Heteroduplex DNA formation and homolog pairing in yeast meiotic mutants

Temporal and Functional Relationship between Synaptonemal Complex Morphogenesis and Recombination during Meiosis

During prophase of meiosis I, programmed double strand breaks (DSBs) are processed into crossovers, a critical requirement for segregation of homologous chromosomes (homologs) and genome haploidization in sexually reproducing organisms. Crossovers form via homologous recombination in close temporospatial association with morphogenesis of the synaptonemal complex (SC), a proteinaceous structure that connects paired homologs along their length during the pachytene stage. Synapsis and recombination are a paradigm for the interplay between higher order chromosome structure and DNA metabolism, yet their temporal and functional relationship remains poorly understood. Probing linkage between these processes in budding yeast, we show that SC assembly is associated with a distinct threshold number of unstable D-loops. The transition from bona fide paranemic D-loops to plectonemic DSB single end invasions (SEIs) is completed during midpachynema, when the SC is fully assembled. Double Holliday junctions (dHJs) form at the time of desynapsis and are resolved into crossovers during diplonema. The SC central element component Zip1 shepherds recombination through three transitions, including DSB first end strand exchange and second end capture, as well as dHJ resolution. Zip1 mediates SEI formation independent of its polymerization whereas precocious Zip1 assembly interferes with double Holliday junction resolution. Together, our findings indicate that the synaptonemal complex controls recombination while assembled but also beyond its disassembly, possibly by establishing spatial constraints at recombination sites.

Meiosis in budding yeast

Meiosis is a specialized cell division program that is essential for sexual reproduction. The two meiotic divisions reduce chromosome number by half, typically generating haploid genomes that are packaged into gametes. To achieve this ploidy reduction, meiosis relies on highly unusual chromosomal processes including the pairing of homologous chromosomes, assembly of the synaptonemal complex, programmed formation of DNA breaks followed by their processing into crossovers, and the segregation of homologous chromosomes during the first meiotic division. These processes are embedded in a carefully orchestrated cell differentiation program with multiple interdependencies between DNA metabolism, chromosome morphogenesis, and waves of gene expression that together ensure the correct number of chromosomes is delivered to the next generation. Studies in the budding yeast Saccharomyces cerevisiae have established essentially all fundamental paradigms of meiosis-specific chromosome metabolism and have uncovered components and molecular mechanisms that underlie these conserved processes. Here, we provide an overview of all stages of meiosis in this key model system and highlight how basic mechanisms of genome stability, chromosome architecture, and cell cycle control have been adapted to achieve the unique outcome of meiosis.

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

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

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

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

Flanking heterozygosity influences the relative probability of different base substitutions in humans

Understanding when, where and which mutations are mostly likely to occur impacts many areas of evolutionary biology, from genetic diseases to phylogenetic reconstruction. Africans and non-African humans differ in the mutability of different triplet base combinations. Africans and non-Africans also differ in mutation rate, possibly because heterozygosity is mutagenic, such that diversity lost when humans expanded out of Africa also lowered the mutation rate. I show that these phenomena are linked: as flanking heterozygosity increases, some triplets become progressively more mutable while others become less so. Africans and non-African show near-identical patterns of dependence on heterozygosity. Thus, the striking differences in triplet mutation frequency between Africans and non-Africans, at least in part, seem to be an emergent property, driven by the way changes in heterozygosity 'out of Africa' have differentially impacted the mutability of different triplets. As heterozygosity decreased, the mutation spectrum outside Africa became enriched for triplet mutations that are favoured by low heterozygosity while those favoured by high heterozygosity became relatively rarer.

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

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

Control of meiotic pairing and recombination by chromosomally tethered 26S proteasome

During meiosis, paired homologous chromosomes (homologs) become linked via the synaptonemal complex (SC) and crossovers. Crossovers mediate homolog segregation and arise from self-inflicted double-strand breaks (DSBs). Here, we identified a role for the proteasome, the multisubunit protease that degrades proteins in the nucleus and cytoplasm, in homolog juxtaposition and crossing over. Without proteasome function, homologs failed to pair and instead remained associated with nonhomologous chromosomes. Although dispensable for noncrossover formation, a functional proteasome was required for a coordinated transition that entails SC assembly between longitudinally organized chromosome axes and stable strand exchange of crossover-designated DSBs. Notably, proteolytic core and regulatory proteasome particles were recruited to chromosomes by Zip3, the ortholog of mammalian E3 ligase RNF212, and SC protein Zip1 . We conclude that proteasome functions along meiotic chromosomes are evolutionarily conserved.

Multiple Pairwise Analysis of Non-homologous Centromere Coupling Reveals Preferential Chromosome Size-Dependent Interactions and a Role for Bouquet Formation in Establishing the Interaction Pattern

During meiosis, chromosomes undergo a homology search in order to locate their homolog to form stable pairs and exchange genetic material. Early in prophase, chromosomes associate in mostly non-homologous pairs, tethered only at their centromeres. This phenomenon, conserved through higher eukaryotes, is termed centromere coupling in budding yeast. Both initiation of recombination and the presence of homologs are dispensable for centromere coupling (occurring in spo11 mutants and haploids induced to undergo meiosis) but the presence of the synaptonemal complex (SC) protein Zip1 is required. The nature and mechanism of coupling have yet to be elucidated. Here we present the first pairwise analysis of centromere coupling in an effort to uncover underlying rules that may exist within these non-homologous interactions. We designed a novel chromosome conformation capture (3C)-based assay to detect all possible interactions between non-homologous yeast centromeres during early meiosis. Using this variant of 3C-qPCR, we found a size-dependent interaction pattern, in which chromosomes assort preferentially with chromosomes of similar sizes, in haploid and diploid spo11 cells, but not in a coupling-defective mutant (spo11 zip1 haploid and diploid yeast). This pattern is also observed in wild-type diploids early in meiosis but disappears as meiosis progresses and homologous chromosomes pair. We found no evidence to support the notion that ancestral centromere homology plays a role in pattern establishment in S. cerevisiae post-genome duplication. Moreover, we found a role for the meiotic bouquet in establishing the size dependence of centromere coupling, as abolishing bouquet (using the bouquet-defective spo11 ndj1 mutant) reduces it. Coupling in spo11 ndj1 rather follows telomere clustering preferences. We propose that a chromosome size preference for centromere coupling helps establish efficient homolog recognition.

