Chromosome choreography: the meiotic ballet

Persistence of histone H2AX phosphorylation after meiotic chromosome synapsis and abnormal centromere cohesion in poly (ADP-ribose) polymerase (Parp-1) null oocytes

In spite of the impact of aneuploidy on human health little is known concerning the molecular mechanisms involved in the formation of structural or numerical chromosome abnormalities during meiosis. Here, we provide novel evidence indicating that lack of PARP-1 function during oogenesis predisposes the female gamete to genome instability. During prophase I of meiosis, a high proportion of Parp-1((-/-)) mouse oocytes exhibit a spectrum of meiotic defects including incomplete homologous chromosome synapsis or persistent histone H2AX phosphorylation in fully synapsed chromosomes at the late pachytene stage. Moreover, the X chromosome bivalent is also prone to exhibit persistent double strand DNA breaks (DSBs). In striking contrast, such defects were not detected in mutant pachytene spermatocytes. In fully-grown wild type oocytes at the germinal vesicle stage, PARP-1 protein associates with nuclear speckles and upon meiotic resumption, undergoes a striking re-localization towards spindle poles as well as pericentric heterochromatin domains at the metaphase II stage. Notably, a high proportion of in vivo matured Parp-1((-/-)) oocytes show lack of recruitment of the kinetochore-associated protein BUB3 to centromeric domains and fail to maintain metaphase II arrest. Defects in chromatin modifications in the form of persistent histone H2AX phosphorylation during prophase I of meiosis and deficient sister chromatid cohesion during metaphase II predispose mutant oocytes to premature anaphase II onset upon removal from the oviductal environment. Our results indicate that PARP-1 plays a critical role in the maintenance of chromosome stability at key stages of meiosis in the female germ line. Moreover, in the metaphase II stage oocyte PARP-1 is required for the regulation of centromere structure and function through a mechanism that involves the recruitment of BUB3 protein to centromeric domains.

Synaptonemal complex assembly and H3K4Me3 demethylation determine DIDO3 localization in meiosis

Synapsis of homologous chromosomes is a key meiotic event, mediated by a large proteinaceous structure termed the synaptonemal complex. Here, we describe a role in meiosis for the murine death-inducer obliterator (Dido) gene. The Dido gene codes for three proteins that recognize trimethylated histone H3 lysine 4 through their amino-terminal plant homeodomain domain. DIDO3, the largest of the three isoforms, localizes to the central region of the synaptonemal complex in germ cells. DIDO3 follows the distribution of the central region protein SYCP1 in Sycp3-/- spermatocytes, which lack the axial elements of the synaptonemal complex. This indicates that synapsis is a requirement for DIDO3 incorporation. Interestingly, DIDO3 is missing from the synaptonemal complex in Atm mutant spermatocytes, which form synapses but show persistent trimethylation of histone H3 lysine 4. In order to further address a role of epigenetic modifications in DIDO3 localization, we made a mutant of the Dido gene that produces a truncated DIDO3 protein. This truncated protein, which lacks the histone-binding domain, is incorporated in the synaptonemal complex irrespective of histone trimethylation status. DIDO3 protein truncation in Dido mutant mice causes mild meiotic defects, visible as gaps in the synaptonemal complex, but allows for normal meiotic progression. Our results indicate that histone H3 lysine 4 demethylation modulates DIDO3 localization in meiosis and suggest epigenetic regulation of the synaptonemal complex.

Phosphorylation-independent regulation of Atf1-promoted meiotic recombination by stress-activated, p38 kinase Spc1 of fission yeast

Background

Stress-activated protein kinases regulate multiple cellular responses to a wide variety of intracellular and extracellular conditions. The conserved, multifunctional, ATF/CREB protein Atf1 (Mts1, Gad7) of fission yeast binds to CRE-like (M26) DNA sites. Atf1 is phosphorylated by the conserved, p38-family kinase Spc1 (Sty1, Phh1) and is required for many Spc1-dependent stress responses, efficient sexual differentiation, and activation of Rec12 (Spo11)-dependent meiotic recombination hotspots like ade6-M26.

Methodology/Principal Findings

We sought to define mechanisms by which Spc1 regulates Atf1 function at the ade6-M26 hotspot. The Spc1 kinase was essential for hotspot activity, but dispensable for basal recombination. Unexpectedly, a protein lacking all eleven MAPK phospho-acceptor sites and detectable phosphorylation (Atf1-11M) was fully proficient for hotspot recombination. Furthermore, tethering of Atf1 to ade6 in the chromosome by a heterologous DNA binding domain bypassed the requirement for Spc1 in promoting recombination.

Conclusions/Significance

The Spc1 protein kinase regulates the pathway of Atf1-promoted recombination at or before the point where Atf1 binds to chromosomes, and this pathway regulation is independent of the phosphorylation status of Atf1. Since basal recombination is Spc1-independent, the principal function of the Spc1 kinase in meiotic recombination is to correctly position Atf1-promoted recombination at hotspots along chromosomes. We also propose new hypotheses on regulatory mechanisms for shared (e.g., DNA binding) and distinct (e.g., osmoregulatory vs. recombinogenic) activities of multifunctional, stress-activated protein Atf1.

Distribution of meiotic recombination events: talking to your neighbors

Accurate chromosome segregation during meiosis is essential for a species' survival. Therefore, a series of events unfold during meiosis, including pairing, synapsis, and recombination between homologous chromosomes, to ultimately ensure the successful completion of this task. This review will focus on how the regulation of crossover recombination events between homologous chromosomes plays a key role in promoting faithful segregation. Although our understanding of the molecular mechanisms by which crossovers are formed has increased significantly, the mechanisms governing the distribution of crossovers along meiotic chromosomes remain largely mysterious. Here, we review the different levels of apparent control of meiotic crossover formation and distribution.

Mechanics and dynamics of X-chromosome pairing at X inactivation

At the onset of X-chromosome inactivation, the vital process whereby female mammalian cells equalize X products with respect to males, the X chromosomes are colocalized along their Xic (X-inactivation center) regions. The mechanism inducing recognition and pairing of the X's remains, though, elusive. Starting from recent discoveries on the molecular factors and on the DNA sequences (the so-called “pairing sites”) involved, we dissect the mechanical basis of Xic colocalization by using a statistical physics model. We show that soluble DNA-specific binding molecules, such as those experimentally identified, can be indeed sufficient to induce the spontaneous colocalization of the homologous chromosomes but only when their concentration, or chemical affinity, rises above a threshold value as a consequence of a thermodynamic phase transition. We derive the likelihood of pairing and its probability distribution. Chromosome dynamics has two stages: an initial independent Brownian diffusion followed, after a characteristic time scale, by recognition and pairing. Finally, we investigate the effects of DNA deletion/insertions in the region of pairing sites and compare model predictions to available experimental data.

