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Results for “Synaptonemal Complex”

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PubMed1995

Chromosome dynamics in rad12 mutants of Coprinus cinereus.

We have characterized the phenotypes of three rad12 mutants of the basidiomycete Coprinus cinereus, which were isolated on the basis of sensitivity to ionizing radiation. Electron microscopic studies of meiotic nuclear spreads showed that all three rad12 mutants are defective in chromosomal synapsis. For rad12-1 and rad12-4, very limited assembly of the synaptonemal complex occurs. The phenotype of rad12-15 is less severe and longer stretches of synapsed chromosomes are formed. However, for all three alleles mutant nuclei arrest in a diffuse state with little synaptonemal complex structure. Observations made of spreads of acridine orange-stained meiotic nuclei correlated with the electron microscopic data. In rad12 strains, chromosomes condense but do not pair, and they later arrest in a decondensed state; very few rad12 cells enter metaphase I. Homozygous dikaryons of rad12 mutants produce fruiting bodies with significantly fewer basidiospores than are found in wild-type dikaryons. The viability of these spores is greatly reduced: all spores produced by rad12-1 and rad12-4 mushrooms fail to germinate, while only 16% of rad12-15 spores are viable. Recombination within the tract of the ribosomal RNA gene repeats was not significantly different in the mutants when compared with a wild-type congenic control. Quantitative measurements of oidial survival indicate that all three rad12 alleles are sensitive to gamma radiation but insensitive to UV radiation relative to wild-type strains.

Cell Nucleus↗

PubMed1982

Meiosis in the foetal mouse ovary. I. An analysis at the light microscope level using surface-spreading.

The identification and progression of the prophase stages of meiosis in the mouse foetal ovary are reported, from d 13 of gestation to d 1 postpartum. Air-dried Giemsa-stained oocyte preparations are compared with surface-spread silver-stained cells. The latter method allows a more detailed quantitative analysis of the pachytene stage. Numbers of synaptonemal complexes can be counted, and the degree of synapsis determined. The progression of cells appears to be relatively synchronous, in agreement with previous reports. The activity of nucleolar organisers, in particular one associated with the shortest synaptonemal complex (chromosome No. 19) is described. At late pachytene the lateral elements of the No. 19 bivalent desynapse precociously with apparent nucleolar involvement.

Animals↗

PubMed1980

Silver-stained accessory structures on human sex chromosomes.

Using a combination of silver-staining and light microscopic techniques on human male meiotic preparations, it is feasible to study the morphology and behavior of both autosomal synaptonemal complexes and sex chromosome axes. During leptotene and early zygotene, the X and Y chromosomes are separate; their axes appearing as thin, filamentous structures. During late zygotene/early pachytene, the sex chromosomes come close to each other and a distinct sex vesicle is formed. We confirm existence of a short synaptonemal complex between the terminal ends of the X and Y chromosomes. In our preparations, a number of accessory structures can be seen along the axes of the sex chromosomes. These structures appear to be similar in morphology to those previously observed in several other mammalian species.

Aged↗

PubMed1979

An experimental analysis of bivalent interlocking in spermatocytes of the new Triturus vulgaris.

Interlocked bivalents at 1st meiotic metaphase are relatively uncommon in spermatocytes of the newt Triturus vulgaris, but their frequency of occurrence can be significantly increased by subjecting newts to a 24-h heat shock. Newt spermatocytes are sensitive to a heart shock at any stage between the end of premeiotic S and mid to late pachytene. The heat shock does not cause evidence desynapsis, nor does it significantly affect chiasma frequency; therefore the interlocked condition induced in spermatocytes which were subjected to a heat shock when they were in zygotene or pachytene is unlikely to be a consequence of synaptic trapping. By way of explanation it is suggested that a heat shock may cause telomers to detach from the nuclear membrane, or from the synaptonemal complex where the latter is attached to the membrane, thus allowing non-homologous chromonemata to become interwined before chiasmata have formed. If this explanation is valid, it is then further suggested that the recombination process which results in chiasma formation probably takes place in chromosomal regions lying outside the synaptonemal complex, rather than inside, between its 2 lateral elements.