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

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

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

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

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

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

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

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

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

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.

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.

N-terminal disordered domain of Saccharomyces cerevisiae Hop1 protein is dispensable for DNA binding, bridging, and synapsis of double-stranded DNA molecules but is necessary for spore formation

The cytological architecture of the synaptonemal complex (SC), a meiosis-specific proteinaceous structure, is evolutionarily conserved among eukaryotes. However, little is known about the biochemical properties of SC components or the mechanisms underlying their roles in meiotic chromosome synapsis and recombination. Functional analysis of Saccharomyces cerevisiae Hop1, a key structural component of SC, has begun to reveal important insights into its function in interhomolog recombination. Previously, we showed that Hop1 is a structure-specific DNA-binding protein, exhibits higher binding affinity for the Holliday junction, and induces structural distortion at the core of the junction. Furthermore, Hop1 promotes DNA condensation and intra- and intermolecular synapsis between duplex DNA molecules. Here, we show that Hop1 possesses a modular domain organization, consisting of an intrinsically disordered N-terminal domain and a protease-resistant C-terminal domain (Hop1CTD). Furthermore, we found that Hop1CTD exhibits strong homotypic as well as heterotypic protein-protein interactions, and its biochemical activities were similar to those of the full-length Hop1 protein. However, Hop1CTD failed to complement the meiotic recombination defects of the Δhop1 strain, indicating that both N- and C-terminal domains of Hop1 are essential for meiosis and spore formation. Altogether, our findings reveal novel insights into the structure-function relationships of Hop1 and help to further our understanding of its role in meiotic chromosome synapsis and recombination.

Variation in heterozygosity predicts variation in human substitution rates between populations, individuals and genomic regions

The "heterozygote instability" (HI) hypothesis suggests that gene conversion events focused on heterozygous sites during meiosis locally increase the mutation rate, but this hypothesis remains largely untested. As humans left Africa they lost variability, which, if HI operates, should have reduced the mutation rate in non-Africans. Relative substitution rates were quantified in diverse humans using aligned whole genome sequences from the 1,000 genomes project. Substitution rate is consistently greater in Africans than in non-Africans, but only in diploid regions of the genome, consistent with a role for heterozygosity. Analysing the same data partitioned into a series of non-overlapping 2 Mb windows reveals a strong, non-linear correlation between the amount of heterozygosity lost "out of Africa" and the difference in substitution rate between Africans and non-Africans. Putative recent mutations, derived variants that occur only once among the 80 human chromosomes sampled, occur preferentially at the centre of 2 Kb windows that have elevated heterozygosity compared both with the same region in a closely related population and with an immediately adjacent region in the same population. More than half of all substitutions appear attributable to variation in heterozygosity. This observation provides strong support for HI with implications for many branches of evolutionary biology.

Multiple opposing constraints govern chromosome interactions during meiosis

Homolog pairing and crossing over during meiosis I prophase is required for accurate chromosome segregation to form euploid gametes. The repair of Spo11-induced double-strand breaks (DSB) using a homologous chromosome template is a major driver of pairing in many species, including fungi, plants, and mammals. Inappropriate pairing and crossing over at ectopic loci can lead to chromosome rearrangements and aneuploidy. How (or if) inappropriate ectopic interactions are disrupted in favor of allelic interactions is not clear. Here we used an in vivo "collision" assay in budding yeast to test the contributions of cohesion and the organization and motion of chromosomes in the nucleus on promoting or antagonizing interactions between allelic and ectopic loci at interstitial chromosome sites. We found that deletion of the cohesin subunit Rec8, but not other chromosome axis proteins (e.g. Red1, Hop1, or Mek1), caused an increase in homolog-nonspecific chromosome interaction, even in the absence of Spo11. This effect was partially suppressed by expression of the mitotic cohesin paralog Scc1/Mdc1, implicating Rec8's role in cohesion rather than axis integrity in preventing nonspecific chromosome interactions. Disruption of telomere-led motion by treating cells with the actin polymerization inhibitor Latrunculin B (Lat B) elevated nonspecific collisions in rec8Δ spo11Δ. Next, using a visual homolog-pairing assay, we found that the delay in homolog pairing in mutants defective for telomere-led chromosome motion (ndj1Δ or csm4Δ) is enhanced in Lat B-treated cells, implicating actin in more than one process promoting homolog juxtaposition. We suggest that multiple, independent contributions of actin, cohesin, and telomere function are integrated to promote stable homolog-specific interactions and to destabilize weak nonspecific interactions by modulating the elastic spring-like properties of chromosomes.

Meiotic development in Caenorhabditis elegans

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

Single-molecule DNA analysis reveals that yeast Hop1 protein promotes DNA folding and synapsis: implications for condensation of meiotic chromosomes

During meiosis, long-range interaction between homologous chromosomes is thought to be crucial for homology recognition, exchange of DNA strands, and production of normal haploid gametes. However, little is known about the identity of the proteins involved and the actual molecular mechanism(s) by which chromosomes recognize and recombine with their appropriate homologous partners. Single-molecule analyses have the potential to provide insights into our understanding of this fascinating and long-standing question. Using atomic force microscopy and magnetic tweezers techniques, we discovered that Hop1 protein, a key structural component of Saccharomyces cerevisiae synaptonemal complex, exhibits the ability to bridge noncontiguous DNA segments into intramolecular stem-loop structures in which the DNA segments appear to be fully synapsed within the filamentous protein stems. Additional evidence suggests that Hop1 folds DNA into rigid protein-DNA filaments and higher-order nucleoprotein structures. Importantly, Hop1 promotes robust intra- and intermolecular synapsis between double-stranded DNA molecules, suggesting that juxtaposition of DNA sequences may assist in strand exchange between homologues by recombination-associated proteins. Finally, the evidence from ensemble experiments is consistent with the notion that Hop1 causes rigidification of DNA molecules. These results provide the first direct evidence for long-range protein-mediated DNA-DNA synapsis, independent of crossover recombination, which is presumed to occur during meiotic recombination.

