Another way to move chromosomes

Heterochromatin in plant meiosis

Heterochromatin is an organizational property of eukaryotic chromosomes, characterized by extensive DNA and histone modifications, that is associated with the silencing of transposable elements and repetitive sequences. Maintaining heterochromatin is crucial for ensuring genomic integrity and stability during the cell cycle. During meiosis, heterochromatin is important for homologous chromosome synapsis, recombination, and segregation, but our understanding of meiotic heterochromatin formation and condensation is limited. In this review, we focus on the dynamics and features of heterochromatin and how it condenses during meiosis in plants. We also discuss how meiotic heterochromatin influences the interaction and recombination of homologous chromosomes during prophase I.

Non-random spatial organization of telomeres varies during the cell cycle and requires LAP2 and BAF

Spatial genome organization within the nucleus influences major biological processes and is impacted by the configuration of linear chromosomes. Here, we applied 3D spatial statistics and modeling on high-resolution telomere and centromere 3D-structured illumination microscopy images in cancer cells. We found a multi-scale organization of telomeres that dynamically evolved from a mixed clustered-and-regular distribution in early G1 to a purely regular distribution as cells progressed through the cell cycle. In parallel, our analysis revealed two pools of peripheral and internal telomeres, the proportions of which were inverted during the cell cycle. We then conducted a targeted screen using MadID to identify the molecular pathways driving or maintaining telomere anchoring to the nuclear envelope observed in early G1. Lamina-associated polypeptide (LAP) proteins were found transiently localized to telomeres in anaphase, a stage where LAP2α initiates the reformation of the nuclear envelope, and impacted telomere redistribution in the next interphase together with their partner barrier-to-autointegration factor (BAF).

The close relationship between oocyte aging and telomere shortening, and possible interventions for telomere protection

As women delay childbearing due to socioeconomic reasons, understanding molecular mechanisms decreasing oocyte quantity and quality during ovarian aging becomes increasingly important. The ovary undergoes biological aging at a higher pace when compared to other organs. As is known, telomeres play crucial roles in maintaining genomic integrity, and their shortening owing to increased reactive oxygen species, consecutive cellular divisions, genetic and epigenetic alterations is associated with loss of developmental competence of oocytes. Novel interventions such as antioxidant treatments and regulation of gene expression are being investigated to prevent or rescue telomere attrition and thereby oocyte aging. Herein, potential factors and molecular mechanisms causing telomere shortening in aging oocytes were comprehensively reviewed. For the purpose of extending reproductive lifespan, possible therapeutic interventions to protect telomere length were also discussed.

Force transmission and SUN-KASH higher-order assembly in the LINC complex models

The linkers of the nucleoskeleton and cytoskeleton (LINC) complex comprises Sad-1 and UNC-84 (SUN) and Klarsicht, ANC-1, SYNE homology (KASH) domain proteins, whose conserved interactions provide a physical coupling between the cytoskeleton and the nucleoskeleton, thereby mediating the transfer of physical forces across the nuclear envelope. The LINC complex can perform distinct cellular functions by pairing various KASH domain proteins with the same SUN domain protein. Recent studies have suggested a higher-order assembly of SUN and KASH instead of a more widely accepted linear trimer model for the LINC complex. In the present study, we use molecular dynamics simulations to investigate the mechanism of force transfer across the two proposed models of LINC complex assembly, namely the 3:3 linear trimer model and the 6:6 higher-order model. Employing steered molecular dynamics simulations with various structures using forces at different rates and directions, we examine the structural stability of the two models under various biologically relevant conditions. Our results suggest that both models can withstand and transfer significant levels of force while retaining their structural integrity. However, the force response of various SUN/KASH assemblies depend on the force direction and pulling rates. Slower pulling rates result in higher mean square fluctuations of the 3:3 assembly compared to the fast pulling. Interestingly, the 6:6 assembly tends to provide an additional range of motion flexibility and might be more advantageous to the structural rigidity and pliability of the nuclear envelope. These findings offer insights into how the SUN and KASH proteins maintain the structural integrity of the nuclear membrane.

Reproductive aging and telomeres: Are women and men equally affected?

Successful reproduction is very important for individuals and for society. Currently, the human health span and lifespan are the object of intense and productive investigation with great achievements, compared to the last century. However, reproduction span does not progress concomitantly with lifespan. Reproductive organs age, decreasing the levels of sexual hormones, which are protectors of health through their action on several organs of the body. Thus, this is the starting point of the organismal decay and infertility. This starting point is easily detected in women. In men, it goes under the surface, undetected, but it goes, nevertheless. Regarding fertility, aging alters the hormonal equilibrium, decreases the potential of reproductive organs, diminishes the quality of the gametes and worsen the reproductive outcomes. All these events happen at a different pace and affecting different organs in women and men. The question is what molecular pathways are involved in reproductive aging and if there is a possible halting or even reversion of the aging events. Answers to all these points will be explained in the present review.

Tell the Difference Between Mitosis and Meiosis: Interplay Between Chromosomes, Cytoskeleton, and Cell Cycle Regulation

Meiosis is a specialized style of cell division conserved in eukaryotes, particularly designed for the production of gametes. A huge number of studies to date have demonstrated how chromosomes behave and how meiotic events are controlled. Yeast substantially contributed to the understanding of the molecular mechanisms of meiosis in the past decades. Recently, evidence began to accumulate to draw a perspective landscape showing that chromosomes and microtubules are mutually influenced: microtubules regulate chromosomes, whereas chromosomes also regulate microtubule behaviors. Here we focus on lessons from recent advancement in genetical and cytological studies of the fission yeast Schizosaccharomyces pombe, revealing how chromosomes, cytoskeleton, and cell cycle progression are organized and particularly how these are differentiated in mitosis and meiosis. These studies illuminate that meiosis is strategically designed to fulfill two missions: faithful segregation of genetic materials and production of genetic diversity in descendants through elaboration by meiosis-specific factors in collaboration with general factors.

