The Chromosomal Courtship Dance-homolog pairing in early meiosis

Meiosis through three centuries

Meiosis is the specialized cellular program that underlies gamete formation for sexual reproduction. It is therefore not only interesting but also a fundamentally important subject for investigation. An especially attractive feature of this program is that many of the processes of special interest involve organized chromosomes, thus providing the possibility to see chromosomes "in action". Analysis of meiosis has also proven to be useful in discovering and understanding processes that are universal to all chromosomal programs. Here we provide an overview of the different historical moments when the gap between observation and understanding of mechanisms and/or roles for the new discovered molecules was bridged. This review reflects also the synergy of thinking and discussion among our three laboratories during the past several decades.

Identifying Chromosome Movement Patterns During Meiosis Using ChroMo

During meiosis, transient associations between the nuclear envelope and telomeres transmit nuclear movements to chromosomes, enabling their pairing and recombination. Recent advances in the field of quantitative cell biology allow a large volume of information about the kinetics of these chromosome movements to be extracted and analyzed with the aim of identifying biologically relevant movement patterns. To this end, we have developed ChroMo, a freely available application for the unsupervised study of chromosome movements in fission yeast meiosis. ChroMo contains a set of time series algorithms to identify chromosome movement motifs that are not easily observable by direct human visualization and to establish causal relationships between phenotypes. In this chapter, we present a detailed protocol for the processing of raw live imaging data from fission yeast and its subsequent analysis in ChroMo.

The scaffold nucleoporins SAR1 and SAR3 are essential for proper meiotic progression in Arabidopsis thaliana

Nuclear Pore Complexes (NPCs) are embedded in the nuclear envelope (NE), regulating macromolecule transport and physically interacting with chromatin. The NE undergoes dramatic breakdown and reformation during plant cell division. In addition, this structure has a specific meiotic function, anchoring and positioning telomeres to facilitate the pairing of homologous chromosomes. To elucidate a possible function of the structural components of the NPCs in meiosis, we have characterized several Arabidopsis lines with mutations in genes encoding nucleoporins belonging to the outer ring complex. Plants defective for either SUPPRESSOR OF AUXIN RESISTANCE1 (SAR1, also called NUP160) or SAR3 (NUP96) present condensation abnormalities and SPO11-dependent chromosome fragmentation in a fraction of meiocytes, which is increased in the double mutant sar1 sar3. We also observed these meiotic defects in mutants deficient in the outer ring complex protein HOS1, but not in mutants affected in other components of this complex. Furthermore, our findings may suggest defects in the structure of NPCs in sar1 and a potential link between the meiotic role of this nucleoporin and a component of the RUBylation pathway. These results provide the first insights in plants into the role of nucleoporins in meiotic chromosome behavior.

Cadmium-induced reproductive toxicity combined with a correlation to the oogenesis process and competing endogenous RNA networks based on a Caenorhabditis elegans model

Accumulation of the heavy metal Cadmium (Cd) in the ovaries and placenta can affect the structure and function of these organs and induce female reproductive toxicity. This toxicity may be due to Cd's similarity to estrogen and its ability to disrupt endocrine systems. However, the exact molecular mechanism by which Cd causes reproductive toxicity at the transcriptome level remains poorly understood. Hence, this study aimed to observe Cd-induced reproductive damage at the gene level, scrutinize the repercussions of Cd exposure on oogenesis, and explicate the putative pathogenesis of Cd-induced oogenesis based on Caenorhabditis elegans (C. elegans) as an in vivo model. The results showed that Cd exposure significantly decreased the number of offspring and prolonged the reproductive span of C. elegans. Cd exposure also reduced the number of cells in mitosis and the pachytene and diakinesis stages of meiosis, thereby disrupting oogenesis. Combined with transcriptional sequencing and bioinformatics analysis, a total of 3167 DEmRNAs were identified. Regarding gene expression, cul-6, mum-2, and vang-1 were found to be related to Cd-induced reproductive toxicity, and their competing endogenous RNA networks were constructed. We observed that mutations of mom-2 and vang-1 in the Wnt pathway could induce susceptibility to Cd-caused meiosis injury. In conclusion, the results indicated that Cd could impair the oogenesis of C. elegans and the Wnt pathway might serve as a protective mechanism against Cd reproductive toxicity. These findings contribute to a better understanding of the damaging effects and molecular biological mechanisms of Cd on the human reproductive system.

Meiosis: Dances Between Homologs

The raison d'être of meiosis is shuffling of genetic information via Mendelian segregation and, within individual chromosomes, by DNA crossing-over. These outcomes are enabled by a complex cellular program in which interactions between homologous chromosomes play a central role. We first provide a background regarding the basic principles of this program. We then summarize the current understanding of the DNA events of recombination and of three processes that involve whole chromosomes: homolog pairing, crossover interference, and chiasma maturation. All of these processes are implemented by direct physical interaction of recombination complexes with underlying chromosome structures. Finally, we present convergent lines of evidence that the meiotic program may have evolved by coupling of this interaction to late-stage mitotic chromosome morphogenesis.

Beyond tradition: exploring the non-canonical functions of telomeres in meiosis

The telomere bouquet is a specific chromosomal configuration that forms during meiosis at the zygotene stage, when telomeres cluster together at the nuclear envelope. This clustering allows cytoskeleton-induced movements to be transmitted to the chromosomes, thereby facilitating homologous chromosome search and pairing. However, loss of the bouquet results in more severe meiotic defects than can be attributed solely to recombination problems, suggesting that the bouquet's full function remains elusive. Despite its transient nature and the challenges in performing in vivo analyses, information is emerging that points to a remarkable suite of non-canonical functions carried out by the bouquet. Here, we describe how new approaches in quantitative cell biology can contribute to establishing the molecular basis of the full function and plasticity of the bouquet, and thus generate a comprehensive picture of the telomeric control of meiosis.

TERRA beyond cancer: the biology of telomeric repeat-containing RNAs in somatic and germ cells

The telomeric noncoding RNA TERRA is a key component of telomeres and it is widely expressed in normal as well as cancer cells. In the last 15 years, several publications have shed light on the role of TERRA in telomere homeostasis and cell survival in cancer cells. However, only few studies have investigated the regulation or the functions of TERRA in normal tissues. A better understanding of the biology of TERRA in non-cancer cells may provide unexpected insights into how these lncRNAs are transcribed and operate in cells, and their potential role in physiological processes, such as aging, age-related pathologies, inflammatory processes and human genetic diseases. In this review we aim to discuss the findings that have advanced our understanding of the biology of TERRA using non-cancer mammalian cells as a model system.

