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Results for “Synaptonemal Complex”

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PubMed2001

A Caenorhabditis elegans cohesion protein with functions in meiotic chromosome pairing and disjunction.

We have studied four Caenorhabditis elegans homologs of the Rad21/Scc1/Rec8 sister-chromatid cohesion protein family. Based on the RNAi phenotype and protein localization, it is concluded that one of them, W02A2.6p, is the likely worm ortholog of yeast Rec8p. The depletion of C. elegans W02A2.6p (called REC-8) by RNAi, induced univalent formation and splitting of chromosomes into sister chromatids at diakinesis. Chromosome synapsis at pachytene was defective, but primary homology recognition seemed unaffected, as a closer-than-random association of homologous fluorescence in situ hybridization (FISH) signals at leptotene/zygotene was observed. Depletion of REC-8 also induced chromosome fragmentation at diakinesis. We interpret these fragments as products of unrepaired meiotic double-stranded DNA breaks (DSBs), because fragmentation was suppressed in a spo-11 background. Thus, REC-8 seems to be required for successful repair of DSBs. The occurrence of DSBs in REC-8-depleted meiocytes suggests that DSB formation does not depend on homologous synapsis. Anti-REC-8 immunostaining decorated synaptonemal complexes (SCs) at pachytene and chromosomal axes in bivalents and univalents at diakinesis. Between metaphase I and metaphase II, REC-8 is partially lost from the chromosomes. The partial loss of REC-8 from chromosomes between metaphase I and metaphase II suggests that worm REC-8 might function similarly to yeast Rec8p. The loss of yeast Rec8p from chromosome arms at meiosis I and centromeres at meiosis II coordinates the disjunction of homologs and sister chromatids at the two meiotic divisions.

Animals↗

PubMed1988

Chromosome pairing and germ cell loss in male and female mice carrying a reciprocal translocation.

Female carriers of the T(5;12)31H reciprocal translocation had an average reduction of 73% in oocyte numbers compared with normal litter mates, which was of a magnitude similar to the reduction in sperm counts of male carriers. Analysis of synaptonemal complexes showed that the translocated chromosomes appeared as quadrivalents, or trivalents and univalents, or bivalents in both sexes. Quadrivalents were of three types: fully synapsed, with asynapsis confined to breakpoints, and with unsynapsed ends. There was more pairing in spermatocytes than in oocytes: 37% of spermatocytes, but only 14% of oocytes, contained a fully synapsed quadrivalent, and trivalents were also more frequently fully synapsed in spermatocytes. When these results are compared with those previously obtained for other chromosome anomalies, it becomes evident that there are considerable differences in chromosome pairing between males and females, and that different chromosome rearrangements differ in the relative amount of pairing failure occurring in male and female carriers.

Animals↗

PubMed1997

Identical megabase transgenes on mouse chromosomes 3 and 4 do not promote ectopic pairing or synapsis at meiosis.

To investigate ectopic interactions at the chromatin level, we examined the meiotic organization of 1-2 mb phage lambda transgenes on mouse chromosomes 3 and 4 by fluorescence in situ hybridization in combination with immunocytology of meiotic chromosomes. At early meiotic prophase, the transgenes are sufficiently dispersed in the nuclear volume to permit potential DNA-DNA interactions, but no synaptonemal complexes form between the sites of transgenes residing on different chromosomes. At later stages, when the chromatin is more condensed, the transgenes on different chromosomes are not preferentially associated as they are when they are on the same chromosome. At diplotene and metaphase I, no formations were observed that could be interpreted as reciprocal crossovers or chiasmata between the transgenes located on chromosomes 3 and 4. It appears that in normal fertile mice, a I- to 2-mb homology is insufficient to initiate synapsis between nonhomologs, and it is concluded that homology is assessed within the broader context of the chromosome to initiate synapsis at meiotic prophase.

Animals↗

PubMed1988

Synaptic behaviour and recombination nodules in the human XY pair.