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.

Finding the correct partner: the meiotic courtship

Homologous chromosomes are usually separated at the entrance of meiosis; how they become paired is one of the outstanding mysteries of the meiotic process. Reduction of spacing between homologues makes possible the occurrence of chromosomal interactions leading to homology detection and the formation of bivalents. In many organisms, telomere-led chromosome movements are generated that bring homologues together. Additional movements produced by chromatin conformational changes at early meiosis may also facilitate homologous contacts. Organisms used in the study of meiosis show a surprising variety of strategies for homology detection. In dipterans, homologous chromosomes remain paired throughout most of development. Pairing seems to arise as a balance between promoter and suppressor pairing genes. Some fungi, plants and animals, use mechanisms based on recombinational interactions. Other mechanisms leading to homology search are recombination-independent and require specialized pairing sites. In the worm Caenorhabditis elegans, each chromosome carries a pairing center consisting of a chromosome-specific DNA-protein complex, and in the fission yeast Schizosaccharomyces pombe, the sme2 locus encodes a meiosis-specific non-coding RNA that mediates on homologous recognition. In addition, mismatch correction plays a relevant role, especially in polyploids, which evolved genetic systems that suppress pairing between non-homologous related (homoeologus) chromosomes.

Temporal characterization of homology-independent centromere coupling in meiotic prophase

BACKGROUND

Over the past thirty years several reports of the pairing or association of non-homologous centromeres during meiotic prophase have appeared in the literature. Recently, the homology-independent pairwise association of centromeres, termed centromere coupling, was also reported in budding yeast. It seems paradoxical that centromeres would pair with non-homologous partners during a process intended to align homologous chromosomes, yet the conservation of this phenomenon across a wide range of species suggests it may play an important role in meiosis.

PRINCIPAL FINDINGS

To better define the role of this phenomenon in budding yeast, experiments were preformed to place centromere coupling within the context of landmark meiotic events. Soon after the initiation of the meiotic program, centromeres were found to re-organize from a single cluster into non-homologous couples. Centromere coupling is detected as soon as chromosome replication is finished and persists while the recombination protein Dmc1 is loaded onto the chromosomes, suggesting that centromere coupling persists through the time of double strand break formation. In the absence of the synaptonemal complex component, Zip1, centromere coupling was undetectable, at all times examined, confirming the essential role of this protein on this process. Finally, the timely release of centromere coupling depends on the recombination-initiating enzyme, Spo11, suggesting a connection between events in homologous pairing/recombination and the regulation of centromere coupling.

CONCLUSIONS

Based on our results we propose a role for centromere coupling in blocking interactions between homologous centromeres as recombination initiation is taking place.

Evidence that MEK1 positively promotes interhomologue double-strand break repair

During meiosis there is an imperative to create sufficient crossovers for homologue segregation. This can be achieved during repair of programmed DNA double-strand breaks (DSBs), which are biased towards using a homologue rather than sister chromatid as a repair template. Various proteins contribute to this bias, one of which is a meiosis specific kinase Mek1. It has been proposed that Mek1 establishes the bias by creating a barrier to sister chromatid repair, as distinct from enforcing strand invasion with the homologue. We looked for evidence that Mek1 positively stimulates strand invasion of the homologue. This was done by analysing repair of DSBs induced by the VMA1-derived endonuclease (VDE) and flanked by directly repeated sequences that can be used for intrachromatid single-strand annealing (SSA). SSA competes with interhomologue strand invasion significantly more successfully when Mek1 function is lost. We suggest the increase in intrachromosomal SSA reflects an opportunistic default repair pathway due to loss of a MEK1 stimulated bias for strand invasion of the homologous chromosome. Making use of an inhibitor sensitive mek1-as1 allele, we found that Mek1 function influences the repair pathway throughout the first4–5 h of meiosis. Perhaps reflecting a particular need to create bias for successful interhomologue events before chromosome pairing is complete.

Heterozygosity and mutation rate: evidence for an interaction and its implications: the potential for meiotic gene conversions to influence both mutation rate and distribution

If natural selection chose where new mutations occur it might well favour placing them near existing polymorphisms, thereby avoiding disruption of areas that work while adding novelty to regions where variation is tolerated or even beneficial. Such a system could operate if heterozygous sites are recognised and 'repaired' during the initial stages of crossing over. Such repairs involve an extra round of DNA replication, providing an opportunity for further mutations, thereby raising the local mutation rate. If so, the changes in heterozygosity that occur when populations grow or shrink could feed back to modulate both the rate and the distribution of mutations. Here, I review evidence from isozymes, microsatellites and single nucleotide polymorphisms that this potential is realised in real populations. I then consider the likely implications, focusing particularly on how these processes might affect microsatellites, concluding that heterozygosity does impact on the rate and distribution of mutations.

Prelude to a division

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

Components of the RNAi machinery that mediate long-distance chromosomal associations are dispensable for meiotic and early somatic homolog pairing in Drosophila melanogaster

Homolog pairing is indispensable for the proper segregation of chromosomes in meiosis but the mechanism by which homologs uniquely pair with each other is poorly understood. In Drosophila, somatic chromosomes also undergo full homolog pairing by an unknown mechanism. It has been recently demonstrated that both insulator function and somatic long-distance interactions between Polycomb response elements (PREs) are stabilized by the RNAi machinery in Drosophila. This suggests the possibility that long-distance pairing interactions between homologs, either during meiosis or in the soma, may be stabilized by a similar mechanism. To test this hypothesis, we have characterized meiotic and early somatic chromosome pairing of homologous chromosomes in flies that are mutant for various components of the RNAi machinery. Despite the identification of a novel role for the piRNA machinery in meiotic progression and synaptonemal complex (SC) assembly, we have found that the components of the RNAi machinery that mediate long-distance chromosomal interactions are dispensable for homologous chromosome pairing. Thus, there appears to be at least two mechanisms that bring homologous sequences together within the nucleus: those that act between dispersed homologous sequences and those that act to align and pair homologous chromosomes.

