Meiotic chromosome pairing and synaptonemal complex transformation in Culex pipiens oocytes

The synaptonemal complexes of the oocytes of the mosquito Culex pipiens quinquefasciatus have been reconstructed from serial sections. A diffuse structure, probably a chromocenter composed of centromeric heterochromatin, was present during pachytene. As no synaptonemal complexes were visible inside the chromocenter the continuity of the 2 arms of a bivalent was lost. The telomeric ends were clustered in a small area of the nuclear membrane in a bouquet arrangement; they were associated in pairs, and sometimes joined through a special structure. One pair was composed of the 2 telomeres of the shortest bivalent and a ring configuration was thus formed. The other 2 chromosomes may form one or two rings. During a short transitional stage, after the disappearence of the synaptonemal complexes, several thousand annuli, 1200–1500 A in diameter, were present in the nuclei. The annuli disappeared as material originating mainly from the transverse filaments of the synaptonemal complexes formed a capsule around the chromosomes during diplotene.     

Meiotic configurations in female trisomy 21 foetuses

Summary 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 plusunivalent. 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.     

Parental origin of the extra chromosome in prenatally diagnosed fetal trisomy 21

Trisomy 21 (Down syndrome) is one of the most common chromosomal abnormalities. Of cases of free trisomy 21 causing Down syndrome, about 95% result from nondisjunction during meiosis, and about 5% are due to mitotic errors in somatic cells. Previous studies using DNA polymorphisms of chromosome 21 showed that paternal origin of trisomy 21 occurred in only 6.7% of cases. However, these studies were conducted in liveborn trisomy 21-affected infants, and the possible impact of fetal death was not taken into account. Using nine distinct DNA polymorphisms, we tested 110 families with a prenatally diagnosed trisomy 21 fetus. Of the 102 informative cases, parental origin was maternal in 91 cases (89.2%) and paternal in 11 (10.8%). This percentage differs significantly from the 7.0% observed in previous studies (P<0.001). In order to test the influence of genomic parental imprinting, we determined the origin of the extra chromosome 21 in relation to different factors: advanced maternal age, maternal serum human chorionic gonadotropin (hormone of placental origin), severity of the disease, gestational age at diagnosis and fetal gender. We found that the increased frequency of paternal origin of nondisjunction in trisomy 21-affected fetuses cannot obviously be explained by factors leading to selective loss of paternal origin fetuses.     

Meiotic chromosome pairing in Actinidia chinensis var. deliciosa

Polyploids are defined as either autopolyploids or allopolyploids, depending on their mode of origin and/or chromosome pairing behaviour. Autopolyploids have chromosome sets that are the result of the duplication or combination of related genomes (e.g., AAAA), while allopolyploids result from the combination of sets of chromosomes from two or more different taxa (e.g., AABB, AABBCC). Allopolyploids are expected to show preferential pairing of homologous chromosomes from within each parental sub-genome, leading to disomic inheritance. In contrast, autopolyploids are expected to show random pairing of chromosomes (non-preferential pairing), potentially leading to polysomic inheritance. The two main cultivated taxa of Actinidia (kiwifruit) are A. chinensis (2x and 4x) and A. chinensis var. deliciosa (6x). There is debate whether A. chinensis var. deliciosa is an autopolyploid derived solely from A. chinensis or whether it is an allopolyploid derived from A. chinensis and one or two other Actinidia taxa. To investigate whether preferential or non-preferential chromosome pairing occurs in A. chinensis var. deliciosa, the inheritance of microsatellite alleles was analysed in the tetraploid progeny of a cross between A. chinensis var. deliciosa and the distantly related Actinidia eriantha Benth. (2x). The frequencies of inherited microsatellite allelic combinations in the hybrids suggested that non-preferential chromosome pairing had occurred in the A. chinensis var. deliciosa parent. Meiotic chromosome analysis showed predominantly bivalent formation in A. chinensis var. deliciosa, but a low frequency of quadrivalent chromosome formations was observed (1 observed in 20 pollen mother cells).     