Some important cellular processes involve homologous chromosome recognition and pairing. A prominent example is the colocalization of X chromosomes occurring at the onset of X chromosome inactivation, the vital process whereby female mammalian cells silence one of their two X chromosomes to equalize the dosage of X products with respect to males (having just one X). The crucial question on how the Xs recognize each other and come together is, however, still open. Starting from important recent experimental discoveries, we propose a quantitative model, from statistical mechanics, which elucidates the mechanical basis of such phenomena. We demonstrate that a set of soluble molecules binding specific DNA sequences are sufficient to induce recognition and colocalization. This is possible, however, only when their binding energy/concentration exceeds a threshold value, and this suggests how the cell could regulate colocalization. The pairing mechanism that we propose is grounded in general thermodynamic principles, so it could apply to other DNA pairing processes. While we also explore the kinetics of X colocalization, we compare our results to available experimental data and produce testable predictions.

Mutations in the chromosomal passenger complex and the condensin complex differentially affect synaptonemal complex disassembly and metaphase I configuration in Drosophila female meiosis

Production of haploid gametes relies on the specially regulated meiotic cell cycle. Analyses of the role of essential mitotic regulators in meiosis have been hampered by a shortage of appropriate alleles in metazoans. We characterized female-sterile alleles of the condensin complex component dcap-g and used them to define roles for condensin in Drosophila female meiosis. In mitosis, the condensin complex is required for sister-chromatid resolution and contributes to chromosome condensation. In meiosis, we demonstrate a role for dcap-g in disassembly of the synaptonemal complex and for proper retention of the chromosomes in a metaphase I-arrested state. The chromosomal passenger complex also is known to have mitotic roles in chromosome condensation and is required in some systems for localization of the condensin complex. We used the QA26 allele of passenger component incenp to investigate the role of the passenger complex in oocyte meiosis. Strikingly, in incenp(QA26) mutants maintenance of the synaptonemal complex is disrupted. In contrast to the dcap-g mutants, the incenp mutation leads to a failure of paired homologous chromosomes to biorient, such that bivalents frequently orient toward only one pole in prometaphase and metaphase I. We show that incenp interacts genetically with ord, suggesting an important functional relationship between them in meiotic chromosome dynamics. The dcap-g and incenp mutations cause maternal effect lethality, with embryos from mutant mothers arrested in the initial mitotic divisions.

FKBP36 is an inherent multifunctional glyceraldehyde-3-phosphate dehydrogenase inhibitor

FKBP36 has been previously shown to be a crucial factor in spermatogenesis because of its interplay with the synaptonemal complex protein SCPI. Here we show that beyond this function, FKBP36 forms complexes with glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 1.2.1.12) and Hsp90. Both proteins bind independently to different sites of the FKBP36 tetratricopeptide repeat domain. The interaction between FKBP36 and GAPDH directly inhibits the catalytic activity of GAPDH. In addition, FKBP36 expression causes a significant reduction of the GAPDH level and activity in COS-7 cells. Particularly in the cytosolic fraction, GAPDH was depleted by FKBP36 expression. Thus, FKBP36 diminishes GAPDH activity by direct interaction and down-regulation of GAPDH, which represents a previously unknown mechanism of GAPDH regulation and a novel function of FKBP36 in testis-specific signaling.

Crossovers trigger a remodeling of meiotic chromosome axis composition that is linked to two-step loss of sister chromatid cohesion

Segregation of homologous chromosomes during meiosis depends on linkages (chiasmata) created by crossovers and on selective release of a subset of sister chromatid cohesion at anaphase I. During Caenorhabditis elegans meiosis, each chromosome pair forms a single crossover, and the position of this event determines which chromosomal regions will undergo cohesion release at anaphase I. Here we provide insight into the basis of this coupling by uncovering a large-scale regional change in chromosome axis composition that is triggered by crossovers. We show that axial element components HTP-1 and HTP-2 are removed during late pachytene, in a crossover-dependent manner, from the regions that will later be targeted for anaphase I cohesion release. We demonstrate correspondence in position and number between chiasmata and HTP-1/2-depleted regions and provide evidence that HTP-1/2 depletion boundaries mark crossover sites. In htp-1 mutants, diakinesis bivalents lack normal asymmetrical features, and sister chromatid cohesion is prematurely lost during the meiotic divisions. We conclude that HTP-1 is central to the mechanism linking crossovers with late-prophase bivalent differentiation and defines the domains where cohesion will be protected until meiosis II. Further, we discuss parallels between the pattern of HTP-1/2 removal in response to crossovers and the phenomenon of crossover interference.

Coprinus cinereus Mer3 is required for synaptonemal complex formation during meiosis

Mer3 is an evolutionarily conserved DNA helicase that has crucial roles in meiotic recombination and crossover formation. We have identified the MER3 homolog in Coprinus cinereus (Ccmer3) and show that it is expressed in zygotene and pachytene meiocytes. Immunostaining analysis indicated that CcMer3 was localized on chromosomes at zygotene and pachytene and CcMer3 foci were more frequent on paired than unpaired chromosomes. We generated a C. cinereus mer3 mutant (#1) and found that it showed abnormal meiosis progression and underwent apoptosis after prophase I. Basidiospore production in #1 was reduced to 0.8% of the wild-type level; the spores showed slower germination at 25 degrees C but were similar to the wild type at 37 degrees C. Electron microscopic analysis of chromosome spreads revealed that axial elements were formed in the mutant but that synapsis was defective, resulting in a reduction in spore production. Our results demonstrate that CcMer3 is required for synaptonemal complex formation after axial elements align and is thus essential for homologous synapsis.

Cohesin and recombination proteins influence the G1-to-S transition in azygotic meiosis in Schizosaccharomyces pombe

To determine whether recombination and/or sister-chromatid cohesion affect the timing of meiotic prophase events, the horsetail stage and S phase were analyzed in Schizosaccharomyces pombe strains carrying mutations in the cohesin genes rec8 or rec11, the linear element gene rec10, the pairing gene meu13, the double-strand-break formation genes rec6, rec7, rec12, rec14, rec15, and mde2, and the recombination gene dmc1. The double-mutant strains rec8 rec11 and rec8 rec12 were also assayed. Most of the single and both double mutants showed advancement of bulk DNA synthesis, start of nuclear movement (horsetail stage), and meiotic divisions by up to 2 hr. Only mde2 and dmc1 deletion strains showed wild-type timing. Contrasting behavior was observed for rec8 deletions (delayed by 1 hr) compared to a rec8 point mutation (advanced by 1 hr). An hypothesis for the role of cohesin and recombination proteins in the control of the G(1)-to-S transition is proposed. Finally, differences between azygotic meiosis and two other types of fission yeast meiosis (zygotic and pat1-114 meiosis) are discussed with respect to possible control steps in meiotic G(1).