Sites of recombination are local determinants of meiotic homolog pairing in Saccharomyces cerevisiae

Trans-acting factors involved in the early meiotic recombination pathway play a major role in promoting homolog pairing during meiosis in many plants, fungi, and mammals. Here we address whether or not allelic sites have higher levels of interaction when in cis to meiotic recombination events in the budding yeast Saccharomyces cerevisiae. We used Cre/loxP site-specific recombination to genetically measure the magnitude of physical interaction between loxP sites located at allelic positions on homologous chromosomes during meiosis. We observed nonrandom coincidence of Cre-mediated loxP recombination events and meiotic recombination events when the two occurred at linked positions. Further experiments showed that a subset of recombination events destined to become crossover products increased the frequency of nearby Cre-mediated loxP recombination. Our results support a simple physical model of homolog pairing in budding yeast, where recombination at numerous genomic positions generally serves to loosely coalign homologous chromosomes, while crossover-bound recombination intermediates locally stabilize interactions between allelic sites.

Phosphorylation of the axial element protein Hop1 by Mec1/Tel1 ensures meiotic interhomolog recombination

An essential feature of meiosis is interhomolog recombination whereby a significant fraction of the programmed meiotic double-strand breaks (DSBs) is repaired using an intact homologous non-sister chromatid rather than a sister. Involvement of Mec1 and Tel1, the budding yeast homologs of the mammalian ATR and ATM kinases, in meiotic interhomlog bias has been implicated, but the mechanism remains elusive. Here, we demonstrate that Mec1 and Tel1 promote meiotic interhomolog recombination by targeting the axial element protein Hop1. Without Mec1/Tel1 phosphorylation of Hop1, meiotic DSBs are rapidly repaired via a Dmc1-independent intersister repair pathway, resulting in diminished interhomolog crossing-over leading to spore lethality. We find that Mec1/Tel1-mediated phosphorylation of Hop1 is required for activation of Mek1, a meiotic paralogue of the DNA-damage effector kinase, Rad53p/CHK2. Thus, Hop1 is a meiosis-specific adaptor protein of the Mec1/Tel1 signaling pathway that ensures interhomolog recombination by preventing Dmc1-independent repair of meiotic DSBs.

A novel nonnull ZIP1 allele triggers meiotic arrest with synapsed chromosomes in Saccharomyces cerevisiae

During meiotic prophase, assembly of the synaptonemal complex (SC) brings homologous chromosomes into close apposition along their lengths. The Zip1 protein is a major building block of the SC in Saccharomyces cerevisiae. In the absence of Zip1, SC fails to form, cells arrest or delay in meiotic prophase (depending on strain background), and crossing over is reduced. We created a novel allele of ZIP1, zip1-4LA, in which four leucine residues in the central coiled-coil domain have been replaced by alanines. In the zip1-4LA mutant, apparently normal SC assembles with wild-type kinetics; however, crossing over is delayed and decreased compared to wild type. The zip1-4LA mutant undergoes strong checkpoint-induced arrest in meiotic prophase; the defect in cell cycle progression is even more severe than that of the zip1 null mutant. When the zip1-4LA mutation is combined with the pch2 checkpoint mutation, cells sporulate with wild-type efficiency and crossing over occurs at wild-type levels. This result suggests that the zip1-4LA defect in recombination is an indirect consequence of cell cycle arrest. Previous studies have suggested that the Pch2 protein acts in a checkpoint pathway that monitors chromosome synapsis. We hypothesize that the zip1-4LA mutant assembles aberrant SC that triggers the synapsis checkpoint.

Analysis of close stable homolog juxtaposition during meiosis in mutants of Saccharomyces cerevisiae

A unique aspect of meiosis is the segregation of homologous chromosomes at the meiosis I division. The pairing of homologous chromosomes is a critical aspect of meiotic prophase I that aids proper disjunction at anaphase I. We have used a site-specific recombination assay in Saccharomyces cerevisiae to examine allelic interaction levels during meiosis in a series of mutants defective in recombination, chromatin structure, or intracellular movement. Red1, a component of the chromosome axis, and Mnd1, a chromosome-binding protein that facilitates interhomolog interaction, are critical for achieving high levels of allelic interaction. Homologous recombination factors (Sae2, Rdh54, Rad54, Rad55, Rad51, Sgs1) aid in varying degrees in promoting allelic interactions, while the Srs2 helicase appears to play no appreciable role. Ris1 (a SWI2/SNF2 related protein) and Dot1 (a histone methyltransferase) appear to play minor roles. Surprisingly, factors involved in microtubule-mediated intracellular movement (Tub3, Dhc1, and Mlp2) appear to play no appreciable role in homolog juxtaposition, unlike their counterparts in fission yeast. Taken together, these results support the notion that meiotic recombination plays a major role in the high levels of homolog interaction observed during budding yeast meiosis.

The many facets of SC function during C. elegans meiosis

Sexually reproducing organisms rely on meiosis for the formation of haploid gametes. This is achieved through two consecutive rounds of cell division (meiosis I and II) after one round of DNA replication. During the meiotic divisions, chromosomes face several challenges to ultimately ensure proper chromosome segregation. Unique events unfold during meiosis I to overcome these challenges. Homologous chromosomes pair, synapse, and recombine. A remarkable feature throughout this process is the formation of an evolutionarily conserved tripartite proteinaceous structure known as the synaptonemal complex (SC). It is comprised of two lateral elements, assembled along each axis of a pair of homologous chromosomes, and a central region consisting of transverse filaments bridging the gap between lateral elements. While the presence of the SC during meiosis has been appreciated now for 50 years (Moses, Biophys Biochem Cytol 2:215-218, 1956; Fawcett, J Biophys Biochem Cytol 2:403-406, 1956), its role(s) remain a matter of intense investigation. This review concentrates on studies performed in Caenorhabditis elegans, a powerful system for investigating meiosis. Studies in this organism are contributing to the unraveling of the various processes leading to the formation of the SC and the various facets of the functions it exerts throughout meiosis.