A sample of 90 XY pairs from men with normal karyotypes has been analyzed by measuring their morphological features in electron micrographs of microspread spermatocytes. The classification of human XY types (Solari, 1980) has been given stricter definitions. Stepwise splitting of the axes is seen in types 1 and 2. The development of axial branches and lengthening of the X axis is seen in type 3. In the two subtypes a and b of type 4 the net-like filamentous array grows in length to a maximum (average = 59.7 microns) in subtype b. The location of the putative Y kinetochore defines a short arm that measures 22.34% of Y axis length, and the kinetochore of the X axis defines a short arm of 38.15% of the axial length. The average number of excrescences in the X axis is 19.9 and in the Y is 4.3. The frequency of a non-homologous, distal end-joining grows steadily from type 0 to type 3. The average length of the synaptonemal complex (SC) in 51 XY pairs of types 1 and 2 is 1.33 microns (SD = 0.65) and it corresponds to 25.54% of the Y axis length. Thus, the average SC covers the short arm of the Y and the pericentromeric region. Maximum lengths of this SC may reach up to 81.8% of the Y axis. 30 recombination nodules (RNs) were located in 26 XY pairs, and 90% of the nodules are located in the distal half of the short arm of the Y axis. Thus, RNs are restricted to a segment much shorter than the length of the average SC. A gradient of decreasing probability of recombination may reach up to the centromeric region of the Y chromosome. Some possible consequences of these facts are discussed.

Humans↗

PubMed2003

Recombination nodules in coleopteran species: Palembus dermestoides (Tenebrionidae) and Epicauta atomaria (Meloidae).

This work describes the first report about the occurrence of recombination nodules (RNs) in spread pachytene cells of two species of Coleoptera: Palembus dermestoides (Tenebrionidae) and Epicauta atomaria (Meloidae). The RNs were observed in preparations contrasted with phosphotungstic acid. Considering RN morphology and its occurrence in pachytene bivalents (one per autosome bivalent) these structures were interpreted to be late RNs. P. dermestoides and E. atomaria have 2n = 20 chromosomes including an Xy(p) sex determination system. In spite of most frequently subtelocentric morphology observed in the autosomes of both species, the occurrence of RNs is limited only to the synaptonemal complex (SC) structure of the long arms. These findings are in agreement with those obtained using light microscopy analysis in which only one chiasma or terminalization event is observed per autosomal bivalent in early or late metaphase I cells. The RNs have the same average width of the SC of each analyzed species, a circular shape, strong electron density, and are observed mainly between the lateral elements of the SC. The RNs of P. dermestoides and E. atomaria have approximately the same average size (width), 180 +/- 20 nm and 160 +/- 80 nm, respectively. The absence of RNs in the short arms and its occurrence in the long arms are discussed considering the short arm pericentromeric and pro-centric heterochromatin.

Animals↗

PubMed1997

Distribution of the Rad51 recombinase in human and mouse spermatocytes.

In vitro, the human Rad51 protein (hRad51) promotes homologous pairing and strand exchange reactions suggestive of a key role in genetic recombination. To analyse its role in this process, polyclonal antibodies raised against hRad51 were used to study the distribution of Rad51 in human and mouse spermatocytes during meiosis I. In human spermatocytes, hRad51 was found to form discrete nuclear foci from early zygotene to late pachytene. The foci always co-localized with lateral element proteins, components of the synaptonemal complex (SC). During zygotene, the largest foci were present in regions undergoing synapsis, suggesting that Rad51 is a component of early recombination nodules. Pachytene nuclei showed a greatly reduced level of Rad51 labelling, with the exceptions of any asynapsed autosomes and XY segments, which were intensely labelled. The distribution of Rad51 in mouse spermatocytes was similar to that found in human spermatocytes, except that in this case Rad51 was detectable at leptotene. From these results, we conclude that the Rad51 protein has a role in the interhomologue interactions that occur during meiotic recombination. These interactions are spatially and temporally associated with synapsis during meiotic prophase I.

Animals↗

PubMed1998

The meiosis-specific Hop2 protein of S. cerevisiae ensures synapsis between homologous chromosomes.