Meiotic chromosome condensation and pairing in Saccharomyces cerevisiae studied by chromosome painting

Non-isotopic high resolution in sity hybridization was applied to cytological preparations of sporulating yeast cells. Ribosomal DNA (rDNA) and chromosome V-specific recombinant lambda clones were used to tag individual chromosomes and chromosome subregions. This allowed the study of chromosome behaviour during early meiotic prophase. It was found that chromatin becomes condensed and homologous DNA sequences then appear to become aligned prior to synaptonemal complex formation.by E.R. Schmidt     

New insights into human nondisjunction of chromosome 21 in oocytes

Nondisjunction of chromosome 21 is the leading cause of Down syndrome. Two risk factors for maternal nondisjunction of chromosome 21 are increased maternal age and altered recombination. In order to provide further insight on mechanisms underlying nondisjunction, we examined the association between these two well established risk factors for chromosome 21 nondisjunction. In our approach, short tandem repeat markers along chromosome 21 were genotyped in DNA collected from individuals with free trisomy 21 and their parents. This information was used to determine the origin of the nondisjunction error and the maternal recombination profile. We analyzed 615 maternal meiosis I and 253 maternal meiosis II cases stratified by maternal age. The examination of meiosis II errors, the first of its type, suggests that the presence of a single exchange within the pericentromeric region of 21q interacts with maternal age-related risk factors. This observation could be explained in two general ways: 1) a pericentromeric exchange initiates or exacerbates the susceptibility to maternal age risk factors or 2) a pericentromeric exchange protects the bivalent against age-related risk factors allowing proper segregation of homologues at meiosis I, but not segregation of sisters at meiosis II. In contrast, analysis of maternal meiosis I errors indicates that a single telomeric exchange imposes the same risk for nondisjunction, irrespective of the age of the oocyte. Our results emphasize the fact that human nondisjunction is a multifactorial trait that must be dissected into its component parts to identify specific associated risk factors.     

Meiotic chromosome pairing is promoted by telomere-led chromosome movements independent of bouquet formation

Chromosome pairing in meiotic prophase is a prerequisite for the high fidelity of chromosome segregation that haploidizes the genome prior to gamete formation. In the budding yeast Saccharomyces cerevisiae, as in most multicellular eukaryotes, homologous pairing at the cytological level reflects the contemporaneous search for homology at the molecular level, where DNA double-strand broken ends find and interact with templates for repair on homologous chromosomes. Synapsis (synaptonemal complex formation) stabilizes pairing and supports DNA repair. The bouquet stage, where telomeres have formed a transient single cluster early in meiotic prophase, and telomere-promoted rapid meiotic prophase chromosome movements (RPMs) are prominent temporal correlates of pairing and synapsis. The bouquet has long been thought to contribute to the kinetics of pairing, but the individual roles of bouquet and RPMs are difficult to assess because of common dependencies. For example, in budding yeast RPMs and bouquet both require the broadly conserved SUN protein Mps3 as well as Ndj1 and Csm4, which link telomeres to the cytoskeleton through the intact nuclear envelope. We find that mutants in these genes provide a graded series of RPM activity: wild-type>mps3-dCC>mps3-dAR>ndj1Δ>mps3-dNT = csm4Δ. Pairing rates are directly correlated with RPM activity even though only wild-type forms a bouquet, suggesting that RPMs promote homologous pairing directly while the bouquet plays at most a minor role in Saccharomyces cerevisiae. A new collision trap assay demonstrates that RPMs generate homologous and heterologous chromosome collisions in or before the earliest stages of prophase, suggesting that RPMs contribute to pairing by stirring the nuclear contents to aid the recombination-mediated homology search.     

The possible role of meiotic pairing anomalies in the atresia of human fetal oocytes

Summary Following a previous study of human fetal oocytes analysed by light and electron microscope microspreading (Speed 1985), a further and more extensive analysis has now been carried out at electron microscope (EM) level. Some new anomalies not previously observed are described. More than one-third of all pachytene oocytes show degeneration (Z-cells) or synaptic errors which might lead to germ cell death. Meiotic pairing anomalies appear to be much more common among oocytes than spermatocytes, and could be a significant factor in the high rate of atresia found between mid-term and birth in the human ovary.     