Synaptonemal complex formation: where does it start?

The synaptonemal complex is a prominent, evolutionarily conserved feature of meiotic prophase. The assembly of this structure is closely linked to meiotic recombination. A recent study in budding yeast reveals an unexpected role in centromere pairing for a protein component of the synaptonemal complex, Zip1. These findings have implications for synaptonemal complex formation.

Meiotic telomere clustering requires actin for its formation and cohesin for its resolution

In diploid organisms, meiosis reduces the chromosome number by half during the formation of haploid gametes. During meiotic prophase, telomeres transiently cluster at a limited sector of the nuclear envelope (bouquet stage) near the spindle pole body (SPB). Cohesin is a multisubunit complex that contributes to chromosome segregation in meiosis I and II divisions. In yeast meiosis, deficiency for Rec8 cohesin subunit induces telomere clustering to persist, whereas telomere cluster-SPB colocalization is defective. These defects are rescued by expressing the mitotic cohesin Scc1 in rec8delta meiosis, whereas bouquet-stage exit is independent of Cdc5 pololike kinase. An analysis of living Saccharomyces cerevisiae meiocytes revealed highly mobile telomeres from leptotene up to pachytene, with telomeres experiencing an actin- but not microtubule-dependent constraint of mobility during the bouquet stage. Our results suggest that cohesin is required for exit from actin polymerization-dependent telomere clustering and for linking the SPB to the telomere cluster in synaptic meiosis.

Meiosis: inducing variation by reduction

A brief introduction is presented with some thought on the origin of meiosis. Subsequently, a sequential overview of the diverse processes that take place during meiosis is provided, with an eye to similarities and differences between the different eukaryotic systems. In the final part, we try to summarize the available core meiotic mutants and make a comprehensive comparison for orthologous genes between fungal, plant, and animal systems.

A synaptonemal complex protein promotes homology-independent centromere coupling

We describe a process in meiotic cells of budding yeast in which chromosomes become joined together in pairs at their centromeres independent of chromosomal homology. These centromeric interactions depend on the synaptonemal complex component Zip1. During meiosis in wild-type diploids, centromere couples are initially nonhomologous and then undergo switching until all couples involve homologs. This transition to homologous coupling depends on Spo11, a protein required for the initiation of meiotic recombination. Regions of synaptonemal complex assembled early in meiosis are often centromere-associated. We propose that centromere coupling facilitates homolog pairing and promotes synapsis initiation.

Multiple branches of the meiotic recombination pathway contribute independently to homolog pairing and stable juxtaposition during meiosis in budding yeast

A unique aspect of meiosis is the segregation of homologous chromosomes at the meiosis I division. Homologs are physically connected prior to segregation by crossing over between nonsister chromatids. Crossovers arise from the repair of induced double-strand breaks (DSBs). In many organisms, more DSBs are formed than crossovers in a given nucleus. It has been previously suggested that repair of DSBs to noncrossover recombination products aids homolog alignment. Here we explore how two modes of the meiotic recombination pathway (crossover and noncrossover) and meiotic telomere reorganization contribute to the pairing and close juxtaposition of homologous chromosomes in budding yeast. We found that intermediates in the DSB repair pathway leading to both crossover and noncrossover recombination products contribute independently to close, stable homolog juxtaposition (CSHJ), a measurable state of homolog pairing. Analysis of the ndj1delta mutant indicates that the effect of meiotic telomere reorganization on CSHJ is exerted through recombination intermediates at interstitial chromosomal loci, perhaps through the noncrossover branch of the DSB repair pathway. We suggest that transient, early DSB-initiated interactions, including those that give rise to noncrossovers, are important for homolog recognition and juxtaposition.

THE GENETICS AND MOLECULAR BIOLOGY OF THE SYNAPTONEMAL COMPLEX

The synaptonemal complex (SC) is a protein lattice that resembles railroad tracks and connects paired homologous chromosomes in most meiotic systems. The two side rails of the SC, known as lateral elements (LEs), are connected by proteins known as transverse filaments. The LEs are derived from the axial elements of the chromosomes and play important roles in chromosome condensation, pairing, transverse filament assembly, and prohibiting double-strand breaks (DSBs) from entering into recombination pathways that involve sister chromatids. The proteins that make up the transverse filaments of the SC also play a much earlier role in committing a subset of DSBs into a recombination pathway, which results in the production of reciprocal meiotic crossovers. Sites of crossover commitment can be observed as locations where the SC initiates and as immunostaining foci for a set of proteins required for the processing of DSBs to mature crossovers. In most (but not all) organisms it is the establishment of sites marking such crossover-committed DSBs that facilitates completion of synapsis (full-length extension of the SC). The function of the mature full-length SC may involve both the completion of meiotic recombination at the DNA level and the exchange of the axial elements of the two chromatids involved in the crossover. However, the demonstration that the sites of crossover formation are designated prior to SC formation, and the finding that these sites display interference, argues against a role of the mature SC in mediating the process of interference. Finally, in at least some organisms, modifications of the SC alone are sufficient to ensure meiotic chromosome segregation in the complete absence of meiotic recombination.