Nuclear Envelope Proteins Modulating the Heterochromatin Formation and Functions in Fission Yeast

The nuclear envelope (NE) consists of the inner and outer nuclear membranes (INM and ONM), and the nuclear pore complex (NPC), which penetrates the double membrane. ONM continues with the endoplasmic reticulum (ER). INM and NPC can interact with chromatin to regulate the genetic activities of the chromosome. Studies in the fission yeast Schizosaccharomyces pombe have contributed to understanding the molecular mechanisms underlying heterochromatin formation by the RNAi-mediated and histone deacetylase machineries. Recent studies have demonstrated that NE proteins modulate heterochromatin formation and functions through interactions with heterochromatic regions, including the pericentromeric and the sub-telomeric regions. In this review, we first introduce the molecular mechanisms underlying the heterochromatin formation and functions in fission yeast, and then summarize the NE proteins that play a role in anchoring heterochromatic regions and in modulating heterochromatin formation and functions, highlighting roles for a conserved INM protein, Lem2.

Chromosome-nuclear envelope tethering - a process that orchestrates homologue pairing during plant meiosis?

During prophase I of meiosis, homologous chromosomes pair, synapse and exchange their genetic material through reciprocal homologous recombination, a phenomenon essential for faithful chromosome segregation. Partial sequence identity between non-homologous and heterologous chromosomes can also lead to recombination (ectopic recombination), a highly deleterious process that rapidly compromises genome integrity. To avoid ectopic exchange, homology recognition must be extended from the narrow position of a crossover-competent double-strand break to the entire chromosome. Here, we review advances on chromosome behaviour during meiotic prophase I in higher plants, by integrating centromere- and telomere dynamics driven by cytoskeletal motor proteins, into the processes of homologue pairing, synapsis and recombination. Centromere-centromere associations and the gathering of telomeres at the onset of meiosis at opposite nuclear poles create a spatially organised and restricted nuclear state in which homologous DNA interactions are favoured but ectopic interactions also occur. The release and dispersion of centromeres from the nuclear periphery increases the motility of chromosome arms, allowing meiosis-specific movements that disrupt ectopic interactions. Subsequent expansion of interstitial synapsis from numerous homologous interactions further corrects ectopic interactions. Movement and organisation of chromosomes, thus, evolved to facilitate the pairing process, and can be modulated by distinct stages of chromatin associations at the nuclear envelope and their collective release.

Phase separation drives pairing of homologous chromosomes

Pairing of homologous chromosomes is crucial for ensuring accurate segregation of chromosomes during meiosis. Molecular mechanisms of homologous chromosome pairing in meiosis have been extensively studied in the fission yeast Schizosaccharomyces pombe. In this organism, meiosis-specific noncoding RNA transcribed from specific genes accumulates at the respective gene loci, and chromosome-associated RNA-protein complexes mediate meiotic pairing of homologous loci through phase separation. Pairing of homologous chromosomes also occurs in somatic diploid cells in certain situations. For example, somatic pairing of homologous chromosomes occurs during the early embryogenesis in diptera, and relies on the transcription-associated chromatin architecture. Earlier models also suggest that transcription factories along the chromosome mediate pairing of homologous chromosomes in plants. These studies suggest that RNA bodies formed on chromosomes mediate the pairing of homologous chromosomes. This review summarizes lessons from S. pombe to provide general insights into mechanisms of homologous chromosome pairing mediated by phase separation of chromosome-associated RNA-protein complexes.

Improved Methods for Preparing the Telomere Tethering Complex Bqt1-Bqt2 for Structural Studies

In eukaryotes, chromosome ends (telomeres) are tethered to the inner nuclear membrane. During the early stages of meiosis, telomeres move along the nuclear membrane and gather near the spindle-pole body, resulting in a bouquet-like arrangement of chromosomes. This chromosomal configuration appears to be widely conserved among eukaryotes, and is assumed to play an important role in the normal progression of meiosis, by mediating the proper pairing of homologous chromosomes. In fission yeast, the Bqt1-Bqt2 protein complex plays a key role in tethering the telomere to the inner nuclear membrane. However, the structural details of the complex required to clarify how telomeres are gathered near the spindle-pole body remain enigmatic. Previously, we devised a preparation procedure for the Schizosaccharomyces japonicus Bqt1-Bqt2 complex, in which a SUMO tag was fused to the N-terminus of the Bqt1 protein. This allowed us to purify the Bqt1-Bqt2 complex from the soluble fraction. In the present study, we found that a maltose-binding protein homolog, Athe_0614, served as a better fusion partner than the SUMO protein, resulting in the marked increase in the solubility of the Bqt1-Bqt2 complex. The Athe_0614 fusion partner may open up new avenues for X-ray crystallographic analyses of the structure of the Bqt1-Bqt2 complex.

The nuclear envelope in higher plant mitosis and meiosis

Mitosis and meiosis in higher plants involve significant reconfiguration of the nuclear envelope and the proteins that interact with it. The dynamic series of events involves a range of interactions, movement, breakdown, and reformation of this complex system. Recently, progress has been made in identifying and characterizing the protein and membrane interactome that performs these complex tasks, including constituents of the nuclear envelope, the cytoskeleton, nucleoskeleton, and chromatin. This review will present the current understanding of these interactions and advances in knowledge of the processes for the breakdown and reformation of the nuclear envelope during cell divisions in plants.

Nuclear Mechanics in the Fission Yeast

In eukaryotic cells, the organization of the genome within the nucleus requires the nuclear envelope (NE) and its associated proteins. The nucleus is subjected to mechanical forces produced by the cytoskeleton. The physical properties of the NE and the linkage of chromatin in compacted conformation at sites of cytoskeleton contacts seem to be key for withstanding nuclear mechanical stress. Mechanical perturbations of the nucleus normally occur during nuclear positioning and migration. In addition, cell contraction or expansion occurring for instance during cell migration or upon changes in osmotic conditions also result innuclear mechanical stress. Recent studies in Schizosaccharomyces pombe (fission yeast) have revealed unexpected functions of cytoplasmic microtubules in nuclear architecture and chromosome behavior, and have pointed to NE-chromatin tethers as protective elements during nuclear mechanics. Here, we review and discuss how fission yeast cells can be used to understand principles underlying the dynamic interplay between genome organization and function and the effect of forces applied to the nucleus by the microtubule cytoskeleton.