The opossum genome: insights and opportunities from an alternative mammal

The strategic importance of the genome sequence of the gray, short-tailed opossum, Monodelphis domestica, accrues from both the unique phylogenetic position of metatherian (marsupial) mammals and the fundamental biologic characteristics of metatherians that distinguish them from other mammalian species. Metatherian and eutherian (placental) mammals are more closely related to one another than to other vertebrate groups, and owing to this close relationship they share fundamentally similar genetic structures and molecular processes. However, during their long evolutionary separation these alternative mammals have developed distinctive anatomical, physiologic, and genetic features that hold tremendous potential for examining relationships between the molecular structures of mammalian genomes and the functional attributes of their components. Comparative analyses using the opossum genome have already provided a wealth of new evidence regarding the importance of noncoding elements in the evolution of mammalian genomes, the role of transposable elements in driving genomic innovation, and the relationships between recombination rate, nucleotide composition, and the genomic distributions of repetitive elements. The genome sequence is also beginning to enlarge our understanding of the evolution and function of the vertebrate immune system, and it provides an alternative model for investigating mechanisms of genomic imprinting. Equally important, availability of the genome sequence is fostering the development of new research tools for physical and functional genomic analyses of M. domestica that are expanding its versatility as an experimental system for a broad range of research applications in basic biology and biomedically oriented research.

Shugoshin-2 is essential for the completion of meiosis but not for mitotic cell division in mice

Shugoshin-2 (SGOL2) is one of the two mammalian orthologs of the Shugoshin/Mei-S322 family of proteins that regulate sister chromatid cohesion by protecting the integrity of the multiprotein cohesin complexes. This protective system is essential for faithful chromosome segregation during mitosis and meiosis, which is the physical basis of Mendelian inheritance. Regardless of its evolutionary conservation from yeast to mammals, little is known about the in vivo relevance and specific role that SGOL2 plays in mammals. Here we show that disruption of the gene encoding mouse SGOL2 does not cause any alteration in sister chromatid cohesion in embryonic cultured fibroblasts and adult somatic tissues. Moreover, mutant mice develop normally and survive to adulthood without any apparent alteration. However, both male and female Sgol2-deficient mice are infertile. We demonstrate that SGOL2 is necessary for protecting centromeric cohesion during mammalian meiosis I. In vivo, the loss of SGOL2 promotes a premature release of the meiosis-specific REC8 cohesin complexes from anaphase I centromeres. This molecular alteration is manifested cytologically by the complete loss of centromere cohesion at metaphase II leading to single chromatids and physiologically with the formation of aneuploid gametes that give rise to infertility.

Meiotic recombination hotspots of fission yeast are directed to loci that express non-coding RNA

Background

Polyadenylated, mRNA-like transcripts with no coding potential are abundant in eukaryotes, but the functions of these long non-coding RNAs (ncRNAs) are enigmatic. In meiosis, Rec12 (Spo11) catalyzes the formation of dsDNA breaks (DSBs) that initiate homologous recombination. Most meiotic recombination is positioned at hotspots, but knowledge of the mechanisms is nebulous. In the fission yeast genome DSBs are located within 194 prominent peaks separated on average by 65-kbp intervals of DNA that are largely free of DSBs.

Methodology/Principal Findings

We compared the genome-wide distribution of DSB peaks to that of polyadenylated ncRNA molecules of the prl class. DSB peaks map to ncRNA loci that may be situated within ORFs, near the boundaries of ORFs and intergenic regions, or most often within intergenic regions. Unconditional statistical tests revealed that this colocalization is non-random and robust (P≤5.5×10⁻⁸). Furthermore, we tested and rejected the hypothesis that the ncRNA loci and DSB peaks localize preferentially, but independently, to a third entity on the chromosomes.

Conclusions/Significance

Meiotic DSB hotspots are directed to loci that express polyadenylated ncRNAs. This reveals an unexpected, possibly unitary mechanism for what directs meiotic recombination to hotspots. It also reveals a likely biological function for enigmatic ncRNAs. We propose specific mechanisms by which ncRNA molecules, or some aspect of RNA metabolism associated with ncRNA loci, help to position recombination protein complexes at DSB hotspots within chromosomes.

Both ATM and ATR promote the efficient and accurate processing of programmed meiotic double-strand breaks

The ATM and ATR protein kinases play central roles in the cellular response to double-strand breaks (DSBs) by regulating DNA repair, cell-cycle arrest and apoptosis. During meiosis, SPO11-dependent DSBs are generated, initiating recombination between homologous chromosomes. Previous studies in mice and plants have shown that defects in ATM result in the appearance of abnormally fragmented chromosomes. However, the role of ATR in promoting normal meiosis has not yet been elucidated. Employing null Arabidopsis mutants of ATR and ATM, we demonstrate here that although atr mutants display no obvious defects in any phase of meiotic progression, the combination of defects in atr and atm exacerbates the fragmentation observed in the atm single mutant, prevents complete synapsis of chromosomes, and results in extensive and persistent interactions between non-homologous DNAs. The observed non-homologous interactions require the induction of programmed breaks: the combination of either the atm single or the atr atm double mutant with a spo11 defect eliminates the ectopic interactions observed in the double mutant, as well as significantly reducing the fragmentation seen in atm or in atr atm. Our results suggest that ATM is required for the efficient processing of SPO11-dependent DSBs during meiosis. They also indicate that ATM and ATR act redundantly to inhibit sustained interactions between non-homologous chromatids, and that these ectopic interactions require SPO11 activity.

The role of specialized transcription factories in chromosome pairing

Homologous chromosomes can pair in somatic and germ line cells, and many mechanisms have been proposed to explain how they do so. One popular class of models involves base-pairing between DNA strands catalyzed by recombination proteins, but pairing still occurs in mutants lacking the relevant functional proteins. We discuss an alternative based on two observations: transcription occurs in factories that specialize in transcribing specific gene sub-sets, and chromosomes only pair when transcribed. Each chromosome in the haploid set has a unique array of transcription units strung along its length; we suggest each is organized into clouds of loops tethered to specialized factories. Only homologs share similar strings of clouds and factories. Pairing begins when a promoter on one chromosome initiates in the homologous and specialized factory organized mainly by its homologous partner. This transiently ties the two homologs together, to increase the chances that adjacent promoters initiate in their homologous factories and that the two homologs will be zipped together. Then, interactions between promoters and RNA polymerases in the factories mediate pairing.