Homologous pairing and chromosome dynamics in meiosis and mitosis

Pairing of homologous chromosomes is an essential feature of meiosis, acting to promote high levels of recombination and to ensure segregation of homologs. However, homologous pairing also occurs in somatic cells, most regularly in Dipterans such as Drosophila, but also to a lesser extent in other organisms, and it is not known how mitotic and meiotic pairing relate to each other. In this article, I summarize results of recent molecular studies of pairing in both mitosis and meiosis, focusing especially on studies using fluorescent in situ hybridization (FISH) and GFP-tagging of single loci, which have allowed investigators to assay the pairing status of chromosomes directly. These approaches have permitted the demonstration that pairing occurs throughout the cell cycle in mitotic cells in Drosophila, and that the transition from mitotic to meiotic pairing in spermatogenesis is accompanied by a dramatic increase in pairing frequency. Similar approaches in mammals, plants and fungi have established that with few exceptions, chromosomes enter meiosis unpaired and that chromosome movements involving the telomeric, and sometimes centromeric, regions often precede the onset of meiotic pairing. The possible roles of proteins involved in homologous recombination, synapsis and sister chromatid cohesion in homolog pairing are discussed with an emphasis on those for which mutant phenotypes have permitted an assessment of effects on homolog pairing. Finally, I consider the question of the distribution and identity of chromosomal pairing sites, using recent data to evaluate possible relationships between pairing sites and other chromosomal sites, such as centromeres, telomeres, promoters and heterochromatin. I cite evidence that may point to a relationship between matrix attachment sites and homologous pairing sites.

Saccharomyces cerevisiae Hop1 zinc finger motif is the minimal region required for its function in vitro

Saccharomyces cerevisiae meiosis-specific HOP1, which encodes a core component of synaptonemal complex, plays a key role in proper pairing of homologous chromosomes and processing of meiotic DNA double strand breaks. Isolation and analysis of hop1 mutants indicated that these functions require Cys(371) of Hop1 embedded in a region (residues 343-378) sharing homology to a zinc finger motif (ZnF). However, the precise biochemical function of Hop1, or its putative ZnF, in these processes is poorly understood. Our previous studies revealed that Hop1 is a DNA-binding protein, showed substantially higher binding affinity for G4 DNA, and enhances its formation. We report herein that ZnF appears to be sufficient for both zinc as well as DNA-binding activities. Molecular modeling studies suggested that Hop1 ZnF differs from the previously characterized natural ZnFs. The zinc-binding assay showed that the affinity for zinc is weaker for C371S ZnF mutant compared with the wild type (WT) ZnF. Analysis of CD spectra indicated that zinc and DNA induce substantial conformational changes in WT ZnF, but not in C371S ZnF mutant. The results from a number of different experimental approaches suggested that the DNA-binding properties of ZnF are similar to those of full-length Hop1 and that interaction with DNA rich in G residues is particularly robust. Significantly, WT ZnF by itself, but not C371S mutant, was able to bind duplex DNA and promote interstitial pairing of DNA double helices via the formation of guanine quartets. Together, these results implicate a direct role for Hop1 in pairing of homologous chromosomes during meiosis.

The importance of genetic recombination for fidelity of chromosome pairing in meiosis

In budding yeast, absence of the Hop2 protein leads to extensive synaptonemal complex (SC) formation between nonhomologous chromosomes, suggesting a crucial role for Hop2 in the proper alignment of homologous chromosomes during meiotic prophase. Genetic analysis indicates that Hop2 acts in the same pathway as the Rad51 and Dmc1 proteins, two homologs of E. coli RecA. Thus, the hop2 mutant phenotype demonstrates the importance of the recombination machinery in promoting accurate chromosome pairing. We propose that the Dmc1/Rad51 recombinases require Hop2 to distinguish homologous from nonhomologous sequences during the homology search process. Thus, when Hop2 is absent, interactions between nonhomologous sequences become inappropriately stabilized and can initiate SC formation. Overexpression of RAD51 largely suppresses the meiotic defects of the dmc1 and hop2 mutants. We conclude that Rad51 is capable of carrying out a homology search independently, whereas Dmc1 requires additional factors such as Hop2.

Telomere attachment, meiotic chromosome condensation, pairing, and bouquet stage duration are modified in spermatocytes lacking axial elements

During the extended prophase to the meiosis I division, chromosomes assemble axial elements (AE) along replicated sister chromatids whose ends attach to the inner nuclear membrane (NM) via a specialized conical thickening. Here, we show at the EM level that in Sycp3(-/-) spermatocyte chromosomes lack the AE and the conical end thickening, but still they attach their telomeres to the inner NM with an electron-dense plate that contains T(2)AG(3) repeats. Immunofluorescence detected telomere proteins, SCP2, and the meiosis-specific cohesin STAG3 at the Sycp3(-/-) telomere. Bouquet stage spermatocytes were approximately threefold enriched, and the number of telomere but not centromere signals was reduced to the haploid in advanced Sycp3(-/-) spermatocytes, which indicates a special mode of homolog pairing at the mammalian telomere. Fluorescence in situ hybridization with mouse chromosome 8- and 12-specific subsatellite probes uncovered reduced levels of regional homolog pairing, whereas painting of chromosomes 13 revealed partial or complete juxtapositioning of homologs; however, condensation of Sycp3(-/-) bivalents was defective. Electron microscopic analysis of AE-deficient spermatocytes revealed that transverse filaments formed short structures reminiscent of the synaptonemal complex central region, which likely mediate stable homolog pairing. It appears that the AE is required for chromosome condensation, rapid exit from the bouquet stage, and fine-tuning of homolog pairing.

Meiotic recombination and chromosome segregation in Drosophila females

In this review, we describe the pathway for generating meiotic crossovers in Drosophila melanogaster females and how these events ensure the segregation of homologous chromosomes. As appears to be common to meiosis in most organisms, recombination is initiated with a double-strand break (DSB). The interesting differences between organisms appear to be associated with what chromosomal events are required for DSBs to form. In Drosophila females, the synaptonemal complex is required for most DSB formation. The repair of these breaks requires several DSB repair genes, some of which are meiosis-specific, and defects at this stage can have effects downstream on oocyte development. This has been suggested to result from a checkpoint-like signaling between the oocyte nucleus and gene products regulating oogenesis. Crossovers result from genetically controlled modifications to the DSB repair pathway. Finally, segregation of chromosomes joined by a chiasma requires a bipolar spindle. At least two kinesin motor proteins are required for the assembly of this bipolar spindle, and while the meiotic spindle lacks traditional centrosomes, some centrosome components are found at the spindle poles.