Meiosis-specific cohesin component, Rec8, promotes the localization of Mps3 SUN domain protein on the nuclear envelope

Proteins in the nuclear envelope (NE) play a role in the dynamics and functions of the nucleus and of chromosomes during mitosis and meiosis. Mps3, a yeast NE protein with a conserved SUN domain, predominantly localizes on a yeast centrosome equivalent, spindle pole body (SPB), in mitotic cells. During meiosis, Mps3, together with SPB, forms a distinct multiple ensemble on NE. How meiosis-specific NE localization of Mps3 is regulated remains largely unknown. In this study, we found that a meiosis-specific component of the protein complex essential for sister chromatid cohesion, Rec8, binds to Mps3 during meiosis and controls Mps3 localization and proper dynamics on NE. Ectopic expression of Rec8 in mitotic yeast cells induced the formation of Mps3 patches/foci on NE. This required the cohesin regulator, WAPL ortholog, Rad61/Wpl1, suggesting that a meiosis-specific cohesin complex with Rec8 controls NE localization of Mps3. We also observed that two domains of the nucleoplasmic region of Mps3 are essential for NE localization of Mps3 in mitotic as well as meiotic cells. We speculate that the interaction of Mps3 with the meiosis-specific cohesin in the nucleoplasm is a key determinant for NE localization/function of Mps3.

Lem2 is retained at the nuclear envelope through its interaction with Bqt4 in fission yeast

Inner nuclear membrane (INM) proteins are thought to play important roles in modulating nuclear organization and function through their interactions with chromatin. However, these INM proteins share redundant functions in metazoans that pose difficulties for functional studies. The fission yeast Schizosaccharomyces pombe exhibits a relatively small number of INM proteins, and molecular genetic tools are available to separate their redundant functions. In S. pombe, it has been reported that among potentially redundant INM proteins, Lem2 displays a unique genetic interaction with another INM protein, Bqt4, which is involved in anchoring telomeres to the nuclear envelope. Double mutations in the lem2 and bqt4 genes confer synthetic lethality during vegetative growth. Here, we show that Lem2 is retained at the nuclear envelope through its interaction with Bqt4, as the loss of Bqt4 results in the exclusive accumulation of Lem2 to the spindle pole body (SPB). An N-terminal nucleoplasmic region of Lem2 bears affinity to both Bqt4 and the SPB in a competitive manner. In contrast, the synthetic lethality of the lem2 bqt4 double mutant is suppressed by the C-terminal region of Lem2. These results indicate that the N-terminal and C-terminal domains of Lem2 show independent functions with respect to Bqt4.

Spatial organization of the Schizosaccharomyces pombe genome within the nucleus

The fission yeast Schizosaccharomyces pombe is a useful experimental system for studying the organization of chromosomes within the cell nucleus. S. pombe has a small genome that is organized into three chromosomes. The small size of the genome and the small number of chromosomes are advantageous for cytological and genome-wide studies of chromosomes; however, the small size of the nucleus impedes microscopic observations owing to limits in spatial resolution during imaging. Recent advances in microscopy, such as super-resolution microscopy, have greatly expanded the use of S. pombe as a model organism in a wide range of studies. In addition, biochemical studies, such as chromatin immunoprecipitation and chromosome conformation capture, have provided complementary approaches. Here, we review the spatial organization of the S. pombe genome as determined by a combination of cytological and biochemical studies. Copyright © 2016 John Wiley & Sons, Ltd.

Inner nuclear membrane protein Lem2 augments heterochromatin formation in response to nutritional conditions

Inner nuclear membrane proteins interact with chromosomes in the nucleus and are important for chromosome activity. Lem2 and Man1 are conserved members of the LEM-domain nuclear membrane protein family. Mutations of LEM-domain proteins are associated with laminopathy, but their cellular functions remain unclear. Here, we report that Lem2 maintains genome stability in the fission yeast Schizosaccharomyces pombe. S. pombe cells disrupted for the lem2(+) gene (lem2∆) showed slow growth and increased rate of the minichromosome loss. These phenotypes were prominent in the rich culture medium, but not in the minimum medium. Centromeric heterochromatin formation was augmented upon transfer to the rich medium in wild-type cells. This augmentation of heterochromatin formation was impaired in lem2∆ cells. Notably, lem2∆ cells occasionally exhibited spontaneous duplication of genome sequences flanked by the long-terminal repeats of retrotransposons. The resulting duplication of the lnp1(+) gene, which encodes an endoplasmic reticulum membrane protein, suppressed lem2∆ phenotypes, whereas the lem2∆ lnp1∆ double mutant showed a severe growth defect. A combination of mutations in Lem2 and Bqt4, which encodes a nuclear membrane protein that anchors telomeres to the nuclear membrane, caused synthetic lethality. These genetic interactions imply that Lem2 cooperates with the nuclear membrane protein network to regulate genome stability.

Meiotic cohesin-based chromosome structure is essential for homologous chromosome pairing in Schizosaccharomyces pombe

Chromosome structure is dramatically altered upon entering meiosis to establish chromosomal architectures necessary for the successful progression of meiosis-specific events. An early meiotic event involves the replacement of the non-SMC mitotic cohesins with their meiotic equivalents in most part of the chromosome, forming an axis on meiotic chromosomes. We previously demonstrated that the meiotic cohesin complex is required for chromosome compaction during meiotic prophase in the fission yeast Schizosaccharomyces pombe. These studies revealed that chromosomes are elongated in the absence of the meiotic cohesin subunit Rec8 and shortened in the absence of the cohesin-associated protein Pds5. In this study, using super-resolution structured illumination microscopy, we found that Rec8 forms a linear axis on chromosomes, which is required for the organized axial structure of chromatin during meiotic prophase. In the absence of Pds5, the Rec8 axis is shortened whereas chromosomes are widened. In rec8 or pds5 mutants, the frequency of homologous chromosome pairing is reduced. Thus, Rec8 and Pds5 play an essential role in building a platform to support the chromosome architecture necessary for the spatial alignment of homologous chromosomes.

Mechanism and regulation of rapid telomere prophase movements in mouse meiotic chromosomes

Telomere-led rapid prophase movements (RPMs) in meiotic prophase have been observed in diverse eukaryote species. A shared feature of RPMs is that the force that drives the chromosomal movements is transmitted from the cytoskeleton, through the nuclear envelope, to the telomeres. Studies in mice suggested that dynein movement along microtubules is transmitted to telomeres through SUN1/KASH5 nuclear envelope bridges to generate RPMs. We monitored RPMs in mouse seminiferous tubules using 4D fluorescence imaging and quantitative motion analysis to characterize patterns of movement in the RPM process. We find that RPMs reflect a combination of nuclear rotation and individual chromosome movements. The telomeres move along microtubule tracks that are apparently continuous with the cytoskeletal network and exhibit characteristic arrangements at different stages of prophase. Quantitative measurements confirmed that SUN1/KASH5, microtubules, and dynein, but not actin, were necessary for RPMs and that defects in meiotic recombination and synapsis resulted in altered RPMs.