The many faces of the bouquet centrosome MTOC in meiosis and germ cell development

Meiotic chromosomal pairing is facilitated by a conserved cytoskeletal organization. Telomeres associate with perinuclear microtubules via Sun/KASH complexes on the nuclear envelope (NE) and dynein. Telomere sliding on perinuclear microtubules contributes to chromosome homology searches and is essential for meiosis. Telomeres ultimately cluster on the NE, facing the centrosome, in a configuration called the chromosomal bouquet. Here, we discuss novel components and functions of the bouquet microtubule organizing center (MTOC) in meiosis, but also broadly in gamete development. The cellular mechanics of chromosome movements and the bouquet MTOC dynamics are striking. The newly identified zygotene cilium mechanically anchors the bouquet centrosome and completes the bouquet MTOC machinery in zebrafish and mice. We hypothesize that various centrosome anchoring strategies evolved in different species. Evidence suggests that the bouquet MTOC machinery is a cellular organizer, linking meiotic mechanisms with gamete development and morphogenesis. We highlight this cytoskeletal organization as a new platform for creating a holistic understanding of early gametogenesis, with direct implications to fertility and reproduction.

Cyclins and CDKs in the regulation of meiosis-specific events

How eukaryotic cells control their duplication is a fascinating example of how a biological system self-organizes specific activities to temporally order cellular events. During cell cycle progression, the cellular level of CDK (Cyclin-Dependent Kinase) activity temporally orders the different cell cycle phases, ensuring that DNA replication occurs prior to segregation into two daughter cells. CDK activity requires the binding of a regulatory subunit (cyclin) to the core kinase, and both CDKs and cyclins are well conserved throughout evolution from yeast to humans. As key regulators, they coordinate cell cycle progression with metabolism, DNA damage, and cell differentiation. In meiosis, the special cell division that ensures the transmission of genetic information from one generation to the next, cyclins and CDKs have acquired novel functions to coordinate meiosis-specific events such as chromosome architecture, recombination, and synapsis. Interestingly, meiosis-specific cyclins and CDKs are common in evolution, some cyclins seem to have evolved to acquire CDK-independent functions, and even some CDKs associate with a non-cyclin partner. We will review the functions of these key regulators in meiosis where variation has specially flourished.

DNA Environment of Centromeres and Non-Homologous Chromosomes Interactions in Mouse

Although the pericentromeric regions of chromosomes that are enriched in tandemly repeated satellite DNA represent a significant part of eukaryotic genomes, they remain understudied, which is mainly due to interdisciplinary knowledge gaps. Recent studies suggest their important role in genome regulation, karyotype stability, and evolution. Thus, the idea of satellite DNA as a junk part of the genome has been refuted. The integration of data regarding molecular composition, chromosome behaviour, and the details of the in situ organization of pericentromeric regions is of great interest. The objective of this work was a cytogenetic analysis of the interactions between pericentromeric regions from non-homologous chromosomes in mouse spermatocytes using immuno-FISH. We analysed two events: the associations between centromeric regions of the X chromosome and autosomes and the associations between the centromeric regions of the autosomal bivalents that form chromocenters. We concluded that the X chromosome forms temporary synaptic associations with different autosomes in early meiotic prophase I, which can normally be found until the pachytene-diplotene, without signs of pachytene arrest. These associations are formed between the satellite-DNA-rich centromeric regions of the X chromosome and different autosomes but do not involve the satellite-DNA-poor centromeric region of the Y chromosome. We suggest the hypothetical model of X chromosome competitive replacement from such associations during synaptic correction. We showed that the centromeric region of the X chromosome in association remains free of γH2Ax-dependent chromatin inactivation, while the Y chromosome is completely inactivated. This finding highlights the predominant role of associations between satellite DNA-rich regions of different chromosomes, including the X chromosome. We suppose that X-autosomal transient associations are a manifestation of an additional synaptic disorder checkpoint. These associations are normally corrected before the late diplotene stage. We revealed that the intense spreading conditions that were applied to the spermatocyte I nuclei did not lead to the destruction of stretched chromatin fibers of elongated chromocenters enriched in satellite DNA. The tight associations that we revealed between the pericentromeric regions of different autosomal bivalents and the X chromosome may represent the basis for a mechanism for maintaining the repeats stability in the autosomes and in the X chromosome. The consequences of our findings are discussed.

Meiotic Chromosome Dynamics in Zebrafish

Recent studies in zebrafish have revealed key features of meiotic chromosome dynamics, including clustering of telomeres in the bouquet configuration, biogenesis of chromosome axis structures, and the assembly and disassembly of the synaptonemal complex that aligns homologs end-to-end. The telomere bouquet stage is especially pronounced in zebrafish meiosis and sub-telomeric regions play key roles in mediating pairing and homologous recombination. In this review, we discuss the temporal progression of these events in meiosis prophase I and highlight the roles of proteins associated with meiotic chromosome architecture in homologous recombination. Finally, we discuss the interplay between meiotic mutants and gonadal sex differentiation and future research directions to study meiosis in living cells, including cell culture.

Bivalent individualization during chromosome territory formation in Drosophila spermatocytes by controlled condensin II protein activity and additional force generators

Reduction of genome ploidy from diploid to haploid necessitates stable pairing of homologous chromosomes into bivalents before the start of the first meiotic division. Importantly, this chromosome pairing must avoid interlocking of non-homologous chromosomes. In spermatocytes of Drosophila melanogaster, where homolog pairing does not involve synaptonemal complex formation and crossovers, associations between non-homologous chromosomes are broken up by chromosome territory formation in early spermatocytes. Extensive non-homologous associations arise from the coalescence of the large blocks of pericentromeric heterochromatin into a chromocenter and from centromere clustering. Nevertheless, during territory formation, bivalents are moved apart into spatially separate subnuclear regions. The condensin II subunits, Cap-D3 and Cap-H2, have been implicated, but the remarkable separation of bivalents during interphase might require more than just condensin II. For further characterization of this process, we have applied time-lapse imaging using fluorescent markers of centromeres, telomeres and DNA satellites in pericentromeric heterochromatin. We describe the dynamics of the disruption of centromere clusters and the chromocenter in normal spermatocytes. Mutations in Cap-D3 and Cap-H2 abolish chromocenter disruption, resulting in excessive chromosome missegregation during M I. Chromocenter persistence in the mutants is not mediated by the special system, which conjoins homologs in compensation for the absence of crossovers in Drosophila spermatocytes. However, overexpression of Cap-H2 precluded conjunction between autosomal homologs, resulting in random segregation of univalents. Interestingly, Cap-D3 and Cap-H2 mutant spermatocytes displayed conspicuous stretching of the chromocenter, as well as occasional chromocenter disruption, suggesting that territory formation might involve forces unrelated to condensin II. While the molecular basis of these forces remains to be clarified, they are not destroyed by inhibitors of F actin and microtubules. Our results indicate that condensin II activity promotes chromosome territory formation in co-operation with additional force generators and that careful co-ordination with alternative homolog conjunction is crucial.