Thermoconditional modulation of the pleiotropic sensitivity phenotype by the Saccharomyces cerevisiae PRP19 mutant allele pso4-1

The conditionally-lethal pso4-1 mutant allele of the spliceosomal-associated PRP19 gene allowed us to study this gene's influence on pre-mRNA processing, DNA repair and sporulation. Phenotypes related to intron-containing genes were correlated to temperature. Splicing reporter systems and RT-PCR showed splicing efficiency in pso4-1 to be inversely correlated to growth temperature. A single amino acid substitution, replacing leucine with serine, was identified within the N-terminal region of the pso4-1 allele and was shown to affect the interacting properties of Pso4-1p. Amongst 24 interacting clones isolated in a two-hybrid screening, seven could be identified as parts of the RAD2, RLF2 and DBR1 genes. RAD2 encodes an endonuclease indispensable for nucleotide excision repair (NER), RLF2 encodes the major subunit of the chromatin assembly factor I, whose deletion results in sensitivity to UVC radiation, while DBR1 encodes the lariat RNA splicing debranching enzyme, which degrades intron lariat structures during splicing. Characterization of mutagen-sensitive phenotypes of rad2Delta, rlf2Delta and pso4-1 single and double mutant strains showed enhanced sensitivity for the rad2Delta pso4-1 and rlf2Delta pso4-1 double mutants, suggesting a functional interference of these proteins in DNA repair processes in Saccharomyces cerevisiae.

Synapsis-dependent and -independent mechanisms stabilize homolog pairing during meiotic prophase in C. elegans

Analysis of Caenorhabditis elegans syp-1 mutants reveals that both synapsis-dependent and -independent mechanisms contribute to stable, productive alignment of homologous chromosomes during meiotic prophase. Early prophase nuclei undergo normal reorganization in syp-1 mutants, and chromosomes initially pair. However, the polarized nuclear organization characteristic of early prophase persists for a prolonged period, and homologs dissociate prematurely; furthermore, the synaptonemal complex (SC) is absent. The predicted structure of SYP-1, its localization at the interface between intimately paired, lengthwise-aligned pachytene homologs, and its kinetics of localization with chromosomes indicate that SYP-1 is an SC structural component. A severe reduction in crossing over together with evidence for accumulated recombination intermediates in syp-1 mutants indicate that initial pairing is not sufficient for completion of exchange and implicates the SC in promoting crossover recombination. Persistence of polarized nuclear organization in syp-1 mutants suggests that SC polymerization may provide a motive force or signal that drives redispersal of chromosomes. Whereas our analysis suggests that the SC is required to stabilize pairing along the entire lengths of chromosomes, striking differences in peak pairing levels for opposite ends of chromosomes in syp-1 mutants reveal the existence of an additional mechanism that can promote local stabilization of pairing, independent of synapsis.

Synapsis-dependent and -independent mechanisms stabilize homolog pairing during meiotic prophase in C. elegans

Analysis of Caenorhabditis elegans syp-1 mutants reveals that both synapsis-dependent and -independent mechanisms contribute to stable, productive alignment of homologous chromosomes during meiotic prophase. Early prophase nuclei undergo normal reorganization in syp-1 mutants, and chromosomes initially pair. However, the polarized nuclear organization characteristic of early prophase persists for a prolonged period, and homologs dissociate prematurely; furthermore, the synaptonemal complex (SC) is absent. The predicted structure of SYP-1, its localization at the interface between intimately paired, lengthwise-aligned pachytene homologs, and its kinetics of localization with chromosomes indicate that SYP-1 is an SC structural component. A severe reduction in crossing over together with evidence for accumulated recombination intermediates in syp-1 mutants indicate that initial pairing is not sufficient for completion of exchange and implicates the SC in promoting crossover recombination. Persistence of polarized nuclear organization in syp-1 mutants suggests that SC polymerization may provide a motive force or signal that drives redispersal of chromosomes. Whereas our analysis suggests that the SC is required to stabilize pairing along the entire lengths of chromosomes, striking differences in peak pairing levels for opposite ends of chromosomes in syp-1 mutants reveal the existence of an additional mechanism that can promote local stabilization of pairing, independent of synapsis.

Close, stable homolog juxtaposition during meiosis in budding yeast is dependent on meiotic recombination, occurs independently of synapsis, and is distinct from DSB-independent pairing contacts

A site-specific recombination system that probes the relative probabilities that pairs of chromosomal loci collide with one another in living cells of budding yeast was used to explore the relative contributions of pairing, recombination, synaptonemal complex formation, and telomere clustering to the close juxtaposition of homologous chromosome pairs during meiosis. The level of Cre-mediated recombination between a pair of loxP sites located at an allelic position on homologous chromosomes was 13-fold greater than that between a pair of loxP sites located at ectopic positions on nonhomologous chromosomes. Mutations affecting meiotic recombination initiation and the processing of DNA double-strand breaks (DSBs) into single-end invasions (SEIs) reduced the levels of allelic Cre-mediated recombination levels by three- to sixfold. The severity of Cre/loxP phenotypes is presented in contrast to relatively weak DSB-independent pairing defects as assayed using fluorescence in situ hybridization for these mutants. Mutations affecting synaptonemal complex (SC) formation or crossover control gave wild-type levels of allelic Cre-mediated recombination. A delay in attaining maximum levels of allelic Cre-mediated recombination was observed for a mutant defective in telomere clustering. None of the mutants affected ectopic levels of recombination. These data suggest that stable, close homolog juxtaposition in yeast is distinct from pre-DSB pairing interactions, requires both DSB and SEI formation, but does not depend on crossovers or SC.