The hop2 mutant of S. cerevisiae displays a novel phenotype: meiotic chromosomes form nearly wild-type amounts of synaptonemal complex, but most chromosomes are engaged in synapsis with nonhomologous partners. The meiosis-specific Hop2 protein localizes to chromosomes prior to and during synapsis and in the absence of the double-strand breaks that initiate recombination. hop2 strains sustain a wild-type level of meiotic double-strand breaks, but these breaks remain unrepaired. The hop2 mutant arrests at the pachytene stage of meiotic prophase with the RecA-like protein Dmc1 located at numerous sites along synapsed chromosomes. We propose that the Hop2 protein functions to prevent synapsis between nonhomologous chromosomes.

Amino Acid Sequence↗

PubMed1994

Egalitarian and the choice of cell fates in Drosophila melanogaster oogenesis.

Oogenesis in Drosophila females begins with the formation of a cyst of 16 interconnected sister cells, one of which eventually becomes the definitive oocyte. The other 15 become nurse cells, performing nutritive roles for the oocyte. There are four alternative developmental pathways in each cyst: winning pro-oocyte, losing pro-oocyte and cells with three ring canals, all of which enter meiosis, and the twelve obligate pro-nurse cells, which do not. In flies homozygous for the female-sterile mutation egalitarian (egl) all 16 cells follow the same intermediate pathway. All nuclei enter meiosis (shown by their attainment of synaptonemal complex of at least mid-zygotene levels and by their attainment of pachytene numbers and locations of recombination nodules), then all exit it and become morphologically indistinguishable from nurse cells in wild-type flies. The wild-type allele of egl therefore performs two active but opposite roles in cell fate choice. Early on it is necessary for inhibition of meiotic entry in the 'obligate' pro-nurse cells; later it is necessary for meiotic maintenance in the 'winning' pro-oocyte. One can account for both roles from a single function by invoking gradients through the early cyst; the egl+ function appears to be required for the normal function of these gradients.

Animals↗

PubMed1998

The X1X2Y sex chromosome system in the fish Hoplias malabaricus. II. Meiotic analyses.

The neotropical fish Hoplias malabaricus shows diversified cytotypes and may represent a group of distinct species. One of these cytotypes is characterized by 2n = 40 and 2n = 39 chromosomes in females and males, respectively, with a multiple sex chromosome system of the X1X1X2X2/X1X2Y type. The Y, representing a large chromosome in male karyotype, is derived from a translocation event between two biarmed chromosomes: one of them similar to X1 chromosome (no. 6) and another one similar to X2 chromosome (probably no. 20). Meiotic data (standard and synaptonemal complexes analyses) show 18 bivalents and one characteristic trivalent in pachytene and metaphase I spermatocytes, as well as two kinds of metaphase II cells with 19 and 20 chromosomes. The trivalent is formed by the Y, X1 and X2 chromosomes and usually presents a complete pairing in pachytene. However, trivalents with partially or fully asynapsed segments are also observed. These segments are assumed to be non-homologous regions of the X1 and X2 chromosomes without correspondence with the Y chromosome, which can heterosynapse. This behaviour of the sex trivalent leading to a fully paired structure, taken together with the close frequencies of the two spermatocyte types at metaphase II, suggests a normal pattern for male H. malabaricus meiosis, representing a stabilized multiple sex chromosome system in this species.

Animals↗

PubMed1990

MER1, a yeast gene required for chromosome pairing and genetic recombination, is induced in meiosis.

The yeast MER1 gene is required for the production of viable meiotic products and for meiotic recombination. Cytological analysis of chromosome spreads from a mer1 mutant indicates that the MER1 gene product is also required for normal chromosome pairing. mer1 strains make axial elements, precursors to the synaptonemal complex; however, the chromosomes in most nuclei do not become fully synapsed. The DNA sequence of the MER1 coding region was determined; the MER1 open reading frame encodes a 270-amino-acid protein with a molecular mass of 31.1 kilodaltons. The MER1 protein shows limited sequence similarity to calmodulin. Expression of the MER1 gene was examined by RNA blot hybridization analysis and through the construction and analysis of mer1::lacZ fusion genes. Expression of the MER1 gene is meiotically induced and required the IME1 gene product. Thus, expression of the MER1 gene early in meiosis is required for proper chromosome pairing and meiotic recombination.

Amino Acid Sequence↗

PubMed1996

Nutritional regulation of late meiotic events in Saccharomyces cerevisiae through a pathway distinct from initiation.