Recombination-independent recognition of DNA homology for meiotic silencing in Neurospora crassa

The pairing of homologous chromosomes represents a critical step of meiosis in nearly all sexually reproducing species. In many organisms, pairing involves chromosomes that remain apparently intact. The mechanistic nature of homology recognition at the basis of such pairing is unknown. Using "meiotic silencing by unpaired DNA" (MSUD) as a model process, we demonstrate the existence of a cardinally different approach to DNA homology recognition in meiosis. The main advantage of MSUD over other experimental systems lies in its ability to identify any relatively short DNA fragment lacking a homologous allelic partner. Here, we show that MSUD does not rely on the canonical mechanism of meiotic recombination, yet it is promoted by REC8, a conserved component of the meiotic cohesion complex. We also show that certain patterns of interspersed homology are recognized as pairable during MSUD. Such patterns need to be colinear and must contain short tracts of sequence identity spaced apart at 21 or 22 base pairs. By using these periodicity values as a guiding parameter in all-atom molecular modeling, we discover that homologous DNA molecules can pair by forming quadruplex-based contacts with an interval of 2.5 helical turns. This process requires right-handed plectonemic coiling and additional conformational changes in the intervening double-helical segments. Our results 1) reconcile genetic and biophysical evidence for the existence of direct homologous double-stranded DNA (dsDNA)-dsDNA pairing, 2) identify a role for this process in initiating RNA interference, and 3) suggest that chromosomes can be cross-matched by a precise mechanism that operates on intact dsDNA molecules.

ChroMo, an Application for Unsupervised Analysis of Chromosome Movements in Meiosis

Nuclear movements during meiotic prophase, driven by cytoskeleton forces, are a broadly conserved mechanism in opisthokonts and plants to promote pairing between homologous chromosomes. These forces are transmitted to the chromosomes by specific associations between telomeres and the nuclear envelope during meiotic prophase. Defective chromosome movements (CMs) harm pairing and recombination dynamics between homologues, thereby affecting faithful gametogenesis. For this reason, modelling the behaviour of CMs and their possible microvariations as a result of mutations or physico-chemical stress is important to understand this crucial stage of meiosis. Current developments in high-throughput imaging and image processing are yielding large CM datasets that are suitable for data mining approaches. To facilitate adoption of data mining pipelines, we present ChroMo, an interactive, unsupervised cloud application specifically designed for exploring CM datasets from live imaging. ChroMo contains a wide selection of algorithms and visualizations for time-series segmentation, motif discovery, and assessment of causality networks. Using ChroMo to analyse meiotic CMs in fission yeast, we found previously undiscovered features of CMs and causality relationships between chromosome morphology and trajectory. ChroMo will be a useful tool for understanding the behaviour of meiotic CMs in yeast and other model organisms.

Redistribution of centrosomal proteins by centromeres and Polo kinase controls partial nuclear envelope breakdown in fission yeast

Proper mitotic progression in Schizosaccharomyces pombe requires partial nuclear envelope breakdown (NEBD) and insertion of the spindle pole body (SPB—yeast centrosome) to build the mitotic spindle. Linkage of the centromere to the SPB is vital to this process, but why that linkage is important is not well understood. Utilizing high-resolution structured illumination microscopy, we show that the conserved Sad1-UNC-84 homology-domain protein Sad1 and other SPB proteins redistribute during mitosis to form a ring complex around SPBs, which is a precursor for localized NEBD and spindle formation. Although the Polo kinase Plo1 is not necessary for Sad1 redistribution, it localizes to the SPB region connected to the centromere, and its activity is vital for redistribution of other SPB ring proteins and for complete NEBD at the SPB to allow for SPB insertion. Our results lead to a model in which centromere linkage to the SPB drives redistribution of Sad1 and Plo1 activation that in turn facilitate partial NEBD and spindle formation through building of a SPB ring structure.

Chromosome Organization in Early Meiotic Prophase

One of the most fascinating aspects of meiosis is the extensive reorganization of the genome at the prophase of the first meiotic division (prophase I). The first steps of this reorganization are observed with the establishment of an axis structure, that connects sister chromatids, from which emanate arrays of chromatin loops. This axis structure, called the axial element, consists of various proteins, such as cohesins, HORMA-domain proteins, and axial element proteins. In many organisms, axial elements are required to set the stage for efficient sister chromatid cohesion and meiotic recombination, necessary for the recognition of the homologous chromosomes. Here, we review the different actors involved in axial element formation in Saccharomyces cerevisiae and in mouse. We describe the current knowledge of their localization pattern during prophase I, their functional interdependence, their role in sister chromatid cohesion, loop axis formation, homolog pairing before meiotic recombination, and recombination. We also address further challenges that need to be resolved, to fully understand the interplay between the chromosome structure and the different molecular steps that take place in early prophase I, which lead to the successful outcome of meiosis I.

Regulation of Meiotic Prophase One in Mammalian Oocytes

In female mammals, meiotic prophase one begins during fetal development. Oocytes transition through the prophase one substages consisting of leptotene, zygotene, and pachytene, and are finally arrested at the diplotene substage, for months in mice and years in humans. After puberty, luteinizing hormone induces ovulation and meiotic resumption in a cohort of oocytes, driving the progression from meiotic prophase one to metaphase two. If fertilization occurs, the oocyte completes meiosis two followed by fusion with the sperm nucleus and preparation for zygotic divisions; otherwise, it is passed into the uterus and degenerates. Specifically in the mouse, oocytes enter meiosis at 13.5 days post coitum. As meiotic prophase one proceeds, chromosomes find their homologous partner, synapse, exchange genetic material between homologs and then begin to separate, remaining connected at recombination sites. At postnatal day 5, most of the oocytes have reached the late diplotene (or dictyate) substage of prophase one where they remain arrested until ovulation. This review focuses on events and mechanisms controlling the progression through meiotic prophase one, which include recombination, synapsis and control by signaling pathways. These events are prerequisites for proper chromosome segregation in meiotic divisions; and if they go awry, chromosomes mis-segregate resulting in aneuploidy. Therefore, elucidating the mechanisms regulating meiotic progression is important to provide a foundation for developing improved treatments of female infertility.