High-resolution mapping of meiotic crossovers and non-crossovers in yeast

Meiotic recombination has a central role in the evolution of sexually reproducing organisms. The two recombination outcomes, crossover and non-crossover, increase genetic diversity, but have the potential to homogenize alleles by gene conversion. Whereas crossover rates vary considerably across the genome, non-crossovers and gene conversions have only been identified in a handful of loci. To examine recombination genome wide and at high spatial resolution, we generated maps of crossovers, crossover-associated gene conversion and non-crossover gene conversion using dense genetic marker data collected from all four products of fifty-six yeast (Saccharomyces cerevisiae) meioses. Our maps reveal differences in the distributions of crossovers and non-crossovers, showing more regions where either crossovers or non-crossovers are favoured than expected by chance. Furthermore, we detect evidence for interference between crossovers and non-crossovers, a phenomenon previously only known to occur between crossovers. Up to 1% of the genome of each meiotic product is subject to gene conversion in a single meiosis, with detectable bias towards GC nucleotides. To our knowledge the maps represent the first high-resolution, genome-wide characterization of the multiple outcomes of recombination in any organism. In addition, because non-crossover hotspots create holes of reduced linkage within haplotype blocks, our results stress the need to incorporate non-crossovers into genetic linkage analysis.

Using a meiosis detection toolkit to investigate ancient asexual "scandals" and the evolution of sex

Sexual reproduction is the dominant reproductive mode in eukaryotes but, in many taxa, it has never been observed. Molecular methods that detect evidence of sex are largely based on the genetic consequences of sexual reproduction. Here we describe a powerful new approach to directly search genomes for genes that function in meiosis. We describe a "meiosis detection toolkit", a set of meiotic genes that represent the best markers for the presence of meiosis. These genes are widely present in eukaryotes, function only in meiosis and can be isolated by degenerate PCR. The presence of most, or all, of these genes in a genome would suggest they have been maintained for meiosis and, implicitly, sexual reproduction. In contrast, their absence would be consistent with the loss of meiosis and asexuality. This approach will help to understand both meiotic gene evolution and the capacity for meiosis and sex in putative obligate asexuals.

Arabidopsis SMG7 protein is required for exit from meiosis

Meiosis consists of two nuclear divisions that are separated by a short interkinesis. Here we show that the SMG7 protein, which plays an evolutionarily conserved role in nonsense-mediated RNA decay (NMD) in animals and yeast, is essential for the progression from anaphase to telophase in the second meiotic division in Arabidopsis. Arabidopsis SMG7 is an essential gene, the disruption of which causes embryonic lethality. Plants carrying a hypomorphic smg7 mutation exhibit an elevated level of transcripts containing premature stop codons. This suggests that the role of SMG7 in NMD is conserved in plants. Furthermore, hypomorphic smg7 alleles render mutant plants sterile by causing an unusual cell-cycle arrest in anaphase II that is characterized by delayed chromosome decondensation and aberrant rearrangement of the meiotic spindle. The smg7 phenotype was mimicked by exposing meiocytes to the proteasome inhibitor MG115. Together, these data indicate that SMG7 counteracts cyclin-dependent kinase (CDK) activity at the end of meiosis, and reveal a novel link between SMG7 and regulation of the meiotic cell cycle.

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.

Cooperative interactions between pairs of homologous chromatids during meiosis in Saccharomyces cerevisiae

We report a novel instance of negative interference during Saccharomyces cerevisiae meiosis, where Cre-mediated recombination between pairs of allelic loxP sites is more frequent than expected. We suggest that endogenous crossover recombination mediates cooperative pairing interactions between all four chromatids of a meiotic bivalent.

Variation in crossover interference levels on individual chromosomes from human males

Crossovers (COs) generated by homologous recombination ensure the proper segregation of chromosomes during meiosis. COs exhibit interference, which leads to widely spaced COs along chromosomes. Strong positive CO interference has been found in humans. However, little is known about the extent of human CO interference. In this study, variations in CO interference over the entire human genome and among individuals were analyzed by immunofluorescence combined with fluorescence in situ hybridization of testicular biopsies from 10 control men. These methods allow for direct identification of the frequency and location of COs in specific chromosomes of pachytene cells. The strength of CO interference was estimated by fitting the frequency distribution of inter-CO distances to the gamma model. Positive interference among CO on chromosomes was observed in these men, and the strength of inter-arm interference was significantly stronger than that for intra-arm CO. In addition, interference was observed to act across the centromere. Significant inter-individual and inter-chromosomal variations in the levels of interference were found, with smaller chromosomes exhibiting stronger interference. Discontinuous chromosome regions (gaps) and unsynapsed chromosome regions (splits) in chromosome 9 had both cis and trans effects on CO interference levels. This is the first report that the interference level varies significantly across the whole genome and that, at least in the human male, anomalies in chromosome synapsis play an important role in altering CO interference levels.

ATM promotes the obligate XY crossover and both crossover control and chromosome axis integrity on autosomes

During meiosis in most sexually reproducing organisms, recombination forms crossovers between homologous maternal and paternal chromosomes and thereby promotes proper chromosome segregation at the first meiotic division. The number and distribution of crossovers are tightly controlled, but the factors that contribute to this control are poorly understood in most organisms, including mammals. Here we provide evidence that the ATM kinase or protein is essential for proper crossover formation in mouse spermatocytes. ATM deficiency causes multiple phenotypes in humans and mice, including gonadal atrophy. Mouse Atm^−/− spermatocytes undergo apoptosis at mid-prophase of meiosis I, but Atm^−/− meiotic phenotypes are partially rescued by Spo11 heterozygosity, such that ATM-deficient spermatocytes progress to meiotic metaphase I. Strikingly, Spo11^+/−Atm^−/− spermatocytes are defective in forming the obligate crossover on the sex chromosomes, even though the XY pair is usually incorporated in a sex body and is transcriptionally inactivated as in normal spermatocytes. The XY crossover defect correlates with the appearance of lagging chromosomes at metaphase I, which may trigger the extensive metaphase apoptosis that is observed in these cells. In addition, control of the number and distribution of crossovers on autosomes appears to be defective in the absence of ATM because there is an increase in the total number of MLH1 foci, which mark the sites of eventual crossover formation, and because interference between MLH1 foci is perturbed. The axes of autosomes exhibit structural defects that correlate with the positions of ongoing recombination. Together, these findings indicate that ATM plays a role in both crossover control and chromosome axis integrity and further suggests that ATM is important for coordinating these features of meiotic chromosome dynamics.