Absence of SUN1 and SUN2 proteins in Arabidopsis thaliana leads to a delay in meiotic progression and defects in synapsis and recombination

The movement of chromosomes during meiosis involves location of their telomeres at the inner surface of the nuclear envelope. Sad1/UNC-84 (SUN) domain proteins are inner nuclear envelope proteins that are part of complexes linking cytoskeletal elements with the nucleoskeleton, connecting telomeres to the force-generating mechanism in the cytoplasm. These proteins play a conserved role in chromosome dynamics in eukaryotes. Homologues of SUN domain proteins have been identified in several plant species. In Arabidopsis thaliana, two proteins that interact with each other, named AtSUN1 and AtSUN2, have been identified. Immunolocalization using antibodies against AtSUN1 and AtSUN2 proteins revealed that they were associated with the nuclear envelope during meiotic prophase I. Analysis of the double mutant Atsun1-1 Atsun2-2 has revealed severe meiotic defects, namely a delay in the progression of meiosis, absence of full synapsis, the presence of unresolved interlock-like structures, and a reduction in the mean cell chiasma frequency. We propose that in Arabidopsis thaliana, overlapping functions of SUN1 and SUN2 ensure normal meiotic recombination and synapsis.

Meiotic nuclear movements in fission yeast are regulated by the transcription factor Mei4 downstream of a Cds1-dependent replication checkpoint pathway

In meiosis, the fission yeast nucleus displays an elongated morphology, moving back and forth within the cell; these nuclear movements continue for approximately 2 h before meiotic nuclear divisions. Meiotic DNA replication occurs in an early phase of the nuclear movements and is followed by meiotic prophase. Here we report that in mutants deficient in meiotic DNA replication, the duration of nuclear movements is strikingly prolonged to four to 5 h. We found that this prolongation was caused by the Cds1-dependent replication checkpoint, which represses expression of the mei4⁺ gene encoding a meiosis-specific transcription factor. In the absence of Mei4, nuclear movements persisted for more than 8 h. In contrast, overproduction of Mei4 accelerated termination of nuclear movements to approximately 30 min. These results show that Mei4 is involved in the termination of nuclear movements and that Mei4-mediated regulatory pathways link a DNA replication checkpoint to the termination of nuclear movements.

The kinesin AtPSS1 promotes synapsis and is required for proper crossover distribution in meiosis

Meiotic crossovers (COs) shape genetic diversity by mixing homologous chromosomes at each generation. CO distribution is a highly regulated process. CO assurance forces the occurrence of at least one obligatory CO per chromosome pair, CO homeostasis smoothes out the number of COs when faced with variation in precursor number and CO interference keeps multiple COs away from each other along a chromosome. In several organisms, it has been shown that cytoskeleton forces are transduced to the meiotic nucleus via KASH- and SUN-domain proteins, to promote chromosome synapsis and recombination. Here we show that the Arabidopsis kinesin AtPSS1 plays a major role in chromosome synapsis and regulation of CO distribution. In Atpss1 meiotic cells, chromosome axes and DNA double strand breaks (DSBs) appear to form normally but only a variable portion of the genome synapses and is competent for CO formation. Some chromosomes fail to form the obligatory CO, while there is an increased CO density in competent regions. However, the total number of COs per cell is unaffected. We further show that the kinesin motor domain of AtPSS1 is required for its meiotic function, and that AtPSS1 interacts directly with WIP1 and WIP2, two KASH-domain proteins. Finally, meiocytes missing AtPSS1 and/or SUN proteins show similar meiotic defects suggesting that AtPSS1 and SUNs act in the same pathway. This suggests that forces produced by the AtPSS1 kinesin and transduced by WIPs/SUNs, are required to authorize complete synapsis and regulate maturation of recombination intermediates into COs. We suggest that a form of homeostasis applies, which maintains the total number of COs per cell even if only a part of the genome is competent for CO formation.

In species that reproduce sexually, diploid individuals have two copies of each chromosome, inherited from their father and mother. During a special cell division called meiosis, these two sets of chromosomes are mixed by homologous recombination to give genetically unique chromosomes that will be transmitted to the next generation. Homologous recombination processes are highly controlled in terms of number and localization of events within and among chromosomes. Disruption of this control (a lack of or improper positioning of homologous recombination events) causes deleterious chromosome associations in the offspring. Using the model plant Arabidopsis thaliana we reveal here that the AtPSS1 gene is required for proper localization of these homologous recombination events along the genome. We also show that AtPSS1, which belongs to a family of proteins able to move along the cytoskeleton, is likely part of a module that allows cytoplasmic forces to be transmitted through the nucleus envelope to promote chromosome movements during homologous recombination progression.

Dynein, microtubule and cargo: a ménage à trois

To exert forces, motor proteins bind with one end to cytoskeletal filaments, such as microtubules and actin, and with the other end to the cell cortex, a vesicle or another motor. A general question is how motors search for sites in the cell where both motor ends can bind to their respective binding partners. In the present review, we focus on cytoplasmic dynein, which is required for a myriad of cellular functions in interphase, mitosis and meiosis, ranging from transport of organelles and functioning of the mitotic spindle to chromosome movements in meiotic prophase. We discuss how dynein targets sites where it can exert a pulling force on the microtubule to transport cargo inside the cell.

The role of chromosomal retention of noncoding RNA in meiosis

Meiosis is a process of fundamental importance for sexually reproducing eukaryotes. During meiosis, homologous chromosomes pair with each other and undergo homologous recombination, ultimately producing haploid sets of recombined chromosomes that will be inherited by the offspring. Compared with the extensive progress that has been made in understanding the molecular mechanisms underlying recombination, how homologous sequences pair with each other is still poorly understood. The diversity of the underlying mechanisms of pairing present in different organisms further increases the complexity of this problem. Involvement of meiosis-specific noncoding RNA in the pairing of homologous chromosomes has been found in the fission yeast Schizosaccharomyces pombe. Although different organisms may have developed other or additional systems that are involved in chromosome pairing, the findings in S. pombe will provide new insights into understanding the roles of noncoding RNA in meiosis.