The Mnd1 protein forms a complex with hop2 to promote homologous chromosome pairing and meiotic double-strand break repair

The hop2 mutant of Saccharomyces cerevisiae arrests in meiosis with extensive synaptonemal complex (SC) formation between nonhomologous chromosomes. A screen for multicopy suppressors of a hop2-ts allele identified the MND1 gene. The mnd1-null mutant arrests in meiotic prophase, with most double-strand breaks (DSBs) unrepaired. A low level of mature recombinants is produced, and the Rad51 protein accumulates at numerous foci along chromosomes. SC formation is incomplete, and homolog pairing is severely reduced. The Mnd1 protein localizes to chromatin throughout meiotic prophase, and this localization requires Hop2. Unlike recombination enzymes such as Rad51, Mnd1 localizes to chromosomes even in mutants that fail to initiate meiotic recombination. The Hop2 and Mnd1 proteins coimmunoprecipitate from meiotic cell extracts. These results suggest that Hop2 and Mnd1 work as a complex to promote meiotic chromosome pairing and DSB repair. The identification of Hop2 and Mnd1 homologs in other organisms suggests that the function of this complex is conserved among eukaryotes.

Mechanism and control of meiotic recombination initiation

Homologous recombination is essential during meiosis in most sexually reproducing organisms. In budding yeast, and most likely in other organisms as well, meiotic recombination proceeds via the formation and repair of DNA double-strand breaks (DSBs). These breaks appear to be formed by the Spo11 protein, with assistance from a large number of other gene products, by a topoisomerase-like transesterase mechanism. Recent studies in fission yeast, multicellular fungi, flies, worms, plants, and mammals indicate that the role of Spo11 in meiotic recombination initiation is highly conserved. This chapter reviews the properties of Spo11 and the other gene products required for meiotic DSB formation in a number of organisms and discusses ways in which recombination initiation is coordinated with other events occurring in the meiotic cell.

Homolog pairing and two kinds of bouquets in the meiotic prophase of rye, Secale cereale

Chromosome configurations and structures during meiotic prophase were investigated by staining large repeated DNA sequences localized in the subtelomeric regions of all the chromosomes in rye, Secale cereale, in order to clarify when and how homolog pairing and bouquet formation occur. The changes of the spatial locations of chromosomes in the nucleus were investigated by the use of laser confocal microscopy, together with the surface-spreading method of silver nitrate staining to detect the formation of the synaptonemal complex. Homolog pairing in which homologs of four chromatids of a pair of homologs were coaligned in parallel but remained distinctly separate was microscopically detected for the first time in the present study. Homolog pairing showed the following characteristics: (1) it occurred at the leptotene-zygotene transition stage, prior to the formation of nodules and the synaptonemal complex; (2) the chromatin structure of chromosomes was in a state of decondensation; (3) it required no telomere clustering. These data suggest that homolog pairing represents a structure that indicates incipient recombination. After the homolog pairing stage, two kinds of bouquet configuration were found in zygotene. The commonly observed type was a loose bouquet, in which the subtelomeric regions were loosely aggregated. The other type was a definite bouquet, in which almost all the subtelomeric regions were conjugated, but this type was observed only in a limited number of the meiotic prophase cells of some individuals. It was concluded that the former represents the configuration of homologous recombination and the latter that of ectopic recombination.

Checkpoints: chromosome pairing takes an unexpected twist

When meiotic cells complete S phase, homologous chromosomes pair, synapse and undergo recombination. A checkpoint protein is somehow required for meiotic chromosome pairing in C. elegans, thus providing a direct link between S phase and the rest of the meiotic program.

A novel meiosis-specific protein of fission yeast, Meu13p, promotes homologous pairing independently of homologous recombination

Meiotic homologous pairing is crucial to proper homologous recombination, which secures subsequent reductional chromosome segregation. We have identified a novel meiosis-specific protein of fission yeast Schizosaccharomyces pombe, Meu13p, to be a molecule that is required for proper homologous pairing and recombination. Rec12p (homologue of Saccharomyces cerevisiae Spo11p), which is essential for the initiation of meiotic recombination, is also shown for the first time to participate in the pairing process of S.pombe. Meu13p, however, contributes to pairing through a recombination-independent mechanism, as disruption of the meu13(+) gene reduces pairing whether the rec12(+) gene is deleted or not. We also demonstrate a dynamic nature of homologous pairing in living meiotic cells, which is markedly affected by meu13 deletion. Meu13p is not required for telomere clustering and the nuclear movement process, which are well known requirements for efficient pairing in S.pombe. Based on these results, together with the localization of Meu13p on meiotic chromatin, we propose that Meu13p directly promotes proper homologous pairing and recombination.

Nuclear reorganization and homologous chromosome pairing during meiotic prophase require C. elegans chk-2

Analysis of mutants defective in meiotic chromosome pairing has uncovered a role for Caenorhabditis elegans chk-2 in initial establishment of pairing between homologous chromosomes during early meiotic prophase. chk-2 is also required for the major spatial reorganization of nuclei that normally accompanies the onset of pairing, suggesting a mechanistic coupling of these two events. Despite failures in pairing, nuclear reorganization, and crossover recombination, chk-2 mutants undergo many other aspects of meiotic chromosome morphogenesis and complete gametogenesis. Although chk-2 encodes a C. elegans ortholog of the Cds1/Chk2 checkpoint protein kinases, germ-line nuclei in chk-2 mutants are competent to arrest proliferation in response to replication inhibition and to trigger DNA damage checkpoint responses to ionizing radiation. However, chk-2 mutants are defective in triggering the pachytene DNA damage checkpoint in response to an intermediate block in the meiotic recombination pathway, suggesting that chk-2 is required either for initiation of meiotic recombination or for monitoring a specific subset of DNA damage lesions. We propose that chk-2 functions during premeiotic S phase to enable chromosomes to become competent for subsequent meiotic prophase events and/or to coordinate replication with entry into prophase.