Meiosis is the specialized cell division that gives rise to reproductive cells such as sperm and eggs. During meiosis in most organisms, genetic information is exchanged between homologous maternal and paternal chromosomes through the process of homologous recombination. This recombination forms connections between homologous chromosomes that allow them to segregate accurately when the meiotic cell divides. Recombination defects can result in reproductive cells with abnormal chromosome numbers, which are a major cause of developmental disorders and spontaneous abortions in humans. Meiotic recombination is tightly controlled such that each pair of chromosomes undergoes at least one crossover recombination event despite a low average number of crossovers per chromosome. Moreover, multiple crossovers on the same chromosome tend to be evenly and widely spaced. Mechanisms of this control are not well understood, but here we provide evidence that ATM protein is required for normal operation of this process(es) in male mice. ATM has long been known to be involved in cellular responses to DNA damage. Our studies reveal a new function for this protein and also provide new insight into the mechanisms by which meiotic cells ensure accurate transmission of genetic material from one generation to the next.

C. elegans germ cells switch between distinct modes of double-strand break repair during meiotic prophase progression

Chromosome inheritance during sexual reproduction relies on deliberate induction of double-strand DNA breaks (DSBs) and repair of a subset of these breaks as interhomolog crossovers (COs). Here we provide a direct demonstration, based on our analysis of rad-50 mutants, that the meiotic program in Caenorhabditis elegans involves both acquisition and loss of a specialized mode of double-strand break repair (DSBR). In premeiotic germ cells, RAD-50 is not required to load strand-exchange protein RAD-51 at sites of spontaneous or ionizing radiation (IR)-induced DSBs. A specialized meiotic DSBR mode is engaged at the onset of meiotic prophase, coincident with assembly of meiotic chromosome axis structures. This meiotic DSBR mode is characterized both by dependence on RAD-50 for rapid accumulation of RAD-51 at DSB sites and by competence for converting DSBs into interhomolog COs. At the mid-pachytene to late pachytene transition, germ cells undergo an abrupt release from the meiotic DSBR mode, characterized by reversion to RAD-50-independent loading of RAD-51 and loss of competence to convert DSBs into interhomolog COs. This transition in DSBR mode is dependent on MAP kinase-triggered prophase progression and coincides temporally with a major remodeling of chromosome architecture. We propose that at least two developmentally programmed switches in DSBR mode, likely conferred by changes in chromosome architecture, operate in the C. elegans germ line to allow formation of meiotic crossovers without jeopardizing genomic integrity. Our data further suggest that meiotic cohesin component REC-8 may play a role in limiting the activity of SPO-11 in generating meiotic DSBs and that RAD-50 may function in counteracting this inhibition.

Faithful inheritance of chromosomes during sexual reproduction depends on the deliberate formation of double-strand DNA breaks (DSBs) and subsequent repair of a subset of these breaks by a mechanism that leads to crossovers between homologous chromosome pairs. The requirement for crossovers to ensure chromosome segregation poses a challenge for sexually reproducing organisms, as DSBs constitute a danger to genomic integrity in other contexts. This manuscript provides insight into the mechanisms that allow germ cells to generate recombination-based linkages that ensure chromosome inheritance while at the same time protecting the integrity of their genomes. Specifically, we provide a direct demonstration, based on our analysis of rad-50 mutants, that the meiotic program in C. elegans involves both acquisition and loss of a specialized meiotic mode of double-strand break repair (DSBR). We propose that the ability to revert to a less constrained DSBR environment at a late stage of meiotic prophase serves as a fail-safe mechanism for safeguarding the genome, as it provides an opportunity to repair any remaining DBSs and restore chromosome integrity prior to chromosome segregation.

The genetics you never knew: a genetics primer

Our understanding about genetics is rapidly changing. The goal of this article is to provide an overview of the basics of and new developments in medical genetics--a Genetics 101 primer for 2008. In this article we review the structure and function of the genes, how genes are packaged, gene replication, gene mutations, and the different modes of inheritance.

To check or not to check? The application of meiotic studies to plant breeding

Understanding the barriers that prevent pairing and recombination of the chromosomes from two parental species is important for crop improvement strategies. It had been generally thought that plants do not possess checkpoint mechanisms during meiosis. However, recent data may question this assumption and suggest that exploitation of such mechanisms could be crucial to breeding.

Fluorescent Arabidopsis tetrads: a visual assay for quickly developing large crossover and crossover interference data sets

In most organisms, one crossover (CO) event inhibits the chances of another nearby event. The term used to describe this phenomenon is 'CO interference'. Here, we describe a protocol for quickly generating large data sets that are amenable to CO interference analysis in the flowering plant, Arabidopsis thaliana. We employ a visual assay that utilizes transgenic marker constructs encoding pollen-expressed fluorescent proteins of three colors in the quartet mutant background. In this genetic background, male meiotic products--the pollen grains--remain physically attached thereby facilitating tetrad analysis. We have developed a library of mapped marker insertions that, when crossed together, create adjacent intervals that can be rapidly and simultaneously screened for COs. This assay system is capable of detecting and differentiating single COs as well as two-, three- and four-strand double COs. We also describe how to analyze the data that are produced by this method. To generate and score a double interval in a wild-type and mutant background using this protocol will take 22-27 weeks.

Molecular cartography: mapping the landscape of meiotic recombination

Mapping recombination hot and cold spots in yeast has previously relied on mutants, which themselves distort the map. Now a new method promises to overcome that problem.

What makes centromeric cohesion resistant to separase cleavage during meiosis I but not during meiosis II?

Segregation of chromosomes during meiosis I is triggered by separase cleavage of the cohesin's Rec8 subunit along chromosome arms. Centromeric cohesin is protected from separase cleavage during meiosis I by Sgo1/MEI-S332 proteins in complex with protein phosphatase 2A (PP2A). This retention of centromeric sister chromatid cohesion is essential for faithful segregation of chromatids during the second meiotic division. While Sgo1/PP2A complex is required for protecting centromeric sister chromatid cohesion during meiosis I, it is not known what renders the centromeric cohesion sensitive to separase cleavage during meiosis II. Our data suggest that the absence of Sgo1 and PP2A from meiosis II centromeres is not sufficient to render centromeric cohesion sensitive to cleavage by separase and additional factors are required to ensure the removal of centromeric cohesion during meiosis II.

Spo11 and the Formation of DNA Double-Strand Breaks in Meiosis

Meiotic recombination is carried out through a specialized pathway for the formation and repair of DNA double-strand breaks made by the Spo11 protein, a relative of archaeal topoisomerase VI. This review summarizes recent studies that provide insight to the mechanism of DNA cleavage by Spo11, functional interactions of Spo11 with other proteins required for break formation, mechanisms that control the timing of recombination initiation, and evolutionary conservation and divergence of these processes.