Chromosomes rein back the spindle pole body during horsetail movement in fission yeast meiosis

In meiosis, pairing and recombination of homologous chromosomes are crucial for the correct segregation of chromosomes, and substantial movements of chromosomes are required to achieve homolog pairing. During this process, it is known that telomeres cluster to form a bouquet arrangement of chromosomes. The fission yeast Schizosaccharomyces pombe provides a striking example of bouquet formation, after which the entire nucleus oscillates between the cell poles (these oscillations are generally called horsetail nuclear movements) while the telomeres remain clustered to the spindle pole body (SPB; a centrosome-equivalent structure in fungi) at the leading edge of the moving nucleus. S. pombe mutants defective in telomere clustering frequently form aberrant spindles, such as monopolar or nonpolar spindles, leading to missegregation of the chromosomes at the subsequent meiotic divisions. Here we demonstrate that such defects in meiotic spindle formation caused by loss of meiotic telomere clustering are rescued when nuclear movement is prevented. On the other hand, stopping nuclear movement does not rescue defects in telomere clustering, nor chromosome missgregation even in cells that have formed a bipolar spindle. These results suggest that movement of the SPB without attachment of telomeres leads to the formation of aberrant spindles, but that recovering bipolar spindles is not sufficient for rescue of chromosome missegregation in mutants lacking telomere clustering.

A role for nuclear envelope-bridging complexes in homology-directed repair

Persistent double-strand DNA breaks (DSBs) are recruited to the nuclear periphery, where they induce formation of associated nuclear envelope–spanning LINC complexes made up of the SUN protein Sad1 and the KASH protein Kms1. The LINC complex couples DSBs within the nucleus to cytoplasmic microtubules, which alters DSB repair pathway choice.

Unless efficiently and faithfully repaired, DNA double-strand breaks (DSBs) cause genome instability. We implicate a Schizosaccharomyces pombe nuclear envelope–spanning linker of nucleoskeleton and cytoskeleton (LINC) complex, composed of the Sad1/Unc84 protein Sad1 and Klarsicht/Anc1/SYNE1 homology protein Kms1, in the repair of DSBs. An induced DSB associates with Sad1 and Kms1 in S/G2 phases of the cell cycle, connecting the DSB to cytoplasmic microtubules. DSB resection to generate single-stranded DNA and the ATR kinase drive the formation of Sad1 foci in response to DNA damage. Depolymerization of microtubules or loss of Kms1 leads to an increase in the number and size of DSB-induced Sad1 foci. Further, Kms1 and the cytoplasmic microtubule regulator Mto1 promote the repair of an induced DSB by gene conversion, a type of homology-directed repair. kms1 genetically interacts with a number of genes involved in homology-directed repair; these same gene products appear to attenuate the formation or promote resolution of DSB-induced Sad1 foci. We suggest that the connection of DSBs with the cytoskeleton through the LINC complex may serve as an input to repair mechanism choice and efficiency.

Recombination-independent mechanisms and pairing of homologous chromosomes during meiosis in plants

Meiosis is the specialized eukaryotic cell division that permits the halving of ploidy necessary for gametogenesis in sexually reproducing organisms. This involves a single round of DNA replication followed by two successive divisions. To ensure balanced segregation, homologous chromosome pairs must migrate to opposite poles at the first meiotic division and this means that they must recognize and pair with each other beforehand. Although understanding of the mechanisms by which meiotic chromosomes find and pair with their homologs has greatly advanced, it remains far from being fully understood. With some notable exceptions such as male Drosophila, the recognition and physical linkage of homologs at the first meiotic division involves homologous recombination. However, in addition to this, it is clear that many organisms, including plants, have also evolved a series of recombination-independent mechanisms to facilitate homolog recognition and pairing. These implicate chromosome structure and dynamics, telomeres, centromeres, and, most recently, small RNAs. With a particular focus on plants, we present here an overview of understanding of these early, recombination-independent events that act in the pairing of homologous chromosomes during the first meiotic division.

Chromosome segregation in plant meiosis

Faithful chromosome segregation in meiosis is essential for ploidy stability over sexual life cycles. In plants, defective chromosome segregation caused by gene mutations or other factors leads to the formation of unbalanced or unreduced gametes creating aneuploid or polyploid progeny, respectively. Accurate segregation requires the coordinated execution of conserved processes occurring throughout the two meiotic cell divisions. Synapsis and recombination ensure the establishment of chiasmata that hold homologous chromosomes together allowing their correct segregation in the first meiotic division, which is also tightly regulated by cell-cycle dependent release of cohesin and monopolar attachment of sister kinetochores to microtubules. In meiosis II, bi-orientation of sister kinetochores and proper spindle orientation correctly segregate chromosomes in four haploid cells. Checkpoint mechanisms acting at kinetochores control the accuracy of kinetochore-microtubule attachment, thus ensuring the completion of segregation. Here we review the current knowledge on the processes taking place during chromosome segregation in plant meiosis, focusing on the characterization of the molecular factors involved.

Mal3, the Schizosaccharomyces pombe homolog of EB1, is required for karyogamy and for promoting oscillatory nuclear movement during meiosis

Two successive rounds of chromosome segregation following a single round of DNA replication enable the production of haploid gametes during meiosis. In the fission yeast Schizosaccharomyces pombe, karyogamy is the process where the nuclei from 2 haploid cells fuse to create a diploid nucleus, which then undergoes meiosis to produce 4 haploid spores. By screening a collection of S. pombe deletion strains, we found that the deletion of 2 genes, mal3 and mto1, leads to the production of asci containing up to 8 spores. Here, we show that Mal3, the fission yeast member of the EB1 family of conserved microtubule plus-end tracking proteins, is required for karyogamy, oscillatory nuclear movement, and proper segregation of chromosomes during meiosis. In the absence of Mal3, meiosis frequently initiates before the completion of karyogamy, thus producing up to 8 nuclei in a single ascus. Our results provide new evidence that fission yeast can initiate meiosis prior to completing karyogamy.

