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PubMed1994

Physical association between nonhomologous chromosomes precedes distributive disjunction in yeast.

During meiosis homologous chromosomes normally pair, undergo reciprocal recombination, and then segregate from each other. Distributive disjunction is the meiotic segregation that is observed in the absence of homologous recombination and can occur for both nonrecombinant homologous chromosomes and completely nonhomologous chromosomes. While the mechanism of distributive disjunction is not known, several models have been presented that either involve or are completely independent of interactions between the segregating chromosomes. In this report, we demonstrate that distributive disjunction in Saccharomyces cerevisiae is preceded by an interaction between nonhomologous chromosomes.

Chromosomes, Fungal↗

PubMed2000

Timing transantarctic disjunctions in the Atherospermataceae (Laurales): evidence from coding and noncoding chloroplast sequences.

Previous studies of the small Southern Hemisphere family Atherospermataceae have drawn contradictory conclusions regarding the number of transantarctic disjunctions and role of transoceanic dispersal in its evolution. Clarification of intergeneric relationships is critical to resolving (1) whether the two Chilean species, Laurelia sempervirens and Laureliopsis philippiana, are related to different Austral-Pacific species, implying two transantarctic disjunctions as suggested by morphology; (2) where the group is likely to have originated; and (3) whether observed disjunctions reflect the breakup of Gondwana. We analyzed chloroplast DNA sequences from six regions (the rbcL gene, the rpl16 intron, and the trnL-trnF, trnT-trnL, psbA-trnH, and atpB-rbcL spacer regions; for all six regions, 4,372 bp) for all genera and most species of Atherospermataceae, using parsimony and maximum likelihood (ML). The family's sister group, the Chilean endemic Gomortega nitida (Gomortegaceae), was used to root the tree. Parsimony and ML yielded identical single best trees that contain three well-supported clades (> or = 75% bootstrap): Daphnandra and Doryphora from south-eastern Australia; Atherosperma and Nemuaron from Australia-Tasmania and New Caledonia, respectively; and Laurelia novac-zelandiac and Laureliopsis philippiana from New Zealand and Chile, respectively. The second Chilean species, Laurelia sempervirens, is sister to this last clade. Likelihood ratio testing did not reject the molecular clock assumption for the rbcL data, which can therefore be used for divergence time estimates. The atherosperm fossil record, which goes back to the Upper Cretaceous, includes pollen, wood, and leaf fossils from Europe, Africa, South America, Antarctica, New Zealand, and Tasmania. Calibration of rbcL substitution rates with the fossils suggests an initial diversification of the family at 100-140 million years ago (MYA), probably in West Gondwana, early entry into Antarctica, and long-distance dispersal to New Zealand and New Caledonia at 50-30 MYA by the ancestors of L. novae-zelandiae and Nemuaron.

Antarctic Regions↗

PubMed1993

The three rows gene of Drosophila melanogaster encodes a novel protein that is required for chromosome disjunction during mitosis.

Zygotic expression of the three rows (thr) gene of Drosophila melanogaster is required for normal cell proliferation during embryogenesis. Mitotic defects in thr mutant embryos begin during mitosis 15, and all subsequent divisions are disrupted. Chromosome disjunction and consequently cytokinesis fail during these defective mitoses, although the initial mitotic processes (chromosome condensation, spindle assembly, metaphase plate formation, and cyclin degradation) are not affected. Despite the failure of chromosome disjunction and cytokinesis, later mitotic events (chromosome decondensation) and subsequent cell cycle progression continue. The thr gene has been isolated and shown to encode a 1209 amino acid protein that shares no extended sequence similarity with known proteins. thr mRNA is present as maternal mRNA that degrades at the time of cellularization. At this and all subsequent times during embryogenesis, zygotic expression correlates with mitotic proliferation. These observations, together with the observation that the zygotic phenotype of thr mutant embryos is influenced by the maternal genotype, suggest that the embryonic phenotype results from exhaustion of the maternal thr contribution and does not reflect a developmentally restricted requirement for thr function. Our results indicate that the novel thr product is required specifically for chromosome disjunction during all mitoses.

Amino Acid Sequence↗

PubMed2000

Genome-wide variation in recombination in female meiosis: a risk factor for non-disjunction of chromosome 21.

Altered recombination patterns along non-disjoined chromosomes is the first molecular correlate identified for non-disjunction in humans. To understand better the factors related to this correlate, we have asked to what extent is recombination altered in an egg with a disomic chromosome: are patterns limited to the non-disjoined chromosome or do they extend to the entire cell? More specifically, we asked whether there is reduced recombination in the total genome of an egg with a non-disjoined chromosome 21 and no detectable recombination. We chose this subclass of non-disjoined chromosomes to enrich potentially for extremes in recombination. We found a statistically significant cell-wide reduction in the mean recombination rate in these eggs with non-disjoined chromosomes 21; no specific chromosomes were driving this effect. Most importantly, we found that this reduction was consistent with normal variation in recombination observed among eggs. Thus, given that recombination is a multifactorial trait, these data suggest that when the number of genome-wide recombination events is less than some threshold, specific chromosomes may be at an increased risk for non-disjunction. Further studies are required to confirm these results, to determine the importance of genetic and environmental factors that regulate recombination and to determine their impact on non-disjunction.