Centromeres are dismantled by foundational meiotic proteins Spo11 and Rec8

Meiotic processes are potentially dangerous to genome stability and could be disastrous if activated in proliferative cells. Here we show that two key meiosis-defining proteins, the topoisomerase Spo11 (which forms double-strand breaks) and the meiotic cohesin Rec8, can dismantle centromeres. This dismantlement is normally observable only in mutant cells that lack the telomere bouquet, which provides a nuclear microdomain conducive to centromere reassembly¹; however, overexpression of Spo11 or Rec8 leads to levels of centromere dismantlement that cannot be countered by the bouquet. Specific nucleosome remodelling factors mediate centromere dismantlement by Spo11 and Rec8. Ectopic expression of either protein in proliferating cells leads to the loss of mitotic kinetochores in both fission yeast and human cells. Hence, while centromeric chromatin has been characterized as extraordinarily stable, Spo11 and Rec8 challenge this stability and may jeopardize kinetochores in cancers that express meiotic proteins.

LINC complex regulation of genome organization and function

The regulation of genomic function is in part mediated through the physical organization and architecture of the nucleus. Disruption to nuclear organization and architecture is increasingly being recognized by its contribution to many diseases. The LINC complexes - protein structures traversing the nuclear envelope, that physically connect the nuclear interior, and hence the genome, to cytoplasmic cytoskeletal networks are an important component in the physical organization of the genome and its function. This connection, potentially allows for the constant detection of environmental mechanical stimuli, resulting in altered regulation of nuclear architecture and genome function, either directly or via the process of mechanotransduction. Here, we review the influences LINC complexes exert on genome functions and their impact on cellular/organismal health.

Interplay of the nuclear envelope with chromatin in physiology and pathology

The nuclear envelope compartmentalizes chromatin in eukaryotic cells. The main nuclear envelope components are lamins that associate with a panoply of factors, including the LEM domain proteins. The nuclear envelope of mammalian cells opens up during cell division. It is reassembled and associated with chromatin at the end of mitosis when telomeres tether to the nuclear periphery. Lamins, LEM domain proteins, and DNA binding factors, as BAF, contribute to the reorganization of chromatin. In this context, an emerging role is that of the ESCRT complex, a machinery operating in multiple membrane assembly pathways, including nuclear envelope reformation. Research in this area is unraveling how, mechanistically, ESCRTs link to nuclear envelope associated factors as LEM domain proteins. Importantly, ESCRTs work also during interphase for repairing nuclear envelope ruptures. Altogether the advances in this field are giving new clues for the interpretation of diseases implicating nuclear envelope fragility, as laminopathies and cancer.

ABBREVIATIONS

na, not analyzed; ko, knockout; kd, knockdown; NE, nuclear envelope; LEM, LAP2-emerin-MAN1 (LEM)-domain containing proteins; LINC, linker of nucleoskeleton and cytoskeleton complexes; Cyt, cytoplasm; Chr, chromatin; MB, midbody; End, endosomes; Tel, telomeres; INM, inner nuclear membrane; NP, nucleoplasm; NPC, Nuclear Pore Complex; ER, Endoplasmic Reticulum; SPB, spindle pole body.

Spatial constraints on chromosomes are instrumental to meiotic pairing

In most eukaryotes, the meiotic chromosomal bouquet (comprising clustered chromosome ends) provides an ordered chromosome arrangement that facilitates pairing and recombination between homologous chromosomes. In the protist Tetrahymena thermophila, the meiotic prophase nucleus stretches enormously, and chromosomes assume a bouquet-like arrangement in which telomeres and centromeres are attached to opposite poles of the nucleus. We have identified and characterized three meiosis-specific genes [meiotic nuclear elongation 1-3 (MELG1-3)] that control nuclear elongation, and centromere and telomere clustering. The Melg proteins interact with cytoskeletal and telomere-associated proteins, and probably repurpose them for reorganizing the meiotic prophase nucleus. A lack of sequence similarity between the Tetrahymena proteins responsible for telomere clustering and bouquet proteins of other organisms suggests that the Tetrahymena bouquet is analogous, rather than homologous, to the conserved eukaryotic bouquet. We also report that centromere clustering is more important than telomere clustering for homologous pairing. Therefore, we speculate that centromere clustering may have been the primordial mechanism for chromosome pairing in early eukaryotes.

Meiotic CENP-C is a shepherd: bridging the space between the centromere and the kinetochore in time and space

While many of the proteins involved in the mitotic centromere and kinetochore are conserved in meiosis, they often gain a novel function due to the unique needs of homolog segregation during meiosis I (MI). CENP-C is a critical component of the centromere for kinetochore assembly in mitosis. Recent work, however, has highlighted the unique features of meiotic CENP-C. Centromere establishment and stability require CENP-C loading at the centromere for CENP-A function. Pre-meiotic loading of proteins necessary for homolog recombination as well as cohesion also rely on CENP-C, as do the main scaffolding components of the kinetochore. Much of this work relies on new technologies that enable in vivo analysis of meiosis like never before. Here, we strive to highlight the unique role of this highly conserved centromere protein that loads on to centromeres prior to M-phase onset, but continues to perform critical functions through chromosome segregation. CENP-C is not merely a structural link between the centromere and the kinetochore, but also a functional one joining the processes of early prophase homolog synapsis to late metaphase kinetochore assembly and signaling.

CDK Regulation of Meiosis: Lessons from S. cerevisiae and S. pombe

Meiotic progression requires precise orchestration, such that one round of DNA replication is followed by two meiotic divisions. The order and timing of meiotic events is controlled through the modulation of the phosphorylation state of proteins. Key components of this phospho-regulatory system include cyclin-dependent kinase (CDK) and its cyclin regulatory subunits. Over the past two decades, studies in budding and fission yeast have greatly informed our understanding of the role of CDK in meiotic regulation. In this review, we provide an overview of how CDK controls meiotic events in both budding and fission yeast. We discuss mechanisms of CDK regulation through post-translational modifications and changes in the levels of cyclins. Finally, we highlight the similarities and differences in CDK regulation between the two yeast species. Since CDK and many meiotic regulators are highly conserved, the findings in budding and fission yeasts have revealed conserved mechanisms of meiotic regulation among eukaryotes.

Meiotic chromosome movement: what's lamin got to do with it?

Active meiotic chromosome movements are a universally conserved feature. They occur at the early stages of prophase of the first meiotic division and support the chromosome pairing process by (1) efficiently installing the synaptonemal complex between homologous chromosomes, (2) discouraging inadvertent chromosome interactions and (3) bringing homologous chromosomes into proximity. Chromosome movements are driven by forces in the cytoplasm, which are passed on to chromosome ends attached to the nuclear periphery by nuclear-membrane-spanning protein modules. In this extra view, we highlight our recent studies into the role of the nuclear lamina during this process to emphasize that it is a highly conserved structure in metazoans. The nuclear lamina forms a rigid proteinaceous network that underlies the inner nuclear membrane to provide stability to the nucleus. Misdemeanors of the nuclear lamina during meiosis has deleterious consequences for the viability and health of the offspring, highlighting the importance of a functional nuclear lamina during this cell cycle stage.