Animals↗

PubMed1990

Unequal crossing over and heterochromatin exchange in the X-Y bivalents of the deer mouse, Peromyscus beatae.

Differences in length of the heterochromatic short arms of the X and Y chromosomes in individuals of Peromyscus beatae are hypothesized to result from unequal crossing over. To test this hypothesis, we examined patterns of synapsis, chiasma formation, and segregation for male P. beatae which were either heterozygous or homozygous for the amount of short-arm sex heterochromatin. Synaptonemal complex analysis demonstrated that mitotic differences in heterochromatic short-arm lengths between the X and Y chromosomes were reflected in early pachynema as corresponding differences in axial element lengths within the pairing region of the sex bivalent. These length differences were subsequently eliminated by synaptic adjustment such that by late pachynema, the synaptonemal complex configurations of the XY bivalent of heterozygotes were not differentiable from those of homozygotes. Crossing over between the heterochromatic short arms of the XY bivalent was documented by the routine appearance of a single chiasma in this region during diakinesis/metaphase I. Sex heterochromatin heterozygotes were characterized by the presence of asymmetrical chiasma between the X and Y short arms at diakinesis/metaphase I and sex chromosomes with unequal chromatid lengths at metaphase II. These data corroborate our hypothesis on the role of unequal crossing over in the production and propagation of X and Y heterochromatin variation and suggest that, in some cases, crossing over can occur during the process of synaptic adjustment.

Animals↗

PubMed2005

[Developing and testing a method of in silico identification and characterization of meiotic DNA].

A method of in silico search for specific repetitive DNA sequences related to the synaptonemal complex (meiDNA) in mammalian genomes was developed. A study of the distribution of these repeats over chromosomes revealed their scarcity on the Y chromosome and a decrease in recombination frequency in regions enriched in meiDNA. The results are discussed in context of the model of the looplike meiotic chromosome organization during the formation of the synaptonemal complex.

DNA↗

PubMed2002

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.

Cell Nucleus↗

PubMed2026

The dynamic pool of Rec8-cohesin is crucial for meiotic recombination and transcription regulation in the yeast Saccharomyces cerevisiae.

Cohesin is a ring-shaped protein complex that mediates sister-chromatid cohesion (SCC) to ensure accurate chromosome segregation during mitosis and meiosis. In Saccharomyces cerevisiae, cohesin consists of four core subunits-Smc1, Smc3, Scc1/Mcd1, and Scc3. During meiosis, the mitotic α-kleisin Scc1/Mcd1 is replaced by the meiosis-specific α-kleisin Rec8. Rec8-containing cohesin is essential for multiple meiotic processes, including chromosome morphogenesis, homologous recombination, axis and synaptonemal complex formation, SCC, and transcriptional regulation. While stable association of Rec8-cohesin with chromatin is required to maintain SCC from premeiotic S phase through anaphase II, dynamic chromatin association is thought to underlie its roles in recombination, chromosome architecture, and transcription via loop extrusion. Whether distinct stable and dynamic pools of Rec8-cohesin coexist during meiosis and how their functions are partitioned remained unclear. Here, we employed an anchor-away strategy to conditionally deplete only the dynamic pool of Rec8-cohesin from the nucleus while preserving the stable pool. Selective depletion reduced sporulation efficiency and spore viability without compromising SCC. Calibrated ChIP-seq revealed a genome-wide reduction in Rec8-cohesin levels rather than locus-specific loss. Functional analyses demonstrated that the dynamic pool of Rec8-cohesin is required for efficient meiotic recombination, establishment of meiosis-specific chromosome architecture and synaptonemal complex formation, and proper transcriptional regulation of key meiotic regulators. In contrast, the stable pool alone was sufficient to maintain spindle pole body cohesion. Together, our findings demonstrate the existence of two functionally distinct pools of Rec8-cohesin during yeast meiosis.

Saccharomyces cerevisiae↗

PubMed1995

Crossover and noncrossover recombination during meiosis: timing and pathway relationships.