Unified mode of centromeric protection by shugoshin in mammalian oocytes and somatic cells

Reductional chromosome segregation in germ cells, where sister chromatids are pulled to the same pole, accompanies the protection of cohesin at centromeres from separase cleavage. Here, we show that mammalian shugoshin Sgo2 is expressed in germ cells and is solely responsible for the centromeric localization of PP2A and the protection of cohesin Rec8 in oocytes, proving conservation of the mechanism from yeast to mammals. However, this role of Sgo2 contrasts with its mitotic role in protecting centromeric cohesin only from prophase dissociation, but never from anaphase cleavage. We demonstrate that, in somatic cells, shugoshin colocalizes with cohesin in prophase or prometaphase, but their localizations become separate when centromeres are pulled oppositely at metaphase. Remarkably, if tension is artificially removed from the centromeres at the metaphase-anaphase transition, cohesin at the centromeres can be protected from separase cleavage even in somatic cells, as in germ cells. These results argue for a unified view of centromeric protection by shugoshin in mitosis and meiosis.

A novel plant gene essential for meiosis is related to the human CtIP and the yeast COM1/SAE2 gene

Obligatory homologous recombination (HR) is required for chiasma formation and chromosome segregation in meiosis I. Meiotic HR is initiated by DNA double-strand breaks (DSBs), generated by Spo11, a homologue of the archaebacterial topoisomerase subunit Top6A. In Saccharomyces cerevisiae, Rad50, Mre11 and Com1/Sae2 are essential to process an intermediate of the cleavage reaction consisting of Spo11 covalently linked to the 5' termini of DNA. While Rad50 and Mre11 also confer genome stability to vegetative cells and are well conserved in evolution, Com1/Sae2 was believed to be fungal-specific. Here, we identify COM1/SAE2 homologues in all eukaryotic kingdoms. Arabidopsis thaliana Com1/Sae2 mutants are sterile, accumulate AtSPO11-1 during meiotic prophase and fail to form AtRAd51 foci despite the presence of unrepaired DSBs. Furthermore, DNA fragmentation in AtCom1 is suppressed by eliminating AtSPO11-1. In addition, AtCOM1 is specifically required for mitomycin C resistance. Interestingly, we identified CtIP, an essential protein interacting with the DNA repair machinery, as the mammalian homologue of Com1/Sae2, with important implications for the molecular role of CtIP.

Interactions between Mei4, Rec114, and other proteins required for meiotic DNA double-strand break formation in Saccharomyces cerevisiae

In most sexually reproducing organisms, meiotic recombination is initiated by DNA double-strand breaks (DSBs) formed by the Spo11 protein. In budding yeast, nine other proteins are also required for DSB formation, but the roles of these proteins and the interactions among them are poorly understood. We report further studies of the behaviors of these proteins. Consistent with other studies, we find that Mei4 and Rec114 bind to chromosomes from leptonema through early pachynema. Both proteins showed only limited colocalization with the meiotic cohesin subunit Rec8, suggesting that Mei4 and Rec114 associated preferentially with chromatin loops. Rec114 localization was independent of other DSB factors, but Mei4 localization was strongly dependent on Rec114 and Mer2. Systematic deletion analysis identified protein regions important for a previously described two-hybrid interaction between Mei4 and Rec114. We also report functional characterization of a previously misannotated 5' coding exon of REC102. Sequences encoded in this exon are essential for DSB formation and for Rec102 interaction with Rec104, Spo11, Rec114, and Mei4. Finally, we also examined genetic requirements for a set of previously described two-hybrid interactions that can be detected only when the reporter strain is induced to enter meiosis. This analysis reveals new functional dependencies for interactions among the DSB proteins. Taken together, these studies support the view that Mei4, Rec114, and Mer2 make up a functional subgroup that is distinct from other subgroups of the DSB proteins: Spo11-Ski8, Rec102-Rec104, and Mre11-Rad50-Xrs2. These studies also suggest that an essential function of Rec102 and Rec104 is to connect Mei4 and Rec114 to Spo11.

Meiotic pairing and disjunction of mini-X chromosomes in drosophila is mediated by 240-bp rDNA repeats and the homolog conjunction proteins SNM and MNM

In most eukaryotes, segregation of homologous chromosomes during meiosis is dependent on crossovers that occur while the homologs are intimately paired during early prophase. Crossovers generate homolog connectors known as chiasmata that are stabilized by cohesion between sister-chromatid arms. In Drosophila males, homologs pair and segregate without recombining or forming chiasmata. Stable pairing of homologs is dependent on two proteins, SNM and MNM, that associate with chromosomes throughout meiosis I until their removal at anaphase I. SNM and MNM localize to the rDNA region of the X-Y pair, which contains 240-bp repeats that have previously been shown to function as cis-acting chromosome pairing/segregation sites. Here we show that heterochromatic mini-X chromosomes lacking native rDNA but carrying transgenic 240-bp repeat arrays segregate preferentially from full-length sex chromosomes and from each other. Mini-X pairs do not form autonomous bivalents but do associate at high frequency with the X-Y bivalent to form trivalents and quadrivalents. Both disjunction of mini-X pairs and multivalent formation are dependent on the presence of SNM and MNM. These results imply that 240-bp repeats function to mediate association of sex chromosomes with SNM and MNM.

Regulation of meiotic cohesion and chromosome core morphogenesis during pachytene in Drosophila oocytes

During meiosis, cohesion between sister chromatids is required for normal levels of homologous recombination, maintenance of chiasmata and accurate chromosome segregation during both divisions. In Drosophila, null mutations in the ord gene abolish meiotic cohesion, although how ORD protein promotes cohesion has remained elusive. We show that SMC subunits of the cohesin complex colocalize with ORD at centromeres of ovarian germ-line cells. In addition, cohesin SMCs and ORD are visible along the length of meiotic chromosomes during pachytene and remain associated with chromosome cores following DNase I digestion. In flies lacking ORD activity, cohesin SMCs fail to accumulate at oocyte centromeres. Although SMC1 and SMC3 localization along chromosome cores appears normal during early pachytene in ord mutant oocytes, the cores disassemble as meiosis progresses. These data suggest that cohesin loading and/or accumulation at centromeres versus arms is under differential control during Drosophila meiosis. Our experiments also reveal that the alpha-kleisin C(2)M is required for the assembly of chromosome cores during pachytene but is not involved in recruitment of cohesin SMCs to the centromeres. We present a model for how chromosome cores are assembled during Drosophila meiosis and the role of ORD in meiotic cohesion, chromosome core maintenance and homologous recombination.

Terminal regions of wheat chromosomes select their pairing partners in meiosis

Many plant species, including important crops like wheat, are polyploids that carry more than two sets of genetically related chromosomes capable of meiotic pairing. To safeguard a diploid-like behavior at meiosis, many polyploids evolved genetic loci that suppress incorrect pairing and recombination of homeologues. The Ph1 locus in wheat was proposed to ensure homologous pairing by controlling the specificity of centromere associations that precede chromosome pairing. Using wheat chromosomes that carry rye centromeres, we show that the centromere associations in early meiosis are not based on homology and that the Ph1 locus has no effect on such associations. Although centromeres indeed undergo a switch from nonhomologous to homologous associations in meiosis, this process is driven by the terminally initiated synapsis. The centromere has no effect on metaphase I chiasmate chromosome associations: homologs with identical or different centromeres, in the presence and absence of Ph1, pair the same. A FISH analysis of the behavior of centromeres and distal chromomeres in telocentric and bi-armed chromosomes demonstrates that it is not the centromeric, but rather the subtelomeric, regions that are involved in the correct partner recognition and selection.