Abbreviations: DSB: DNA double strand break; LEM: LAP2, Emerin, MAN1; LINC: LInker of the Nucleoskeleton and Cytoskeleton; RPM: rapid prophase movement; SUN/KASH: Sad1p, UNC-84/Klarsicht, ANC-1, Syne Homology

"The nuclear envelope, a meiotic jack-of-all-trades"

The nucleus is one of the membrane-bound organelles that are a distinguishing feature between eukaryotes and prokaryotes. During meiosis, the nuclear envelope takes on functions beyond separating the nucleoplasm from the cytoplasm. These include associations with meiotic chromosomes to mediate pairing, being a sensor for recombination progression, and supportive of enormous nuclear growth during oocyte formation. In this review, we highlight recent results that have contributed to our understanding of meiotic nuclear envelope function and dynamics.

Centromeric SMC1 promotes centromere clustering and stabilizes meiotic homolog pairing

During meiosis, each chromosome must selectively pair and synapse with its own unique homolog to enable crossover formation and subsequent segregation. How homolog pairing is maintained in early meiosis to ensure synapsis occurs exclusively between homologs is unknown. We aimed to further understand this process by examining the meiotic defects of a unique Drosophila mutant, Mcm5^A7. We found that Mcm5^A7 mutants are proficient in homolog pairing at meiotic onset yet fail to maintain pairing as meiotic synapsis ensues, causing seemingly normal synapsis between non-homologous loci. This pairing defect corresponds with a reduction of SMC1-dependent centromere clustering at meiotic onset. Overexpressing SMC1 in this mutant significantly restores centromere clustering, homolog pairing, and crossover formation. These data indicate that the initial meiotic pairing of homologs is not sufficient to yield synapsis exclusively between homologs and provide a model in which meiotic homolog pairing must be stabilized by centromeric SMC1 to ensure proper synapsis.

Sexually reproducing organisms must produce gametes (sperm and eggs) that have one copy of each chromosome. This is accomplished through a special cell division called meiosis. Each chromosome replicates to generate identical sister chromatids, then finds and pairs with its unique partner chromosome. A well-regulated recombination process then generates crossovers between paired maternal/paternal partners; these crossovers ensure accurate chromosome segregation in meiosis. The pairing process is very poorly understood. The Drosophila melanogaster (fruit fly) Mcm5^A7 mutation was previously shown to reduce crossovers but we show here that this is due to defects in meiotic chromosome pairing. We trace the primary defect to failure to load cohesins, which hold sister chromatids together but have additional roles in meiosis, at the centromere–the region that will later direct chromosome segregation. Thus, defects in centromeric cohesion lead to loss of chromosome pairing and loss of recombination along the arms of the chromosomes, and ultimately loss of fidelity during chromosome segregation.

Meiosis: the chromosomal foundation of reproduction

Meiosis is the chromosomal foundation of reproduction, with errors in this important process leading to aneuploidy and/or infertility. In this review celebrating the 50th anniversary of the founding of the Society for the Study of Reproduction, the important chromosomal structures and dynamics contributing to genomic integrity across generations are highlighted. Critical unsolved biological problems are identified, and the advances that will lead to their ultimate resolution are predicted.

Drawing and the dynamic nature of living systems

Representing the dynamic nature of biological processes is a challenge. This article describes a collaborative project in which the authors – a philosopher of biology, an artist and a cell biologist – explore how best to represent the entire process of cell division in one connected image. This involved a series of group Drawing Labs, one-to-one sessions, and discussions between the authors. The drawings generated during the collaboration were then reviewed by four experts in cell division. We propose that such an approach has value, both in communicating the dynamic nature of biological processes and in generating new insights and hypotheses that can be tested by artists and scientists.

Meiotic chromosomes in motion: a perspective from Mus musculus and Caenorhabditis elegans

Vigorous chromosome movement during the extended prophase of the first meiotic division is conserved in most eukaryotes. The movement is crucial for the faithful segregation of homologous chromosomes into daughter cells, and thus for fertility. A prerequisite for meiotic chromosome movement is the stable and functional attachment of telomeres or chromosome ends to the nuclear envelope and their cytoplasmic coupling to the cytoskeletal forces responsible for generating movement. Important advances in understanding the components, mechanisms, and regulation of chromosome end attachment and movement have recently been made. This review focuses on insights gained from experiments into two major metazoan model organisms: the mouse, Mus musculus, and the nematode, Caenorhabditis elegans.

Meiotic drive of female-inherited supernumerary chromosomes in a pathogenic fungus

Meiosis is a key cellular process of sexual reproduction that includes pairing of homologous sequences. In many species however, meiosis can also involve the segregation of supernumerary chromosomes, which can lack a homolog. How these unpaired chromosomes undergo meiosis is largely unknown. In this study we investigated chromosome segregation during meiosis in the haploid fungus Zymoseptoria tritici that possesses a large complement of supernumerary chromosomes. We used isogenic whole chromosome deletion strains to compare meiotic transmission of chromosomes when paired and unpaired. Unpaired chromosomes inherited from the male parent as well as paired supernumerary chromosomes in general showed Mendelian inheritance. In contrast, unpaired chromosomes inherited from the female parent showed non-Mendelian inheritance but were amplified and transmitted to all meiotic products. We concluded that the supernumerary chromosomes of Z. tritici show a meiotic drive and propose an additional feedback mechanism during meiosis, which initiates amplification of unpaired female-inherited chromosomes.

A cytological approach to studying meiotic recombination and chromosome dynamics in Arabidopsis thaliana male meiocytes in three dimensions

During meiotic prophase I chromosomes undergo dramatic conformational changes that accompany chromosome condensation, pairing and recombination between homologs. These changes include the anchoring of telomeres to the nuclear envelope and their clustering to form a bouquet. In plants, these events have been studied and illustrated in intact meiocytes of species with large genomes. Arabidopsis thaliana is an excellent genetic model in which major molecular pathways that control synapsis and recombination between homologs have been uncovered. Yet the study of chromosome dynamics is hampered by current cytological methods that disrupt the three-dimensional (3D) architecture of the nucleus. Here we set up a protocol to preserve the 3D configuration of A. thaliana meiocytes. We showed that this technique is compatible with the use of a variety of antibodies that label structural and recombination proteins and were able to highlight the presence of clustered synapsis initiation centers at the nuclear periphery. By using fluorescence in situ hybridization we also studied the behavior of chromosomes during pre-meiotic G₂ and prophase I, revealing the existence of a telomere bouquet during A. thaliana male meiosis. In addition we showed that the number of telomeres in a bouquet and its volume vary greatly, thus revealing the complexity of telomere behavior during meiotic prophase I. Finally, by using probes that label subtelomeric regions of individual chromosomes, we revealed differential localization behaviors of chromosome ends. Our protocol opens new areas of research for investigating chromosome dynamics in A. thaliana meiocytes.