During meiosis, crossovers occur at a high level, but the level of noncrossover recombinants is even higher. The biological rationale for the existence of the latter events is not known. It has been suggested that a noncrossover-specific pathway exists specifically to mediate chromosome pairing. Using a physical assay that monitors both crossovers and noncrossovers in cultures of yeast undergoing synchronous meiosis, we find that both types of products appear at essentially the same time, after chromosomes are fully synapsed at pachytene. We have also analyzed a situation in which commitment to meiotic recombination and formation of the synaptonemal complex are coordinately suppressed (mer1 versus mer1 MER2++). We find that suppression is due primarily to restoration of meiosis-specific double-strand breaks, a characteristic of the major meiotic recombination pathway. Taken together, the observations presented suggest that there probably is no noncrossover-specific pathway and that restoration of intermediate events in a single pairing/recombination pathway promotes synaptonemal complex formation. The biological significant of noncrossover recombination remains to be determined, however.

Crossing Over, Genetic↗

PubMed1992

Role of the pseudoautosomal region in sex-chromosome pairing during male meiosis: meiotic studies in a man with a deletion of distal Xp.

Meiotic studies were undertaken in a 24-year-old male patient with short stature, chondrodysplasia punctata, ichthyosis, steroid sulfatase deficiency, and mild mental retardation with an inherited cytologically visible deletion of distal Xp. Molecular investigations showed that the pseudoautosomal region as well as the steroid sulfatase gene were deleted, but telomeric sequences were present at the pter on the deleted X chromosome. A complete failure of sex-chromosome pairing was observed in the primary spermatocytes of the patient. Telomeric approaches between the sex chromosomes were made at zygotene in some cells, but no XY synaptonemal complex was formed. The sex chromosomes were present as univalents at metaphase I, and germ-cell development was arrested between metaphase I and metaphase II in the vast majority of cells, consistent with the azoospermia observed in the patient. The failure of XY pairing in this individual indicates that the pseudoautosomal sequences play an important role in initiating XY pairing and formation of synaptonemal complex at meiosis.

Adult↗

PubMed2006

Two levels of interference in mouse meiotic recombination.

During meiosis, homologous chromosomes (homologs) undergo recombinational interactions, which can yield crossovers (COs) or noncrossovers. COs exhibit interference; they are more evenly spaced along the chromosomes than would be expected if they were placed randomly. The protein complexes involved in recombination can be visualized as immunofluorescent foci. We have analyzed the distribution of such foci along meiotic prophase chromosomes of the mouse to find out when interference is imposed and whether interference manifests itself at a constant level during meiosis. We observed strong interference among MLH1 foci, which mark CO positions in pachytene. Additionally, we detected substantial interference well before this point, in late zygotene, among MSH4 foci, and similarly, among replication protein A (RPA) foci. MSH4 foci and RPA foci both mark interhomolog recombinational interactions, most of which do not yield COs in the mouse. Furthermore, this zygotene interference did not depend on SYCP1, which is a transverse filament protein of mouse synaptonemal complexes. Interference is thus not specific to COs but may occur in other situations in which the spatial distribution of events has to be controlled. Differences between the distributions of MSH4/RPA foci and MLH1 foci along synaptonemal complexes might suggest that CO interference occurs in two successive steps.

Animals↗

PubMed2004

The Drosophila meiotic kleisin C(2)M functions before the meiotic divisions.

Stepwise and regionally controlled resolution of sister chromatid cohesion is thought to be crucial for faithful chromosome segregation during meiotic divisions. In yeast, the meiosis-specific alpha-kleisin subunit of the cohesin complex, Rec8, is protected from cleavage by separase but only during meiosis I and specifically within the pericentromeric region. While the Drosophila genome does not contain an obvious Rec8 orthologue, as other animal and plant genomes, it includes c(2)M, which encodes a distant alpha-kleisin family member involved in female meiosis. C(2)M associates in vivo with the Smc3 cohesin subunit, as previously shown for yeast Rec8. In contrast to Rec8, however, C(2)M accumulates predominantly after the pre-meiotic S-phase. Moreover, after association with the synaptonemal complex, it disappears again and cannot be detected on meiotic chromosomes by metaphase I. C(2)M cleavage fragments are not observed during completion of the meiotic divisions, and mutations within putative separase cleavage sites do not interfere with meiotic chromosome segregation. Therefore, C(2)M appears to function within the synaptonemal complex during prophase I but possibly not thereafter. This suggests that C(2)M may not confer sister chromatid cohesion needed for meiosis I and II chromosome segregation.