Collaborative homologous pairing during C. elegans meiosis

In preparation for meiotic chromosome segregation, homologous chromosomes need to pair, synapse (i.e., assemble the synaptonemal complex, SC), and then recombine to generate a physical linkage (i.e., chiasma) between them. In many organisms meiotic pairing capacity distributed along the entire chromosome length supports presynaptic alignment. In contrast, the prevailing model for C. elegans proposes that presynaptic homologous pairing is performed solely by a master pairing-site, the pairing center (PC). In this model, the remaining chromosomal regions (the non-PC regions) are not actively involved in presynaptic pairing, and the SC assembling from the PC aligns the homologous chromosomes along non-PC regions and holds them together. Our recent work, however, demonstrates that C. elegans chromosomes establish presynaptic alignment along the entire chromosome length, suggesting that the non-PC regions are also actively involved in the presynaptic pairing process. Furthermore, we have also discovered that the chromodomain protein MRG-1 facilitates this presynaptic non-PC pairing. The phenotype of the mrg-1 mutant indicates that the PC and the non-PC collaborate in successful pairing and synapsis. Therefore, homologous pairing mechanisms in C. elegans possibly share more similarity with those in other organisms than previously thought. Here, we elaborate on these observations and discuss a hypothetical model for presynaptic pairing in C. elegans based on our novel findings.

A mammalian KASH domain protein coupling meiotic chromosomes to the cytoskeleton

A complex of KASH5 and Sun1 is required for meiotic homologous chromosome pairing through the coupling of telomere attachment sites to cytoplasmic dynein and microtubules.

Chromosome pairing is an essential meiotic event that ensures faithful haploidization and recombination of the genome. Pairing of homologous chromosomes is facilitated by telomere-led chromosome movements and formation of a meiotic bouquet, where telomeres cluster to one pole of the nucleus. In metazoans, telomere clustering is dynein and microtubule dependent and requires Sun1, an inner nuclear membrane protein. Here we provide a functional analysis of KASH5, a mammalian dynein-binding protein of the outer nuclear membrane that forms a meiotic complex with Sun1. This protein is related to zebrafish futile cycle (Fue), a nuclear envelope (NE) constituent required for pronuclear migration. Mice deficient in this Fue homologue are infertile. Males display meiotic arrest in which pairing of homologous chromosomes fails. These findings demonstrate that telomere attachment to the NE is insufficient to promote pairing and that telomere attachment sites must be coupled to cytoplasmic dynein and the microtubule system to ensure meiotic progression.

The diverse functional LINCs of the nuclear envelope to the cytoskeleton and chromatin

The nuclear envelope (NE) is connected to the different types of cytoskeletal elements by linker of nucleoskeleton and cytoskeleton (LINC) complexes. LINC complexes exist from yeast to humans, and have preserved their general architecture throughout evolution. They are composed of SUN and KASH domain proteins of the inner and the outer nuclear membrane, respectively. These SUN–KASH bridges are used for the transmission of forces across the NE and support diverse biological processes. Here, we review the function of SUN and KASH domain proteins in various unicellular and multicellular species. Specifically, we discuss their influence on nuclear morphology and cytoskeletal organization. Further, emphasis is given on the role of LINC complexes in nuclear anchorage and migration as well as in genome organization.

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

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

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

One identity or more for telomeres?

A major issue in telomere research is to understand how the integrity of chromosome ends is controlled. The fact that different types of nucleoprotein complexes have been described at the telomeres of different organisms raises the question of whether they have in common a structural identity that explains their role in chromosome protection. We will review here how telomeric nucleoprotein complexes are structured, comparing different organisms and trying to link these structures to telomere biology. It emerges that telomeres are formed by a complex and specific network of interactions between DNA, RNA, and proteins. The fact that these interactions and associated activities are reinforcing each other might help to guarantee the robustness of telomeric functions across the cell cycle and in the event of cellular perturbations. We will also discuss the recent notion that telomeres have evolved specific systems to overcome the DNA topological stress generated during their replication and transcription. This will lead to revisit the way we envisage the functioning of telomeric complexes since the regulation of topology is central to DNA stability, replication, recombination, and transcription as well as to chromosome higher-order organization.

Gene copy-number alterations: a cost-benefit analysis

Changes in DNA copy number, whether confined to specific genes or affecting whole chromosomes, have been identified as causes of diseases and developmental abnormalities and as sources of adaptive potential. Here, we discuss the costs and benefits of DNA copy-number alterations. Changes in DNA copy number are largely detrimental. Amplifications or deletions of specific genes can elicit discrete defects. Large-scale changes in DNA copy number can also cause detrimental phenotypes that are due to the cumulative effects of copy-number alterations of many genes simultaneously. On the other hand, studies in microorganisms show that DNA copy-number alterations can be beneficial, increasing survival under selective pressure. As DNA copy-number alterations underlie many human diseases, we will end with a discussion of gene copy-number changes as therapeutic targets.

LINCing the nuclear envelope to gametogenesis

Gametogenesis combines two important features: reduction of the genome content from diploid to haploid by carefully partitioning chromosomes, and the subsequent differentiation into fertilization-competent gametes, which in males is characterized by profound nuclear restructuring. These are quite difficult tasks and require a tight coordination of different cellular mechanisms. Recent studies in the field established a key role for LINC complexes in both meiosis and sperm head formation. LINC complexes comprise SUN and KASH domain proteins that form nuclear envelope (NE) bridges, linking the nucleoskeleton to the cytoskeleton. They are well known for their crucial roles in diverse cellular and developmental processes, such as nuclear positioning and cell polarization. In this review, we highlight key roles ascribed to LINC complexes and to the nucleocytoskeletal connection in gametogenesis. First, we give a short overview about the general features of LINC components and the profound reorganization of the NE in germ cells. We then focus on specific roles of LINC complexes in meiotic chromosome dynamics and their impact on pairing, synapsis, and recombination. Finally, we provide an update of the mechanisms controlling sperm head formation and discuss the role of sperm-specific LINC complexes in nuclear shaping and their relation to specialized cytoskeletal structures that form concurrently with nuclear restructuring and sperm elongation.

Human telomeres are tethered to the nuclear envelope during postmitotic nuclear assembly

Telomeres are essential for nuclear organization in yeast and during meiosis in mice. Exploring telomere dynamics in living human cells by advanced time-lapse confocal microscopy allowed us to evaluate the spatial distribution of telomeres within the nuclear volume. We discovered an unambiguous enrichment of telomeres at the nuclear periphery during postmitotic nuclear assembly, whereas telomeres were localized more internally during the rest of the cell cycle. Telomere enrichment at the nuclear rim was mediated by physical tethering of telomeres to the nuclear envelope, most likely via specific interactions between the shelterin subunit RAP1 and the nuclear envelope protein Sun1. Genetic interference revealed a critical role in cell-cycle progression for Sun1 but no effect on telomere positioning for RAP1. Our results shed light on the dynamic relocalization of human telomeres during the cell cycle and suggest redundant pathways for tethering telomeres to the nuclear envelope.