Meta-analysis of chromosome-scale crossover rate variation in eukaryotes and its significance to evolutionary genomics

Understanding the distribution of crossovers along chromosomes is crucial to evolutionary genomics because the crossover rate determines how strongly a genome region is influenced by natural selection on linked sites. Nevertheless, generalities in the chromosome-scale distribution of crossovers have not been investigated formally. We fill this gap by synthesizing joint information on genetic and physical maps across 62 animal, plant and fungal species. Our quantitative analysis reveals a strong and taxonomically widespread reduction of the crossover rate in the centre of chromosomes relative to their peripheries. We demonstrate that this pattern is poorly explained by the position of the centromere, but find that the magnitude of the relative reduction in the crossover rate in chromosome centres increases with chromosome length. That is, long chromosomes often display a dramatically low crossover rate in their centre, whereas short chromosomes exhibit a relatively homogeneous crossover rate. This observation is compatible with a model in which crossover is initiated from the chromosome tips, an idea with preliminary support from mechanistic investigations of meiotic recombination. Consequently, we show that organisms achieve a higher genome-wide crossover rate by evolving smaller chromosomes. Summarizing theory and providing empirical examples, we finally highlight that taxonomically widespread and systematic heterogeneity in crossover rate along chromosomes generates predictable broad-scale trends in genetic diversity and population differentiation by modifying the impact of natural selection among regions within a genome. We conclude by emphasizing that chromosome-scale heterogeneity in crossover rate should urgently be incorporated into analytical tools in evolutionary genomics, and in the interpretation of resulting patterns.

Stretching, scrambling, piercing and entangling: Challenges for telomeres in mitotic and meiotic chromosome segregation

The consequences of telomere loss or dysfunction become most prominent when cells enter the nuclear division stage of the cell cycle. At this climactic stage when chromosome segregation occurs, telomere fusions or entanglements can lead to chromosome breakage, wreaking havoc on genome stability. Here we review recent progress in understanding the mechanisms of detangling and breaking telomere associations at mitosis, as well as the unique ways in which telomeres are processed to allow regulated sister telomere separation. Moreover, we discuss unexpected roles for telomeres in orchestrating nuclear envelope breakdown and spindle formation, crucial processes for nuclear division. Finally, we discuss the discovery that telomeres create microdomains in the nucleus that are conducive to centromere assembly, cementing the unexpectedly influential role of telomeres in mitosis.

Nuclear Envelope-Associated Chromosome Dynamics during Meiotic Prophase I

Chromosome dynamics during meiotic prophase I are associated with a series of major events such as chromosomal reorganization and condensation, pairing/synapsis and recombination of the homologs, and chromosome movements at the nuclear envelope (NE). The NE is the barrier separating the nucleus from the cytoplasm and thus plays a central role in NE-associated chromosomal movements during meiosis. Previous studies have shown in various species that NE-linked chromosome dynamics are actually driven by the cytoskeleton. The linker of nucleoskeleton and cytoskeleton (LINC) complexes are important constituents of the NE that facilitate in the transfer of cytoskeletal forces across the NE to individual chromosomes. The LINCs consist of the inner and outer NE proteins Sad1/UNC-84 (SUN), and Klarsicht/Anc-1/Syne (KASH) domain proteins. Meiosis-specific adaptations of the LINC components and unique modifications of the NE are required during chromosomal movements. Nonetheless, the actual role of the NE in chromosomic dynamic movements in plants remains elusive. This review summarizes the findings of recent studies on meiosis-specific constituents and modifications of the NE and corresponding nucleoplasmic/cytoplasmic adaptors being involved in NE-associated movement of meiotic chromosomes, as well as describes the potential molecular network of transferring cytoplasm-derived forces into meiotic chromosomes in model organisms. It helps to gain a better understanding of the NE-associated meiotic chromosomal movements in plants.

Nuclear movement in fungi

Nuclear movement within a cell occurs in a variety of eukaryotic organisms including yeasts and filamentous fungi. Fungal molecular genetic studies identified the minus-end-directed microtubule motor cytoplasmic dynein as a critical protein for nuclear movement or orientation of the mitotic spindle contained in the nucleus. Studies in the budding yeast first indicated that dynein anchored at the cortex via its anchoring protein Num1 exerts pulling force on an astral microtubule to orient the anaphase spindle across the mother-daughter axis before nuclear division. Prior to anaphase, myosin V interacts with the plus end of an astral microtubule via Kar9-Bim1/EB1 and pulls the plus end along the actin cables to move the nucleus/spindle close to the bud neck. In addition, pushing or pulling forces generated from cortex-linked polymerization or depolymerization of microtubules drive nuclear movements in yeasts and possibly also in filamentous fungi. In filamentous fungi, multiple nuclei within a hyphal segment undergo dynein-dependent back-and-forth movements and their positioning is also influenced by cytoplasmic streaming toward the hyphal tip. In addition, nuclear movement occurs at various stages of fungal development and fungal infection of plant tissues. This review discusses our current understanding on the mechanisms of nuclear movement in fungal organisms, the importance of nuclear positioning and the regulatory strategies that ensure the proper positioning of nucleus/spindle.

Telomeric TERB1-TRF1 interaction is crucial for male meiosis

During meiotic prophase, the meiosis-specific telomere-binding protein TERB1 regulates chromosome movement required for homologous pairing and recombination by interacting with the telomeric shelterin subunit TRF1. Here, we report the crystal structure of the TRF1-binding motif of human TERB1 in complex with the TRFH domain of TRF1. Notably, specific disruption of the TERB1-TRF1 interaction by a point mutation in the mouse Terb1 gene results in infertility only in males. We find that this mutation causes an arrest in the zygotene-early pachytene stage and mild telomere abnormalities of autosomes but unpaired X and Y chromosomes in pachytene, leading to massive spermatocyte apoptosis. We propose that the loss of telomere structure mediated by the TERB1-TRF1 interaction significantly affects homologous pairing of the telomere-adjacent pseudoautosomal region (PAR) of the X and Y chromosomes in mouse spermatocytes. Our findings uncover a specific mechanism of telomeres that surmounts the unique challenges of mammalian X-Y pairing in meiosis.