Meiotic competence of bovine fetal oocytes following in vitro maturation

The in vitro ability between fetal and cow oocytes to resume meiosis and progression to metaphase-II (M-II) was compared. Cumulus oocyte complexes (COCs) were harvested from 2 to 6 mm follicles from ovaries of 7.5 month to term fetuses and adult cows. Cumulus cells were removed using 3 mg/ml hyaluronidase and repeated pipetting. Denuded oocytes were fixed in 3% glutaraldehyde, stained with DAPI and evaluated under fluorescent microscopy for nuclear status before in vitro maturation (IVM). COCs from fetal and adult ovaries were also matured in 200 microl droplets of medium 199 supplemented with 10 microg/ml FSH, 10/ml LH, 1.5 microg/ml estradiol, 75 microg/ml streptomycin, 100 IU/ml penicillin, 10 mM hepes and 10% FBS for 24 h at 39 degrees C and 5% CO(2). Matured oocytes were fixed, stained and evaluated as explained above for nuclear status namely stage of germinal vesicle (GV) development and subsequent meiotic competence. Data were analyzed using chi-square analysis. The majority of fetal oocytes (P<0.05) before IVM were at GV stages GV-I (27.7%), GV-II (37.6%) and GV-V (22.8%) compared to cow oocytes, which were at GV stages IV (28.3%) and V (46.7%). After IVM, fewer fetal oocytes were at earlier stages of GV development and majority (P<0.05) were at GV-V (24.0%), premetaphase (17.4%) and metaphase-I (M-I: 7.2%) stages. However, after IVM, more cow oocytes matured to M-II than did fetal oocytes (93.7% versus 26.9%; P<0.05). In conclusion, fetal oocytes do not mature in vitro as well as cow oocytes. Our findings suggest that the low meiotic competence of fetal oocytes can be attributed to their being at earlier stages of GV development before IVM.     

Origin of the extra chromosome in trisomy 21

Summary Of 61 families of children with trisomy 21, polymorphism of chromosome 21 elucidating the origin of the extra chromosome was found in 42. Nondisjunction was of paternal origin in 8 cases (19.04%) and the anomaly occurred with equal frequency during the first and second meiotic divisions. Maternal nondisjunction was demonstrated in 34 cases (80.95%), in which nondisjunction occurred by far the most often during the first meiotic division (29 cases).These results are in agreement with data from the literature, and suggest the existence of at least two different causes for chromosomal nondisjunction, the first being the same in both sexes and occurring in both meiotic divisions and the second specifically limited to the first meiotic division in the mother.Attachée de Recherche au CNRSAttachée de Recherche à l'INSERM     

Epigenetic-genetic chromosome dosage approach for fetal trisomy 21 detection using an autosomal genetic reference marker

Background

The putative promoter of the holocarboxylase synthetase (HLCS) gene on chromosome 21 is hypermethylated in placental tissues and could be detected as a fetal-specific DNA marker in maternal plasma. Detection of fetal trisomy 21 (T21) has been demonstrated by an epigenetic-genetic chromosome dosage approach where the amount of hypermethylated HLCS in maternal plasma is normalized using a fetal genetic marker on the Y chromosome as a chromosome dosage reference marker. We explore if this method can be applied on both male and female fetuses with the use of a paternally-inherited fetal single nucleotide polymorphism (SNP) allele on a reference chromosome for chromosome dosage normalization.

Methodology

We quantified hypermethylated HLCS molecules using methylation-sensitive restriction endonuclease digestion followed by real-time or digital PCR analyses. For chromosome dosage analysis, we compared the amount of digestion-resistant HLCS to that of a SNP allele (rs6636, a C/G SNP) that the fetus has inherited from the father but absent in the pregnant mother.

Principal Findings

Using a fetal-specific SNP allele on a reference chromosome, we analyzed 20 euploid and nine T21 placental tissue samples. All samples with the fetal-specific C allele were correctly classified. One sample from each of the euploid and T21 groups were misclassified when the fetal-specific G allele was used as the reference marker. We then analyzed 33 euploid and 14 T21 maternal plasma samples. All but one sample from each of the euploid and T21 groups were correctly classified using the fetal-specific C allele, while correct classification was achieved for all samples using the fetal-specific G allele as the reference marker.

Conclusions

As a proof-of-concept study, we have demonstrated that the epigenetic-genetic chromosome dosage approach can be applied to the prenatal diagnosis of trisomy 21 for both male and female fetuses.     