Identification of the functional domains of the telomere protein Rap1 in Schizosaccharomyces pombe

The telomere at the end of a linear chromosome plays crucial roles in genome stability. In the fission yeast Schizosaccharomyces pombe, the Rap1 protein, one of the central players at the telomeres, associates with multiple proteins to regulate various telomere functions, such as the maintenance of telomere DNA length, telomere end protection, maintenance of telomere heterochromatin, and telomere clustering in meiosis. The molecular bases of the interactions between Rap1 and its partners, however, remain largely unknown. Here, we describe the identification of the interaction domains of Rap1 with its partners. The Bqt1/Bqt2 complex, which is required for normal meiotic progression, Poz1, which is required for telomere length control, and Taz1, which is required for the recruitment of Rap1 to telomeres, bind to distinct domains in the C-terminal half of Rap1. Intriguingly, analyses of a series of deletion mutants for rap1⁺ have revealed that the long N-terminal region (1–456 a.a. [amino acids]) of Rap1 (full length: 693 a.a.) is not required for telomere DNA length control, telomere end protection, and telomere gene silencing, whereas the C-terminal region (457–693 a.a.) containing Poz1- and Taz1-binding domains plays important roles in those functions. Furthermore, the Bqt1/Bqt2- and Taz1-binding domains are essential for normal spore formation after meiosis. Our results suggest that the C-terminal half of Rap1 is critical for the primary telomere functions, whereas the N-terminal region containing the BRCT (BRCA1 C-terminus) and Myb domains, which are evolutionally conserved among the Rap1 family proteins, does not play a major role at the telomeres.

A conserved KASH domain protein associates with telomeres, SUN1, and dynactin during mammalian meiosis

In yeasts and worms, KASH (Klarsicht/ANC-1/Syne/homology) domain and SUN (Sad-1/UNC-84) domain nuclear envelope (NE) proteins play a crucial role in meiotic chromosome movement and homologue pairing. However, although the vertebrate SUN domain protein SUN1 is involved in these processes, its partner has remained identified. Based on subcellular localization screening in mouse spermatocytes, we identified a novel germ cell-specific protein, KASH5, that localized exclusively at telomeres from the leptotene to diplotene stages in both spermatocytes and oocytes. KASH5 possesses hitherto unknown KASH-related sequences that directly interacted with SUN1 and mediated telomere localization. Thus, KASH5 is a mammalian meiosis-specific KASH domain protein. We show that meiotic chromosome movement depended on microtubules and that KASH5 interacted with the microtubule-associated dynein-dynactin complex. These results suggest that KASH5 connects the telomere-associated SUN1 protein to the cytoplasmic force-generating mechanism involved in meiotic chromosome movement. Our study strongly suggests that the meiotic homologue-pairing mechanism mediated by the SUN-KASH NE bridge is highly conserved among eukaryotes.

Structural insights into SUN-KASH complexes across the nuclear envelope

Linker of the nucleoskeleton and the cytoskeleton (LINC) complexes are composed of SUN and KASH domain-containing proteins and bridge the inner and outer membranes of the nuclear envelope. LINC complexes play critical roles in nuclear positioning, cell polarization and cellular stiffness. Previously, we reported the homotrimeric structure of human SUN2. We have now determined the crystal structure of the human SUN2-KASH complex. In the complex structure, the SUN domain homotrimer binds to three independent "hook"-like KASH peptides. The overall conformation of the SUN domain in the complex closely resembles the SUN domain in its apo state. A major conformational change involves the AA'-loop of KASH-bound SUN domain, which rearranges to form a mini β-sheet that interacts with the KASH peptide. The PPPT motif of the KASH domain fits tightly into a hydrophobic pocket on the homotrimeric interface of the SUN domain, which we termed the BI-pocket. Moreover, two adjacent protomers of the SUN domain homotrimer sandwich the KASH domain by hydrophobic interaction and hydrogen bonding. Mutations of these binding sites disrupt or reduce the association between the SUN and KASH domains in vitro. In addition, transfection of wild-type, but not mutant, SUN2 promotes cell migration in Ovcar-3 cells. These results provide a structural model of the LINC complex, which is essential for additional study of the physical and functional coupling between the cytoplasm and the nucleoplasm.

Meiosis-specific noncoding RNA mediates robust pairing of homologous chromosomes in meiosis

Pairing and recombination of homologous chromosomes are essential for ensuring reductional segregation in meiosis. However, the mechanisms by which chromosomes recognize their homologous partners are poorly understood. Here, we report that the sme2 gene encodes a meiosis-specific noncoding RNA that mediates homologous recognition in the fission yeast Schizosaccharomyces pombe. The sme2 locus shows robust pairing from early in meiotic prophase. The sme2 RNA transcripts accumulate at their respective gene loci and greatly enhance pairing of homologous loci: Deletion of the sme2 sequence eliminates this robust pairing, whereas transposition to other chromosomal sites confers robust pairing at those ectopic sites. Thus, we propose that RNA transcripts retained on the chromosome play an active role in recognition of homologous chromosomes for pairing.

Microtubule organization: a pericentriolar material-like structure in yeast meiosis

During meiotic prophase in fission yeast, the nucleus undergoes dramatic oscillatory movements. A newly identified structure, the radial microtubule organizing center (rMTOC), mediates these movements and shares some of the features of the pericentriolar material in higher eukaryotes.

Transient structure associated with the spindle pole body directs meiotic microtubule reorganization in S. pombe

Background

Vigorous chromosome movements driven by cytoskeletal assemblies are a widely conserved feature of sexual differentiation to facilitate meiotic recombination. In fission yeast, this process involves the dramatic conversion of arrays of cytoplasmic microtubules (MTs), generated from multiple MT organizing centers (MTOCs), into a single radial MT (rMT) array associated with the spindle pole body (SPB), the major MTOC during meiotic prophase. The rMT is then dissolved upon the onset of meiosis I when a bipolar spindle emerges to conduct chromosome segregation. Structural features and molecular mechanisms that govern these dynamic MT rearrangements are poorly understood.