Big Lessons from Little Yeast: Budding and Fission Yeast Centrosome Structure, Duplication, and Function

Centrosomes are a functionally conserved feature of eukaryotic cells that play an important role in cell division. The conserved γ-tubulin complex organizes spindle and astral microtubules, which, in turn, separate replicated chromosomes accurately into daughter cells. Like DNA, centrosomes are duplicated once each cell cycle. Although in some cell types it is possible for cell division to occur in the absence of centrosomes, these divisions typically result in defects in chromosome number and stability. In single-celled organisms such as fungi, centrosomes [known as spindle pole bodies (SPBs)] are essential for cell division. SPBs also must be inserted into the membrane because fungi undergo a closed mitosis in which the nuclear envelope (NE) remains intact. This poorly understood process involves events similar or identical to those needed for de novo nuclear pore complex assembly. Here, we review how analysis of fungal SPBs has advanced our understanding of centrosomes and NE events.

The proteasome enters the meiotic prophase fray

The segregation of homologous chromosomes in meiosis depends on their ability to locate one another in the nucleus and establish a physical association through crossing over. A tightly regulated number of crossovers (COs) emerges following repair of induced DNA double-strand breaks by homologous recombination (HR), but the process of how HR intermediates transition into COs is still poorly understood. Two recent studies by Ahuja et al. and Rao et al. have revealed a role for chromosomally localized proteasomes in choreographing both homologous chromosome pairing and the evolution of HR intermediates into segregation-competent COs. Using chemical inhibition of the proteasome and mutant analysis, the collective data reveal conserved functions for both the proteasome and a family of E3 ligases that can direct or compete with its activity in ensuring CO formation. Here, we review these findings and the impact of the discovery that protein modification dynamics and proteasomal activity cooperate to regulate key meiotic processes.

Dual effect of the wheat Ph1 locus on chromosome synapsis and crossover

Allopolyploids must possess a mechanism for facilitating synapsis and crossover (CO) between homologues, in preference to homoeologues (related chromosomes), to ensure successful meiosis. In hexaploid wheat, the Ph1 locus has a major effect on the control of these processes. Studying a wheat mutant lacking Ph1 provides an opportunity to explore the underlying mechanisms. Recently, it was proposed that Ph1 stabilises wheat during meiosis, both by promoting homologue synapsis during early meiosis and preventing MLH1 sites on synapsed homoeologues from becoming COs later in meiosis. Here, we explore these two effects and demonstrate firstly that whether or not Ph1 is present, synapsis between homoeologues does not take place during the telomere bouquet stage, with only homologous synapsis taking place during this stage. Furthermore, in wheat lacking Ph1, overall synapsis is delayed with respect to the telomere bouquet, with more synapsis occurring after the bouquet stage, when homoeologous synapsis is also possible. Secondly, we show that in the absence of Ph1, we can increase the number of MLH1 sites progressing to COs by altering environmental growing conditions; we show that higher nutrient levels in the soil or lower temperatures increase the level of both homologue and homoeologue COs. These observations suggest opportunities to improve the exploitation of the Ph1 wheat mutant in breeding programmes.

Chromosomes Orchestrate Their Own Liberation: Nuclear Envelope Disassembly

The mammalian nuclear division cycle is coordinated with nuclear envelope breakdown (NEBD), in which the entire nuclear envelope (NE) is dissolved to allow chromosomes to access their segregation vehicle, the spindle. In other eukaryotes, complete NEBD is replaced by localized disassembly or remodeling of the NE. Although the molecular mechanisms controlling NE disassembly are incompletely understood, coordinated cycles of modification of specific NE components drive breakdown. Here, we review the current state of knowledge regarding NE disassembly and argue for a role of chromosome-NE contacts in triggering initiation of NE disassembly and thereby, cell division.

Control of Meiotic Crossovers: From Double-Strand Break Formation to Designation

Meiosis, the mechanism of creating haploid gametes, is a complex cellular process observed across sexually reproducing organisms. Fundamental to meiosis is the process of homologous recombination, whereby DNA double-strand breaks are introduced into the genome and are subsequently repaired to generate either noncrossovers or crossovers. Although homologous recombination is essential for chromosome pairing during prophase I, the resulting crossovers are critical for maintaining homolog interactions and enabling accurate segregation at the first meiotic division. Thus, the placement, timing, and frequency of crossover formation must be exquisitely controlled. In this review, we discuss the proteins involved in crossover formation, the process of their formation and designation, and the rules governing crossovers, all within the context of the important landmarks of prophase I. We draw together crossover designation data across organisms, analyze their evolutionary divergence, and propose a universal model for crossover regulation.

Mitotic Nuclear Envelope Breakdown and Spindle Nucleation Are Controlled by Interphase Contacts between Centromeres and the Nuclear Envelope

Faithful genome propagation requires coordination between nuclear envelope (NE) breakdown, spindle formation, and chromosomal events. The conserved linker of nucleoskeleton and cytoskeleton (LINC) complex connects fission yeast centromeres and the centrosome, across the NE, during interphase. During meiosis, LINC connects the centrosome with telomeres rather than centromeres. We previously showed that loss of telomere-LINC contacts compromises meiotic spindle formation. Here, we define the precise events regulated by telomere-LINC contacts and address the analogous possibility that centromeres regulate mitotic spindle formation. We develop conditionally inactivated LINC complexes in which the conserved SUN-domain protein Sad1 remains stable but severs interphase centromere-LINC contacts. Strikingly, the loss of such contacts abolishes spindle formation. We pinpoint the defect to a failure in the partial NE breakdown required for centrosome insertion into the NE, a step analogous to mammalian NE breakdown. Thus, interphase chromosome-LINC contacts constitute a cell-cycle control device linking nucleoplasmic and cytoplasmic events.

Recombination-independent recognition of DNA homology for repeat-induced point mutation