Meiotic drive of t haplotypes: chromosome segregation in mice with tertiary trisomy

The properties of the t haplotypes, specific mutant states of the proximal region of chromosomes 17 in the house mouse, are of continuing interest. One such property is increased transmission of the t haplotype by heterozygous t/+ males to offspring. Using the reciprocal translocation T(16;17)43H we have constructed males with tertiary trisomy of chromosome 17 (+T43/+ +/Rb7+) carrying the Robertsonian translocation Rb(16.17)7Bnr. Only the progeny of these males which had inherited either T43/+ or Rb7 from their male parent were viable. The segregation patterns in the offspring of t-bearing trisomics were analysed on days 16-18 of embryonic development. It was found that, when the t12 haplotype is in the normal acrocentric (males+ +T43/+ t12 + /Rb7+ +), its presence in the gamete +t12+/+ + T43 does not produce meiotic drive. However, when t6 is in Rb7, meiotic drive was observed: 80% of offspring carried the t haplotype. It is concluded that the meiotic drive is probably inhibited by the presence of a normal homologue of chromosome 17 in the same sperm. Possible mechanisms for the t haplotype effect are discussed.     

Meiotic chromosome distribution in Drosophila oocytes: Roles of two kinesin-related proteins

Summary The recent finding that two proteins required for proper chromosome distribution in Drosophila oocytes are related to the microtubule motor protein, kinesin, provides new insights into the forces involved in meiotic chromosome movement. ncd is a spindle motor in meiosis but may perform a different role in the early mitotic divisions of the embryo. nod, until recently, has been thought to be a component of the distributive process of chromosome segregation. The finding that nod is a kinesin protein provides an alternative explanation of the effect of mutants on nonexchange chromosomes and suggests that nonexchange chromosomes segregate with exchange chromosomes in a single process, rather than via a two-step distributive system.     

Meiotic chromosome pairing behaviour of natural tetraploids and induced autotetraploids of Actinidia chinensis

Key message

Non-preferential chromosome pairing was identified in tetraploid Actinidia chinensis and a higher mean multivalent frequency in pollen mother cells was found in colchine-induced tetraploids of A. chinensis compared with naturally occurring tetraploids.

Abstract

Diploid and tetraploid Actinidia chinensis are used for the development of kiwifruit cultivars. Diploid germplasm can be exploited in a tetraploid breeding programme via unreduced (2n) gametes and chemical-induced chromosome doubling of diploid cultivars and selections. Meiotic chromosome behaviour in diploid A. chinensis ‘Hort16A’ and colchicine-induced tetraploids from ‘Hort16A’ was analysed and compared with that in a diploid male and tetraploid males of A. chinensis raised from seeds sourced from the wild in China. Both naturally occurring and induced tetraploids formed multivalents, but colchicine-induced tetraploids showed a higher mean multivalent frequency in the pollen mother cells. Lagging chromosomes at anaphase I and II were observed at low frequencies in the colchicine-induced tetraploids. To investigate whether preferential or non-preferential chromosome pairing occurs in tetraploid A. chinensis, the inheritance of microsatellite alleles was analysed in the tetraploid progeny of crosses between A. chinensis (4x) and A. arguta (4x). The frequencies of inherited microsatellite allelic combinations in the hybrids suggested that non-preferential chromosome pairing had occurred in the tetraploid A. chinensis parent.     

Mass-spectrometrical analysis of proteins encoded on chromosome 21 in human fetal brain

Summary. Overexpression of chromosome 21 genes is directly or indirectly responsible for the Down syndrome phenotype. In order to analyse chromosome 21 gene products (Chr21Ps), we extracted proteins from fetal human brain cortex and applied an ultracentrifugal and chromatographic prefractionation principle followed by two-dimensional gel electrophoresis (2-DE) and mass-spectrometrical analysis using high-throughput automated MALDI-TOF/TOF. Nine Chr21Ps were identified: pyridoxal kinase; superoxide dismutase [Cu/Zn] 1; carbonyl reductase 1; ES1 protein homolog, mitochondrial [Precursor]; cystathionine-beta-synthetase; T-complex protein 1, theta subunit; cystatin B; 6-phosphofructokinase; glycinamide ribonucleotide synthetase. Mass-spectrometric characterisation of Chr21Ps following separation in 2-DE gels is a useful tool for the analysis of these structures in brain, independent of antibody availability and specificity.