Meiotic recombination in Caenorhabditis elegans

The faithful segregation of homologous chromosomes during meiosis is dependent on the formation of physical connections (chiasma) that form following reciprocal exchange of DNA molecules during meiotic recombination. Here we review the current knowledge in the Caenorhabditis elegans meiotic recombination field. We discuss recent developments that have improved our understanding of the crucial steps that must precede the initiation and propagation of meiotic recombination. We summarize the pathways that impact on meiotic prophase entry and the current understanding of how chromosomes reorganize and interact to promote homologous chromosome pairing and subsequent synapsis. We pay particular attention to the mechanisms that contribute to meiotic DNA double-strand break (DSB) formation and strand exchange processes, and how the C. elegans system compares with other model organisms. Finally, we highlight current and future areas of research that are likely to further our understanding of the meiotic recombination process.

Bi-orientation of achiasmatic chromosomes in meiosis I oocytes contributes to aneuploidy in mice

The spindle assembly checkpoint guards against chromosomal missegregation but does not induce arrest in oocytes that contain a few achiasmatic chromosomes (univalents). We followed the fate of univalents in oocytes during the first meiotic division, and although these preserved a meiotic kinetochore structure, they were also bi-oriented in a mitotic manner. The hybrid chromosomal configuration attained by univalents allows them to evade the spindle assembly checkpoint and contribute to aneuploidy in oocytes.

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

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

Meiotic parthenogenesis in a root-knot nematode results in rapid genomic homozygosity

Many isolates of the plant-parasitic nematode Meloidogyne hapla reproduce by facultative meiotic parthenogenesis. Sexual crosses can occur, but, in the absence of males, the diploid state appears to be restored by reuniting sister chromosomes of a single meiosis. We have crossed inbred strains of M. hapla that differ in DNA markers and produced hybrids and F(2) lines. Here we show that heterozygous M. hapla females, upon parthenogenetic reproduction, produce progeny that segregate 1:1 for the presence or absence of dominant DNA markers, as would be expected if sister chromosomes are rejoined, rather than the 3:1 ratio typical of a Mendelian cross. Codominant markers also segregate 1:1 and heterozygotes are present at low frequency (<3%). Segregation patterns and recombinant analysis indicate that a homozygous condition is prevalent for markers flanking recombination events, suggesting that recombination occurs preferentially as four-strand exchanges at similar locations between both pairs of non-sister chromatids. With this mechanism, meiotic parthenogenesis would be expected to result in rapid genomic homozygosity. This type of high negative crossover interference coupled with positive chromatid interference has not been observed in fungal or other animal systems in which it is possible to examine the sister products of a single meiosis and may indicate that meiotic recombination in this nematode has novel features.

Meiosis-driven genome variation in plants

Meiosis includes two successive divisions of the nucleus with one round of DNA replication and leads to the formation of gametes with half of the chromosomes of the mother cell during sexual reproduction. It provides a cytological basis for gametogenesis and nheritance in eukaryotes. Meiotic cell division is a complex and dynamic process that involves a number of molecular and cellular events, such as DNA and chromosome replication, chromosome pairing, synapsis and recombination, chromosome segregation, and cytokinesis. Meiosis maintains genome stability and integrity over sexual life cycles. On the other hand, meiosis generates genome variations in several ways. Variant meiotic recombination resulting from specific genome structures induces deletions, duplications, and other rearrangements within the genic and non-genic genomic regions and has been considered a major driving force for gene and genome evolution in nature. Meiotic abnormalities in chromosome segregation lead to chromosomally imbalanced gametes and aneuploidy. Meiotic restitution due to failure of the first or second meiotic division gives rise to unreduced gametes, which triggers polyploidization and genome expansion. This paper reviews research regarding meiosis-driven genome variation, including deletion and duplication of genomic regions, aneuploidy, and polyploidization, and discusses the effect of related meiotic events on genome variation and evolution in plants. Knowledge of various meiosis-driven genome variations provides insight into genome evolution and genetic variability in plants and facilitates plant genome research.

Two distinct surveillance mechanisms monitor meiotic chromosome metabolism in budding yeast

Meiotic recombination is initiated by Spo11-generated DNA double-strand breaks (DSBs) . A fraction of total DSBs is processed into crossovers (CRs) between homologous chromosomes, which promote their accurate segregation at meiosis I (MI) . The coordination of recombination-associated events and MI progression is governed by the "pachytene checkpoint", which in budding yeast requires Rad17, a component of a PCNA clamp-like complex, and Pch2, a putative AAA-ATPase . We show that two genetically separable pathways monitor the presence of distinct meiotic recombination-associated lesions: First, delayed MI progression in the presence of DNA repair intermediates is suppressed when RAD17 or SAE2, encoding a DSB-end processing factor , is deleted. Second, delayed MI progression in the presence of aberrant synaptonemal complex (SC) is suppressed when PCH2 is deleted. Importantly, ZIP1, encoding the central element of the SC , is required for PCH2-dependent checkpoint activation. Analysis of the rad17Deltapch2Delta double mutant revealed a redundant function regulating interhomolog CR formation. These findings suggest a link between the surveillance of distinct recombination-associated lesions, control of CR formation kinetics, and regulation of MI timing. A PCH2-ZIP1-dependent checkpoint in meiosis is likely conserved among synaptic organisms from yeast to human .

Pollen tetrad-based visual assay for meiotic recombination in Arabidopsis

Recombination, in the form of cross-overs (COs) and gene conversion (GC), is a highly conserved feature of meiosis from fungi to mammals. Recombination helps ensure chromosome segregation and promotes allelic diversity. Lesions in the recombination machinery are often catastrophic for meiosis, resulting in sterility. We have developed a visual assay capable of detecting Cos and GCs and measuring CO interference in Arabidopsis thaliana. This flexible assay utilizes transgene constructs encoding pollen-expressed fluorescent proteins of three different colors in the qrt1 mutant background. By observing the segregation of the fluorescent alleles in 92,489 pollen tetrads, we demonstrate (i) a correlation between developmental position and CO frequency, (ii) a temperature dependence for CO frequency, (iii) the ability to detect meiotic GC events, and (iv) the ability to rapidly assess CO interference.