The IME1 gene is essential for initiation of meiosis in the yeast Saccharomyces cerevisiae, although it is not required for growth. Here we report that in stationary-phase cultures containing low concentration of glucose, cells overexpressing IME1 undergo the early meiotic events, including DNA replication, commitment to recombination, and synaptonemal complex formation and dissolution. In contrast, later meiotic events, such as chromosome segregation, commitment to meiosis, and spore formation, do not occur. Thus, nutrients can repress the late stages of meiosis independently of their block of initiation. Cells arrested at this midpoint in meiosis are relatively stable and can resume meiotic differentiation if transferred to sporulation conditions. Resumption of meiosis does not require repression of IME1 expression, since IME1 RNA levels stay high after transfer of the arrested cells to sporulation medium. These results suggest that meiosis in S. cerevisiae is a paradigm of a differentiation pathway regulated by signal transduction at both early and late stages.

Amino Acids↗

PubMed1988

Pachytene pairing in relation to sperm and oocyte numbers in a male-fertile reciprocal translocation in the mouse.

In adult males carrying the male-fertile reciprocal translocation T(2;4)13H, body weights, testis weights, and sperm counts were higher in heterozygotes than in homozygotes. Heterozygotes whose mothers were C3H/He exceeded their reciprocal counterparts in the same criteria. At 3-4 days of age, no significant differences between homozygous and heterozygous females were found in body weight, ovarian volume, or oocyte numbers, although mean oocyte volumes were somewhat larger in heterozygotes than in homozygotes. In homozygous males and females the synaptonemal complexes of rearranged chromosomes appeared as bivalents that were indistinguishable from normal bivalents. In most gametocytes of heterozygotes, the translocation was present in the form of a quadrivalent. The degree of pairing failure was greater in oocytes than in spermatocytes. Terminal asynapsis of quadrivalents was very rare in spermatocytes, but it affected one quarter of the oocytes. Only very few translocation configurations were associated with the XY bivalent. It is concluded that the number of sperm produced in male heterozygotes can match the general increase in vigor by the formation of a high level of fully paired quadrivalents, whereas a greater degree of terminal asynapsis in the quadrivalents of oocytes may indicate a slightly more deleterious effect of this translocation on oogenesis.

Animals↗

PubMed2003

Imaging of human meiotic chromosomes by scanning near-field optical microscopy (SNOM).

Centromeres and telomeres are key structures of mitotic and meiotic chromosomes. Especially telomeres develop particular structural properties at meiosis. Here, we investigated the feasibility of scanning near-field optical microscopy (SNOM) for light-microscopic imaging of meiotic telomeres in the sub-hundred nanometer resolution regime. SNOM was applied to visualise the synaptonemal complex (SC) and telomere proteins (TRF1, TRF2) after differential immuno-fluorescent labelling. We tested and compared two different preparation protocols for their applicability in a SNOM setting using micro-fabricated silicon nitride aperture tips. Protocol I consisted of differential labelling of meiotic chromosome cores (SC) by SCP3 immuno-fluorescence and telomeres by TRF1 or TRF2 immuno-fluorescence, while protocol II combined absorption labelling with alkaline phosphatase substrates of cores with fluorescent labelling of telomeres. The results obtained indicate that protocol I reveals a better visualisation of structural (topographic) details than protocol II. By means of SNOM, meiotic chromosome cores could be visualised at a resolution overtopping that of far-field light microscopy.

Chromosomes, Human↗

PubMed1998

Meiotic recombination in C. elegans initiates by a conserved mechanism and is dispensable for homologous chromosome synapsis.

Chromosome segregation at meiosis I depends on pairing and crossing-over between homologs. In most eukaryotes, pairing culminates with formation of the proteinaceous synaptonemal complex (SC). In budding yeast, recombination initiates through double-strand DNA breaks (DSBs) and is thought to be essential for SC formation. Here, we examine whether this mechanism for initiating meiotic recombination is conserved, and we test the dependence of homologous chromosome synapsis on recombination in C. elegans. We find that a homolog of the yeast DSB-generating enzyme, Spo11p, is required for meiotic exchange in this metazoan, and that radiation-induced breaks partially alleviate this dependence. Thus, initiation of recombination by DSBs is apparently conserved. However, homologous synapsis is independent of recombination in the nematode, since it occurs normally in a C. elegans spo-11 null mutant.