Animals↗

PubMed2004

Cohesin component dynamics during meiotic prophase I in mammalian oocytes.

Cohesins are chromosomal proteins that form complexes involved in the maintenance of sister chromatid cohesion during division of somatic and germ cells. Three meiosis-specific cohesin subunits have been reported in mammals, REC8, STAG3 and SMC1 beta; their expression in mouse spermatocytes has also been described. Here we studied the localization of different meiotic and mitotic cohesin components during prophase I in human and murine female germ cells. In normal and atretic human fetal oocytes, from leptotene to diplotene stages, REC8 and STAG3 colocalize in fibers. In murine oocytes, SMC1beta, SMC3 and STAG3 are localized along fibers that correspond first to the chromosome axis and then to the synaptonemal complex in pachytene. Mitotic cohesin subunit RAD21 is also found in fibers that decorate the SC during prophase I in mouse oocytes, suggesting a role for this cohesin in mammalian sister chromatid cohesion in female meiosis. We observed that, unlike human oocytes, murine synaptonemal complex protein SYCP3 localizes to nucleoli throughout prophase I stages, and centromeres cluster in discrete locations from leptotene to dictyate. At difference from meiosis in male mice, the cohesin axis is progressively lost during the first week after birth in females with a parallel destruction of the axial elements at dictyate arrest, demonstrating sexual dimorphism in sister chromatid cohesion in meiosis.

Animals↗

PubMed1980

Chromatin organization during meiotic prophase of Bombyx mori.

Chromatin organization during the early stages of male meiotic prophase in Bombyx mori was investigated by electron microscopy. The analysis of nuclei prepared by the Miller spreading procedure, suggests that chromatin fibers which are 200-300 A in diameter undergo an orderly folding coincident with the formation of the synaptonemal complex. In very early stages the chromatin is released in linear arrays typical of interphase chromatin material. With time loops containing 5-25 mu of B conformation DNA, initially visualized at the periphery of early meiotic prophase nuclei, aggregate into discrete foci. These foci coalesce to form the longitudinal axis of the chromosome in conjunction with the initial appearance of the axial elements of the synaptonemal complex. At pachytene, the loops are evenly distributed along the length of the chromosome and extend radially so that in well spread preparations the chromosome has a brush-like appearance. Throughout this period nascent RNP-fibers were visualized along some of the loops.

Animals↗

PubMed1975

The synaptic behaviour of the X and Y chromosomes in the marsupial Monodelphis dimidiata.

The pairing behaviour of the X and Y chromosomes of Monodelphis dimidiata was studied with light and electron microscopy. Pairing of the sex chromosomes is delayed with respect to autosome synapsis. Both the X and the minute Y chromosome show an axis attached by its two ends to the nuclear envelope. Synapsis of the sex chromosomes occurs by the joining of the chromatin sheaths that surround the axes and by a small, three-layered structure close to the nuclear envelope. The X and Y chromosomes remain joined to each other during the diffuse stage and diplotene-diakinesis but they do not show a synaptonemal complex. During the diffuse stage a dense plate is formed at the boundary between the X-Y body and the nuclear envelope. During early metaphase a folded sheet is attached to the periphery of the X-Y body. This sheet is formed by a piece of the nuclear envelope carrying the dense plate and it shows transverse fibrils and a central element similar to synaptonemal-complex remains. No evidence of a non-chiasmate segregation mechanism was observed. Polarization of the axial ends of the sex chromosomes is observed after X-Y synapsis. These important departures from the X-Y pairing pattern of eutherian mammals are discussed and assumed to present a special mechanism for holding the minute Y joined to the X chromosome in this marsupial.

Animals↗

PubMed1989

The ultrastructure and reversibility of testicular atrophy induced by ethylene glycol monomethyl ether (EGME) in the rat.