Chromosome malorientations after meiosis II arrest cause nondisjunction

This study investigated the basis of meiosis II nondisjunction. Cold arrest induced a fraction of meiosis II crane fly spermatocytes to form (n + 1) and (n - 1) daughters during recovery. Live-cell liquid crystal polarized light microscope imaging showed nondisjunction was caused by chromosome malorientation. Whereas amphitely (sister kinetochore fibers to opposite poles) is normal, cold recovery induced anaphase syntely (sister fibers to the same pole) and merotely (fibers to both poles from 1 kinetochore). Maloriented chromosomes had stable metaphase positions near the equator or between the equator and a pole. Syntelics were at the spindle periphery at metaphase; their sisters disconnected at anaphase and moved all the way to a centrosome, as their strongly birefringent kinetochore fibers shortened. The kinetochore fibers of merotelics shortened little if any during anaphase, making anaphase lag common. If one fiber of a merotelic was more birefringent than the other, the less birefringent fiber lengthened with anaphase spindle elongation, often permitting inclusion of merotelics in a daughter nucleus. Meroamphitely (near amphitely but with some merotely) caused sisters to move in opposite directions. In contrast, syntely and merosyntely (near syntely but with some merotely) resulted in nondisjunction. Anaphase malorientations were more frequent after longer arrests, with particularly long arrests required to induce syntely and merosyntely.

Effect of colchicine and telocentric chromosome conformation on centromere and telomere dynamics at meiotic prophase I in wheat-rye additions

Association of telomeres in a bouquet and clustering of centromere regions have been proposed to be involved in the search and recognition of homologous partners. We have analysed the role of these structures in meiotic chromosome pairing in wheat-rye addition lines by applying colchicine for disturbing presynaptic telomere movements and by modifying the centromere position from submetacentric to telocentric for studying centromere effects. Rye chromosomes, wheat and rye centromeres, and telomeres were identified by fluorescence in-situ hybridization. Presynaptic association of centromeres in pairs or in more complex structures involved mainly non-homologous chromosomes as deduced from the behaviour of rye centromeres. While centromere association was not affected by colchicine, colchicine inhibited bouquet formation, which caused failure of homologous synapsis. Homologous centromeres of rye telocentrics associated earlier than those of rye submetacentric chromosomes, indicating that migration of the telocentrics' centromeres to the telomere pole during bouquet formation facilitated their association. Homologous chromosomes associated in premeiotic interphase can recognize each other and initiate synapsis at zygotene. However, telomere convergence is needed for bringing together the majority of homologous pairs that normally occupy separate territories in premeiotic nuclei.

Role of the mod(mdg4) common region in homolog segregation in Drosophila male meiosis

Homologous chromosomes must pair and establish stable connections during prophase I of meiosis to segregate reliably from each other at anaphase I. In most organisms, the stable connections, called chiasmata, arise from crossovers. In Drosophila males, homologs pair and segregate without crossing over. Chiasmata are replaced by a homolog conjunction complex that includes the Stromalin in Meiosis (SNM) and Modifier of Mdg4 in Meiosis (MNM) proteins. MNM is one of 31 alternative splice products of mod(mdg4), all of which share a common 402-amino-acid N terminus and differ at their C termini. Previous data demonstrated that an MNM-specific exon is required for homolog conjunction, but did not address whether the N-terminal common region, which includes a BTB domain that can mediate coalescence of protein-DNA complexes, is also required. Here we describe a mutation in the common region of mod(mdg4), Z3-3401, that causes qualitatively similar phenotypes as the MNM-specific alleles but disrupts X-Y segregation much more drastically than autosomal segregation. The mutant MNM protein in Z3-3401 is expressed throughout prophase I in spermatocytes but the protein is confined to the cytoplasm, suggesting that the Z3-3401 mutation disrupts a signal required for nuclear localization or retention. Z3-3401 fails to complement a large battery of lethal and semilethal alleles in the common region for meiotic nondisjunction, including an allele containing an amino acid substitution at a conserved residue in the BTB/POZ domain, consistent with a general requirement for the mod(mdg4) common region in homolog segregation.

Centromere-proximal crossovers are associated with precocious separation of sister chromatids during meiosis in Saccharomyces cerevisiae

In most organisms, meiotic chromosome segregation is dependent on crossovers (COs), which enable pairs of homologous chromosomes to segregate to opposite poles at meiosis I. In mammals, the majority of meiotic chromosome segregation errors result from a lack of COs between homologs. Observations in Homo sapiens and Drosophila melanogaster have revealed a second class of exceptional events in which a CO occurred near the centromere of the missegregated chromosome. We show that in wild-type strains of Saccharomyces cerevisiae, most spore inviability is due to precocious separation of sister chromatids (PSSC) and that PSSC is often associated with centromere-proximal crossing over. COs, as opposed to nonreciprocal recombination events (NCOs), are preferentially associated with missegregation. Strains mutant for the RecQ homolog, SGS1, display reduced spore viability and increased crossing over. Much of the spore inviability in sgs1 results from PSSC, and these events are often associated with centromere-proximal COs, just as in wild type. When crossing over in sgs1 is reduced by the introduction of a nonnull allele of SPO11, spore viability is improved, suggesting that the increased PSSC is due to increased crossing over. We present a model for PSSC in which a centromere-proximal CO promotes local loss of sister-chromatid cohesion.

Lsh is required for meiotic chromosome synapsis and retrotransposon silencing in female germ cells

Lymphoid specific helicase (Lsh) is a major epigenetic regulator that is essential for DNA methylation and transcriptional silencing of parasitic elements in the mammalian genome. However, whether Lsh is involved in the regulation of chromatin-mediated processes during meiosis is not known. Here, we show that Lsh is essential for the completion of meiosis and transcriptional repression of repetitive elements in the female gonad. Oocytes from Lsh knockout mice exhibit demethylation of transposable elements and tandem repeats at pericentric heterochromatin, as well as incomplete chromosome synapsis associated with persistent RAD51 foci and gammaH2AX phosphorylation. Failure to load crossover-associated foci results in the generation of non-exchange chromosomes. The severe oocyte loss observed and lack of ovarian follicle formation, together with the patterns of Lsh nuclear compartmentalization in the germ line, demonstrate that Lsh has a critical and previously unidentified role in epigenetic gene silencing and maintenance of genomic stability during female meiosis.

Initiation of homologous chromosome pairing during meiosis

Following pre-meiotic DNA replication, homologous chromosomes must be paired and become tightly linked to ensure reductional segregation during meiosis I. Therefore initiation of homologous chromosome pairing is vital for meiosis to proceed correctly. A number of factors contribute to the initiation of homologous chromosome pairing including telomere and centromere dynamics, pairing centres, checkpoint proteins and components of the axial element. The present review briefly summarizes recent progress in our understanding of initiation of homologous chromosome pairing during meiosis and discusses the differences that are observed between research organisms.