Animals↗

PubMed2005

Crossing over is coupled to late meiotic prophase bivalent differentiation through asymmetric disassembly of the SC.

Homologous chromosome pairs (bivalents) undergo restructuring during meiotic prophase to convert a configuration that promotes crossover recombination into one that promotes bipolar spindle attachment and localized cohesion loss. We have imaged remodeling of meiotic chromosome structures after pachytene exit in Caenorhabditis elegans. Chromosome shortening during diplonema is accompanied by coiling of chromosome axes and highly asymmetric departure of synaptonemal complex (SC) central region proteins SYP-1 and SYP-2, which diminish over most of the length of each desynapsing bivalent while becoming concentrated on axis segments distal to the single emerging chiasma. This and other manifestations of asymmetry along chromosomes are lost in synapsis-proficient crossover-defective mutants, which often retain SYP-1,2 along the full lengths of coiled diplotene axes. Moreover, a gamma-irradiation treatment that restores crossovers in the spo-11 mutant also restores asymmetry of SYP-1 localization. We propose that crossovers or crossover precursors serve as symmetry-breaking events that promote differentiation of subregions of the bivalent by triggering asymmetric disassembly of the SC.

Animals↗

PubMed1991

Chromosome pairing and recombination in mice heterozygous for different translocations in chromosomes 16 and 17.

In order to clarify the relationship between meiotic pairing and recombination, an electron microscopic (EM) study of synaptonemal complexes (SC) and an analysis of chiasma frequency and distribution were made in male mice singly and doubly heterozygous for Robertsonian [Rb(16.17)7Bnr] and reciprocal [T(16:17)43H] translocations and also in tertiary trisomics for the proximal region of chromosome 17. In all these genotypes an extensive zone of asynapsis/desynapsis around the breakpoints was revealed. At the same time a high frequency of non-homologous pairing was observed in precentromeric regions of acrocentric chromosomes. The presence in the proximal region of chromosome 17 of the t haplotype did not affect the synaptic behaviour of this region. Chiasma frequency in the proximal region of chromosome 17 in the T(16:17)43H heterozygotes and trisomics was increased when compared with that in Robertsonian heterozygotes.

Animals↗

PubMed2002

Female germ cell aneuploidy and embryo death in mice lacking the meiosis-specific protein SCP3.

Aneuploidy (trisomy or monosomy) is the leading genetic cause of pregnancy loss in humans and results from errors in meiotic chromosome segregation. Here, we show that the absence of synaptonemal complex protein 3 (SCP3) promotes aneuploidy in murine oocytes by inducing defective meiotic chromosome segregation. The abnormal oocyte karyotype is inherited by embryos, which die in utero at an early stage of development. In addition, embryo death in SCP3-deficient females increases with advancing maternal age. We found that SCP3 is required for chiasmata formation and for the structural integrity of meiotic chromosomes, suggesting that altered chromosomal structure triggers nondisjunction. SCP3 is thus linked to inherited aneuploidy in female germ cells and provides a model system for studying age-dependent degeneration in oocytes.

Aneuploidy↗

PubMedDate not supplied

Crossing over analysis at pachytene in man.

The distribution of anti-MLH1 (MutL homologue 1) antibody labelling was studied in human prophase meiocytes. A labelling pattern consisting of distinct foci, always associated with the synaptonemal complex (SC) and never in closely juxtaposed pairs, was observed. Comparison of the number and general positions of autosomal foci with previous studies of the number and positions of autosomal chiasmata indicates that the anti-MLH1 antibody marks sites of crossing over in human pachytene spermatocytes. A mean number of 50.9 autosomal foci was observed from 46 human pachytene spermatocytes corresponding to a genetic length of 2545 cm. Division of these spermatocytes into sub-stages revealed that the number of foci remains stable throughout pachytene. A focus was found on the XY bivalent in 56.5% of the nuclei. The presence or absence of foci from the XY bivalent could not be correlated to pachytene sub-stage.

Adaptor Proteins, Signal Transducing↗