Numerous cytogenetic observations have shown that homologous chromosomes (or individual chromosomal loci) can engage in specific pairing interactions in the apparent absence of DNA breakage and recombination, suggesting that canonical recombination-mediated mechanisms may not be the only option for sensing DNA/DNA homology. One proposed mechanism for such recombination-independent homology recognition involves direct contacts between intact double-stranded DNA molecules. The strongest in vivo evidence for the existence of such a mechanism is provided by the phenomena of homology-directed DNA modifications in fungi, known as repeat-induced point mutation (RIP, discovered in Neurospora crassa) and methylation-induced premeiotically (MIP, discovered in Ascobolus immersus). In principle, Neurospora RIP can detect the presence of gene-sized DNA duplications irrespectively of their origin, underlying nucleotide sequence, coding capacity or relative, as well as absolute positions in the genome. Once detected, both sequence copies are altered by numerous cytosine-to-thymine (C-to-T) mutations that extend specifically over the duplicated region. We have recently shown that Neurospora RIP does not require MEI-3, the only RecA/Rad51 protein in this organism, consistent with a recombination-independent mechanism. Using an ultra-sensitive assay for RIP mutation, we have defined additional features of this process. We have shown that RIP can detect short islands of homology of only three base-pairs as long as many such islands are arrayed with a periodicity of 11 or 12 base-pairs along a pair of DNA molecules. While the presence of perfect homology is advantageous, it is not required: chromosomal segments with overall sequence identity of only 35-36 % can still be recognized by RIP. Importantly, in order for this process to work efficiently, participating DNA molecules must be able to co-align along their lengths. Based on these findings, we have proposed a model, in which sequence homology is detected by direct interactions between slightly-extended double-stranded DNAs. As a next step, it will be important to determine if the uncovered principles also apply to other processes that involve recombination-independent interactions between homologous chromosomal loci in vivo as well as to protein-free DNA/DNA interactions that were recently observed under biologically relevant conditions in vitro.

Meiotic chromosome mobility in fission yeast is resistant to environmental stress

The formation of healthy gametes requires pairing of homologous chromosomes (homologs) as a prerequisite for their correct segregation during meiosis. Initially, homolog alignment is promoted by meiotic chromosome movements feeding into intimate homolog pairing by homologous recombination and/or synaptonemal complex formation. Meiotic chromosome movements in the fission yeast, Schizosaccharomyces pombe, depend on astral microtubule dynamics that drag the nucleus through the zygote; known as horsetail movement. The response of microtubule-led meiotic chromosome movements to environmental stresses such as ionizing irradiation (IR) and associated reactive oxygen species (ROS) is not known. Here, we show that, in contrast to budding yeast, the horsetail movement is largely radiation-resistant, which is likely mediated by a potent antioxidant defense. IR exposure of sporulating S. pombe cells induced misrepair and irreparable DNA double strand breaks causing chromosome fragmentation, missegregation and gamete death. Comparing radiation outcome in fission and budding yeast, and studying meiosis with poisoned microtubules indicates that the increased gamete death after IR is innate to fission yeast. Inhibition of meiotic chromosome mobility in the face of IR failed to influence the course of DSB repair, indicating that paralysis of meiotic chromosome mobility in a genotoxic environment is not a universal response among species.

Finding a place in the SUN: telomere maintenance in a diverse nuclear landscape

Telomeres function in the context of a complex nuclear milieu in which telomeres tend to occupy distinct subnuclear regions. Indeed, regulation of the subnuclear positioning of telomeres is conserved from yeast to human, raising the age-old question: to what extent is location important for function? In mitotically dividing cells, the positioning of telomeres affects their epigenetic state and influences telomere processing and synthesis. In meiotic cells, telomere location is important for homologue pairing, centromere assembly and spindle formation. Here we focus on recent insights into the functions of telomere positioning in maintaining genome integrity.

A few of our favorite things: Pairing, the bouquet, crossover interference and evolution of meiosis

Meiosis presents many important mysteries that await elucidation. Here we discuss two such aspects. First, we consider how the current meiotic program might have evolved. We emphasize the central feature of this program: how homologous chromosomes find one another ("pair") so as to create the connections required for their regular segregation at Meiosis I. Points of emphasis include the facts that: (i) the classical "bouquet stage" is not required for initial homolog contacts in the current evolved meiotic program; and (ii) diverse observations point to commonality between molecules that mediate meiotic inter-homolog interactions and molecules that are integral to centromeres and/or to microtubule organizing centers (a.k.a. spindle pole bodies or centrosomes). Second, we provide an overview of the classical phenomenon of crossover (CO) interference in an effort to bridge the gap between description on the one hand versus logic and mechanism on the other.

Models of germ cell development and their application for toxicity studies

Germ cells are unique in their ability to transfer genetic information and traits from generation to generation. As such, the proper development of germ cells and the integrity of their genome are paramount to the health of organisms and the survival of species. Germ cells are also exquisitely sensitive to environmental influences although the testing of germ cell toxicity, especially in females, has proven particularly challenging. In this review, we first describe the remarkable odyssey of germ cells in mammals, with an emphasis on the female germline, from their initial specification early during embryogenesis to the generation of mature gametes in adults. We also describe the current methods used in germ cell toxicity testing and their limitations in examining the complex features of mammalian germ cell development. To bypass these challenges, we propose the use of alternative model systems such as Saccharomyces cerevisiae, Drosophila melanogaster, Caenorhabditis elegans, and in vitro germ cell methods that have distinct advantages over traditional toxicity models. We discuss the benefits and limitations of each approach, their application to germ cell toxicity studies, and the need for computational approaches to maximize the usefulness of these models. Together, the inclusion of these alternative germ cell toxicity models will be invaluable for the examination of stages not easily accessible in mammals as well as the large scale, high-throughput investigation of germ cell toxicity.

Centromere Associations in Meiotic Chromosome Pairing

Production of gametes of halved ploidy for sexual reproduction requires a specialized cell division called meiosis. The fusion of two gametes restores the original ploidy in the new generation, and meiosis thus stabilizes ploidy across generations. To ensure balanced distribution of chromosomes, pairs of homologous chromosomes (homologs) must recognize each other and pair in the first meiotic division. Recombination plays a key role in this in most studied species, but it is not the only actor and particular chromosomal regions are known to facilitate the meiotic pairing of homologs. In this review, we focus on the roles of centromeres and in particular on the clustering and pairwise associations of nonhomologous centromeres that precede stable pairing between homologs. Although details vary from species to species, it is becoming increasingly clear that these associations play active roles in the meiotic chromosome pairing process, analogous to those of the telomere bouquet.

CEP63 deficiency promotes p53-dependent microcephaly and reveals a role for the centrosome in meiotic recombination

CEP63 is a centrosomal protein that facilitates centriole duplication and is regulated by the DNA damage response. Mutations in CEP63 cause Seckel syndrome, a human disease characterized by microcephaly and dwarfism. Here we demonstrate that Cep63 deficient mice recapitulate Seckel syndrome pathology. The attrition of neural progenitor cells involves p53-dependent cell death and brain size is rescued by the deletion of p53. Cell death is not the result of an aberrant DNA damage response but is triggered by centrosome-based mitotic errors. In addition, Cep63 loss severely impairs meiotic recombination, leading to profound male infertility. Cep63 deficient spermatocytes display numerical and structural centrosome aberrations, chromosome entanglements and defective telomere clustering, suggesting that a reduction in centrosome-mediated chromosome movements underlies recombination failure. Our results provide novel insight into the molecular pathology of microcephaly and establish a role for the centrosome in meiotic recombination.