Inhalation exposure to 300 ppm ethylene glycol monomethyl ether (EGME) for 3 days produced degenerative changes in spermatocytes of pachytene and meiotic division at spermatogenic stage XIV in rats. However, a wide range of germ cell types including spermatogonia was affected and the stage-specific damage was not discernible after 2 weeks exposure to 300 ppm EGME. The stage-specific damage was related to exposure concentration-time course. In early stages, degenerating spermatocytes showed nuclear chromatin clumping around synaptonemal complexes, cytoplasmic vesiculation with electron-dense material deposition, and disruption of the plasma membrane. Chromosomal microtubules in the meiotic division of spermatocytes were discontinued with deposition of electron-dense chromatin material. Sertoli cells showed cytoplasmic vacuolization, contact loss to germ cells, and cytoplasmic processes fragmentation with disrupted microtubules. Degenerative pachytene or meiotic spermatocytes were associated with disrupted Sertoli-germ cell relationship, chromosomal microtubules, and synaptonemal complexes. Spermatid degeneration and giant cell formation were observed after spermatocyte degeneration. Spermatid degeneration appeared to be a secondary change resulting from disrupted Sertoli-to-germ cell association. After 14 days post-exposure (PE) following 2 weeks exposure, some tubules were lined with regenerating spermatocytes with or without round spermatids. By 42 days PE, many tubules regained normal germinal epithelium, but some tubules were still atrophic even after 84 days PE. Reversibility of testicular atrophy was inversely proportional to severity of damaged stem cells.

Administration, Inhalation↗

PubMed1984

Meiotic configurations in female trisomy 21 foetuses.

Analysis of the meiotic configurations formed by the three No 21 chromosomes in oocytes from two trisomy 21 foetuses was undertaken using a spreading technique. Light microscope analysis of the first gave limited resolving power, such that over half the oocytes could not be classified as to presence or absence of trivalent or bivalent plus univalent. In the second, investigated at the electron microscope level, all 65 cells analysed were informative and precise detail of meiotic pairing in trivalents could be obtained. Two principal forms of trivalent occurred, one in which pairing was initiated at opposite ends of the three No 21's, each initiation point involving only two of the three homologous lateral elements; the other in which pairing was initiated by all three elements at the same end, a triple synaptonemal complex being formed. Only in one oocyte out of the 65 analysed at EM level, however, did triple pairing occur along the entire length of the No 21 trivalent. All others showed splitting into bivalent and univalent at some point along the structure. Unpaired regions within trivalents and all univalents were consistently seen to be thickened and dark staining with silver over the whole period from pachytene to diplotene. This contrasted with the desynapsing lateral elements of previously paired synaptonemal complexes which appeared thin by comparison at diplotene. The significance of the thickening remains, as yet, obscure.

Amniocentesis↗

PubMed2006

[Abnormal meiosis in bisporic mutants of white button mushroom Agaricus bisporus (Lange) Imbach].

A formerly developed method of obtaining spread preparations of mushroom basidial nuclei was applied to study of meiotic prophase I in bisporic white button mushroom (Agaricus bisporus) strains. Meiotic recombination and assemblage of axial structures (axial elements and synaptonemal complexes) of chromosomes in meiotic prophase I are interrelated. It is known that the frequency of meiotic recombination is reduced in the bisporic A. bisporus variety. We showed that formation of axial structures of meiotic chromosomes in bisporic strains of this mushroom was disrupted. The phenotypes of disruptions in spread prophase nuclei are diverse. In leptotene and early zygotene, many nuclei contain abnormal, often short, and, as a rule, few chromosomal axial elements. The abnormalities in the formation of synaptonemal complexes at the zygotene-diplotene stage are of the same kind and even more pronounced. We discovered an important feature of meiosis in A. bisporus associated with fruit-body morphogenesis. Meiosis starting in basidia (meiocytes) of young closed fruit bodies is accompanied by disruption of chromatin condensation in prophase I and, probably, is arrested. After indusium breakage, the course of meiosis normalizes. Preparations with clearly observable chromosomal axial structures can be obtained only at this stage of fruit-body development.

Agaricus↗