Results

Electron tomography of the SPBs showed that the rMT emanates from a newly recognized amorphous structure, which we term the rMTOC. The rMTOC, which resides at the cytoplasmic side of the SPB, is highly enriched in γ-tubulin reminiscent of the pericentriolar material of higher eukaryotic centrosomes. Formation of the rMTOC depends on Hrs1/Mcp6, a meiosis-specific SPB component that is located at the rMTOC. At the onset of meiosis I, Hrs1/Mcp6 is subject to strict downregulation by both proteasome-dependent degradation and phosphorylation leading to complete inactivation of the rMTOC. This ensures rMT dissolution and bipolar spindle formation.

Conclusions

Our study reveals the molecular basis for the transient generation of a novel MTOC, which triggers a program of MT rearrangement that is required for meiotic differentiation.

The chromodomain protein MRG-1 facilitates SC-independent homologous pairing during meiosis in Caenorhabditis elegans

Homologous chromosome pairing is a prerequisite to establish physical linkage between homologs, which is critical for faithful chromosome segregation during meiosis I. The establishment of pairing is genetically separable from subsequent synapsis, defined as stabilization of pairing by the synaptonemal complex (SC). The underlying mechanism of presynaptic pairing is poorly understood. In the nematode Caenorhabditis elegans, a unique cis-acting element, the pairing center (PC), is essential for presynaptic pairing; however, it is not known whether and how the remainder of the chromosome contributes to presynaptic pairing. Here we report direct evidence for presynaptic pairing activity intrinsic to non-PC regions, which is facilitated by a conserved chromodomain protein, MRG-1. In mrg-1 loss-of-function mutants, pairing is compromised specifically in non-PC regions, leading to nonhomologous SC assembly. Our data support a model in which presynaptic alignment in non-PC regions collaborates with initial PC pairing to ensure correct homologous synapsis.

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.

Nuclear mechanics in differentiation and development

The nucleus is by far one of the stiffest organelles within cells of higher eukaryotes. Its mechanical properties are determined by contributions from the nuclear lamina and chromatin. Together they allow a viscoelastic response of the nucleus to applied stresses, where the lamina is thought to behave as an elastic shell, while the nucleoplasm contributes as a largely viscous material. Nuclear mechanics changes during differentiation and development. Altered nuclear mechanics reflects but might also influence global re-arrangements in chromatin architecture, which take place when cells commit themselves into distinct lineages. Thus it is likely that the mechanical characteristics of nuclei significantly contribute to proper differentiation.

Chromosome painting reveals asynaptic full alignment of homologs and HIM-8-dependent remodeling of X chromosome territories during Caenorhabditis elegans meiosis

During early meiotic prophase, a nucleus-wide reorganization leads to sorting of chromosomes into homologous pairs and to establishing associations between homologous chromosomes along their entire lengths. Here, we investigate global features of chromosome organization during this process, using a chromosome painting method in whole-mount Caenorhabditis elegans gonads that enables visualization of whole chromosomes along their entire lengths in the context of preserved 3D nuclear architecture. First, we show that neither spatial proximity of premeiotic chromosome territories nor chromosome-specific timing is a major factor driving homolog pairing. Second, we show that synaptonemal complex-independent associations can support full lengthwise juxtaposition of homologous chromosomes. Third, we reveal a prominent elongation of chromosome territories during meiotic prophase that initiates prior to homolog association and alignment. Mutant analysis indicates that chromosome movement mediated by association of chromosome pairing centers (PCs) with mobile patches of the nuclear envelope (NE)–spanning SUN-1/ZYG-12 protein complexes is not the primary driver of territory elongation. Moreover, we identify new roles for the X chromosome PC (X-PC) and X-PC binding protein HIM-8 in promoting elongation of X chromosome territories, separable from their role(s) in mediating local stabilization of pairing and association of X chromosomes with mobile SUN-1/ZYG-12 patches. Further, we present evidence that HIM-8 functions both at and outside of PCs to mediate chromosome territory elongation. These and other data support a model in which synapsis-independent elongation of chromosome territories, driven by PC binding proteins, enables lengthwise juxtaposition of chromosomes, thereby facilitating assessment of their suitability as potential pairing partners.

Successful sexual reproduction relies on the ability of germ cells to faithfully segregate homologous chromosomes in meiosis, which requires accurate sorting of chromosomes into homologous pairs and alignment of homologs along their entire lengths. The mechanisms underlying homolog sorting and alignment are not well understood, partly because of a scarcity of studies investigating homolog alignment at the level of whole chromosomes. This study provides a global view of the organization of chromosome territories during early meiotic prophase in the nematode Caenorhabditis elegans. We applied chromosome painting to visualize the entire lengths of chromosomes. Our study provides several conceptual advances. First, our study excluded several possible mechanisms as primary drivers of chromosome sorting. Second, our analysis has revealed both a robust capacity for full-lengthwise alignment between homologous chromosomes prior to the stabilization of pairing by the synaptonemal complex as well as a dramatic elongation of chromosome territories that could enable this alignment. We also identified a factor required for the elongation of chromosome territories. Elongation of chromosome territories could enable lengthwise juxtaposition of chromosomes, thereby facilitating assessment of their suitability as potential pairing partners by promoting utilization of information about chromosome identity that is distributed along the length of a chromosome.

Microtubule-induced nuclear envelope fluctuations control chromatin dynamics in Drosophila embryos

Nuclear shape is different in stem cells and differentiated cells and reflects important changes in the mechanics of the nuclear envelope (NE). The current framework emphasizes the key role of the nuclear lamina in nuclear mechanics and its alterations in disease. Whether active stress controls nuclear deformations and how this stress interplays with properties of the NE to control NE dynamics is unclear. We address this in the early Drosophila embryo, in which profound changes in NE shape parallel the transcriptional activation of the zygotic genome. We show that microtubule (MT) polymerization events produce the elementary forces necessary for NE dynamics. Moreover, large-scale NE deformations associated with groove formation require concentration of MT polymerization in bundles organized by Dynein. However, MT bundles cannot produce grooves when the farnesylated inner nuclear membrane protein Kugelkern (Kuk) is absent. Although it increases stiffness of the NE, Kuk also stabilizes NE deformations emerging from the collective effect of MT polymerization forces concentrated in bundles. Finally, we report that MT-induced NE deformations control the dynamics of chromatin and its organization at steady state. Thus, the NE is a dynamic organelle, fluctuations of which increase chromatin dynamics. We propose that such mechanical regulation of chromatin dynamics by MTs might be important for gene regulation.