binds ATP but cannot hydrolyze it: pch-2E253Q. In vitro, this mutant can bind a known substrate but is unable to remodel it. This mutation results in some non-homologous synapsis and impaired crossover assurance. Surprisingly, worms with a null mutation in PCH-2's adapter protein, CMT-1, the ortholog of p31comet, localize PCH-2 to meiotic chromosomes, exhibit non-homologous synapsis and lose crossover assurance. The similarity in phenotypes between cmt-1 and pch-2E253Q mutants suggest that PCH-2 can bind its meiotic substrates in the absence of CMT-1, in contrast to its role during the spindle checkpoint, but requires its adapter to hydrolyze ATP and remodel them.

NHJ-1 Is Required for Canonical Nonhomologous End Joining in Caenorhabditis elegans

DNA double-strand breaks (DSBs) are a particularly lethal form of DNA damage that must be repaired to restore genomic integrity. Canonical nonhomologous end joining (NHEJ), is a widely conserved pathway that detects and directly ligates the broken ends to repair the DSB. These events globally require the two proteins that form the Ku ring complex, Ku70 and Ku80, and the terminal ligase LIG4. While the NHEJ pathway in vertebrates is elaborated by more than a dozen factors of varying conservation and is similarly complex in other eukaryotes, the entire known NHEJ toolkit in Caenorhabditis elegans consists only of the core components CKU-70, CKU-80, and LIG-4 Here, we report the discovery of the first accessory NHEJ factor in C. elegans Our analysis of the DNA damage response in young larvae revealed that the canonical wild-type N2 strain consisted of two lines that exhibited a differential phenotypic response to ionizing radiation (IR). Following the mapping of the causative locus to a candidate on chromosome V and clustered regularly interspaced short palindromic repeats-Cas9 mutagenesis, we show that disruption of the nhj-1 sequence induces IR sensitivity in the N2 line that previously exhibited IR resistance. Using genetic and cytological analyses, we demonstrate that nhj-1 functions in the NHEJ pathway to repair DSBs. Double mutants of nhj-1 and lig-4 or cku-80 do not exhibit additive IR sensitivity, and the post-IR somatic and fertility phenotypes of nhj-1 mimic those of the other NHEJ factors. Furthermore, in com-1 mutants that permit repair of meiotic DSBs by NHEJ instead of restricting their repair to the homologous recombination pathway, loss of nhj-1 mimics the consequences of loss of lig-4 Diakinesis-stage nuclei in nhj-1; com-1 and nhj-1; lig-4 mutant germlines exhibit increased numbers of DAPI-staining bodies, consistent with increased chromosome fragmentation in the absence of NHEJ-mediated meiotic DSB repair. Finally, we show that NHJ-1 and LIG-4 localize to somatic nuclei in larvae, but are excluded from the germline progenitor cells, consistent with NHEJ being the dominant DNA repair pathway in the soma. nhj-1 shares no sequence homology with other known eukaryotic NHEJ factors and is taxonomically restricted to the Rhabditid family, underscoring the evolutionary plasticity of even highly conserved pathways.

ASY1 acts as a dosage-dependent antagonist of telomere-led recombination and mediates crossover interference in Arabidopsis

During meiosis, interhomolog recombination produces crossovers and noncrossovers to create genetic diversity. Meiotic recombination frequency varies at multiple scales, with high subtelomeric recombination and suppressed centromeric recombination typical in many eukaryotes. During recombination, sister chromatids are tethered as loops to a polymerized chromosome axis, which, in plants, includes the ASY1 HORMA domain protein and REC8-cohesin complexes. Using chromatin immunoprecipitation, we show an ascending telomere-to-centromere gradient of ASY1 enrichment, which correlates strongly with REC8-cohesin ChIP-seq data. We mapped crossovers genome-wide in the absence of ASY1 and observe that telomere-led recombination becomes dominant. Surprisingly, asy1/+ heterozygotes also remodel crossovers toward subtelomeric regions at the expense of the pericentromeres. Telomeric recombination increases in asy1/+ occur in distal regions where ASY1 and REC8 ChIP enrichment are lowest in wild type. In wild type, the majority of crossovers show interference, meaning that they are more widely spaced along the chromosomes than expected by chance. To measure interference, we analyzed double crossover distances, MLH1 foci, and fluorescent pollen tetrads. Interestingly, while crossover interference is normal in asy1/+, it is undetectable in asy1 mutants, indicating that ASY1 is required to mediate crossover interference. Together, this is consistent with ASY1 antagonizing telomere-led recombination and promoting spaced crossover formation along the chromosomes via interference. These findings provide insight into the role of the meiotic axis in patterning recombination frequency within plant genomes.

Identification of novel synaptonemal complex components in C. elegans

The synaptonemal complex (SC) is a tripartite protein scaffold that forms between homologous chromosomes during meiosis. Although the SC is essential for stable homologue pairing and crossover recombination in diverse eukaryotes, it is unknown how individual components assemble into the highly conserved SC structure. Here we report the biochemical identification of two new SC components, SYP-5 and SYP-6, in Caenorhabditis elegans. SYP-5 and SYP-6 are paralogous to each other and play redundant roles in synapsis, providing an explanation for why these genes have evaded previous genetic screens. Superresolution microscopy reveals that they localize between the chromosome axes and span the width of the SC in a head-to-head manner, similar to the orientation of other known transverse filament proteins. Using genetic redundancy and structure-function analyses to truncate C-terminal tails of SYP-5/6, we provide evidence supporting the role of SC in both limiting and promoting crossover formation.

Crossover Position Drives Chromosome Remodeling for Accurate Meiotic Chromosome Segregation

Interhomolog crossovers (COs) are a prerequisite for achieving accurate chromosome segregation during meiosis [1, 2]. COs are not randomly positioned, occurring at distinct genomic intervals during meiosis in all species examined [3-10]. The role of CO position as a major determinant of accurate chromosome segregation has not been previously directly analyzed in a metazoan. Here, we use spo-11 mutants, which lack endogenous DNA double-strand breaks (DSBs), to induce a single DSB by Mos1 transposon excision at defined chromosomal locations in the C. elegans germline and show that the position of the resulting CO directly affects the formation of distinct chromosome subdomains during meiotic chromosome remodeling. CO formation in the typically CO-deprived center region of autosomes leads to premature loss of sister chromatid cohesion and chromosome missegregation, whereas COs at an off-centered position, as in wild type, can result in normal remodeling and accurate segregation. Ionizing radiation (IR)-induced DSBs lead to the same outcomes, and modeling of IR dose-response reveals that the CO-unfavorable center region encompasses up to 6% of the total chromosome length. DSBs proximal to telomeres rarely form COs, likely because of formation of unstable recombination intermediates that cannot be sustained as chiasmata until late prophase. Our work supports a model in which regulation of CO position early in meiotic prophase is required for proper designation of chromosome subdomains and normal chromosome remodeling in late meiotic prophase I, resulting in accurate chromosome segregation and providing a mechanism to prevent aneuploid gamete formation.

Sycp2 is essential for synaptonemal complex assembly, early meiotic recombination and homologous pairing in zebrafish spermatocytes

Meiotic recombination is essential for faithful segregation of homologous chromosomes during gametogenesis. The progression of recombination is associated with dynamic changes in meiotic chromatin structures. However, whether Sycp2, a key structural component of meiotic chromatin, is required for the initiation of meiotic recombination is still unclear in vertebrates. Here, we describe that Sycp2 is required for assembly of the synaptonemal complex and early meiotic events in zebrafish spermatocytes. Our genetic screening by N-ethyl-N-nitrosourea mutagenesis revealed that ietsugu (its), a mutant zebrafish line with an aberrant splice site in the sycp2 gene, showed a defect during meiotic prophase I. The its mutation appeared to be a hypomorphic mutation compared to sycp2 knockout mutations generated by TALEN mutagenesis. Taking advantage of these sycp2 hypomorphic and knockout mutant lines, we demonstrated that Sycp2 is required for the assembly of the synaptonemal complex that is initiated in the vicinity of telomeres in wild-type zebrafish spermatocytes. Accordingly, homologous pairing, the foci of the meiotic recombinases Dmc1/Rad51 and RPA, and γH2AX signals were largely diminished in sycp2 knockout spermatocytes. Taken together, our data indicate that Sycp2 plays a critical role in not only the assembly of the synaptonemal complex, but also early meiotic recombination and homologous pairing, in vertebrates.

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

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

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

Synaptonemal Complex Central Region Proteins Promote Localization of Pro-crossover Factors to Recombination Events During Caenorhabditis elegans Meiosis

Crossovers (COs) between homologous chromosomes are critical for meiotic chromosome segregation and form in the context of the synaptonemal complex (SC), a meiosis-specific structure that assembles between aligned homologs. During Caenorhabditis elegans meiosis, central region components of the SC (SYP proteins) are essential to repair double-strand DNA breaks (DSBs) as COs. Here, we investigate the relationships between the SYP proteins and conserved pro-CO factors by examining the immunolocalization of these proteins in meiotic mutants where SYP proteins are absent, reduced, or mislocalized. Although COs do not form in syp null mutants, pro-CO factors COSA-1, MSH-5, and ZHP-3 nevertheless colocalize at DSB-dependent sites during late prophase, reflecting an inherent affinity of these factors for DSB repair sites. In contrast, in mutants where SYP proteins are present but form aggregates or display abnormal synapsis, pro-CO factors consistently track with SYP-1 localization. Further, pro-CO factors usually localize to a single site per SYP-1 structure, even in SYP aggregates or in mutants where the SC forms between sister chromatids, suggesting that CO regulation occurs within these aberrant SC structures. Moreover, we find that the meiotic cohesin REC-8 is required to ensure that SC formation occurs between homologs and not sister chromatids. Taken together, our findings support a model in which SYP proteins promote CO formation by promoting the localization of pro-CO factors to recombination events within an SC compartment, thereby ensuring that pro-CO factors identify a recombination event within an SC structure and that CO maturation occurs only between properly aligned homologous chromosomes.