Adult↗

PubMedDate not supplied

Estimating meiotic disjunction frequencies in chicken translocation heterozygotes based on embryonic mortality.

Chickens heterozygous for a chromosomal translocation [MN t(1;4)] were intercrossed and the progeny were analyzed for their chromosome complement. A ratio of 1 homozygous translocation carrier to 4 heterozygous translocation carriers to 1 homozygous standard chromosome carrier was noted (n = 520), rather than the 1:2:1 ratio expected from Mendelian segregation. The excess of heterozygous carriers was apparently caused by union of complementary duplication/deficient gametes. Embryonic death occurred in 68% of fertile zygotes. This finding fits very closely to expectations if alternate and adjacent meiotic disjunctions occurred at equal frequencies. As alternate disjunction frequencies increase from 0.5 to 1.0 in inter se matings, the proportion of inviable zygotes among fertilized ova will decrease from 0.625 to 0 and the proportion of translocation heterozygotes among viable progeny will decrease from 0.667 to 0.5. In instances where alternate and adjacent disjunction occur at equal frequency, preferential recovery of translocation carriers will occur. This may contribute to chromosomal diversity within a species, and possibly lead to speciation.

Animals↗

PubMed1999

Direct and indirect non-disjunction in the origin of trisomy in cultured human lymphocytes.

The aim of the present work was to investigate the processes involved in the origin of trisomic karyotypes, i.e. co-migration of sister chromatids (mitotic non-disjunction, MND) and recovery of micronuclei (MN) originating from lagging chromosomes/chromatids at anaphase (mitotic indirect non-disjunction, MIND), and to evaluate their relative contribution to aneuploidy in human lymphocytes mitotically activated in vitro. Therefore, phytohaemagglutinin-stimulated human lymphocytes from one donor were treated with 10 and 25 nM colchicine and analysed through two cell cycles by means of both molecular (FISH with centromeric DNA probes specific for chromosomes 7 and 11) and classical cytogenetic techniques. The following events were analysed: (i) chromosome/chromatid loss (a MN-generating event) in M(1) bipolar ana-telophases; (ii) MN recovery in M(2+) prophases; (iii) non-disjunction and loss of chromosomes 7 and 11 by FISH analysis in cytochalasin B-induced binucleate cells; (iv) spontaneous frequency of trisomic cells by chromosome counting and FISH analysis in M(1) c-metaphases; (v) induced frequency of trisomic cells by chromosome counting and FISH analysis in M(2) c-metaphases. Our results indicate that MND plays a major role compared with MIND in the origin of trisomic karyotypes, being approximately 4- to 5-fold higher in colchicine-treated cells. Moreover, remarkable reductions in the observed frequencies of trisomic cells were recorded in comparison with the expected ones, with an observed/expected frequency ratio of trisomic M(2) c-metaphases ranging between 1/3 and 1/6.

Adult↗

PubMed1991

Chromosome analysis in human oocytes unfertilized in vitro: a mathematical model for the estimation of the first meiotic non-disjunction frequency.

The results of cytogenetic analysis of unfertilized human oocytes in an in-vitro fertilization programme are to a large extent influenced by the methodology used. Exact conclusions cannot be drawn due to technical and interpretational errors. A mathematical model was constructed, therefore, to estimate the weight of these errors and to calculate the exact frequency of first meiotic non-disjunction in IVF oocytes. Of 246 prepared oocytes only 105 could be karyotyped exactly. Using the classical method of calculation, an aneuploidy frequency of 39% was obtained. By applying the maximum likelihood method a correlation between the frequency of first meiotic non-disjunction and the interpretational error level was found. Loss of chromosomes by anaphase lagging seemed to be undetectable, but the probability of chromosome loss due to fixation and unhomogeneous spreading was found to be 20%. Accepting an error level of 10-20%, the true non-disjunction frequency expected should range between 20 and 40%. This model yields a standard curve which can offer a basis for comparison of results obtained in different studies.

Anaphase↗

PubMed2005

Phylogeographical disjunction in abundant high-dispersal littoral gastropods.

Abstract Phylogeographical disjunctions in high-dispersal marine taxa are variously ascribed to palaeogeographical conditions or contemporary ecological factors. Associated biogeographical studies, however, seldom incorporate the sampling design required to confidently discriminate among such competing hypotheses. In the current study, over 7800 gastropod specimens were examined for operculum colour, and 129 specimens genetically, to test ecological and historical biogeographical hypotheses relating to biogeographical disjunction in the Southern Hemisphere, and to southern Australia in particular. Mitochondrial DNA sequence analysis of the high-dispersal intertidal gastropod Nerita atramentosa in southern Australia (88 specimens; 18 localities) revealed an east-west phylogeographical split involving two highly divergent clades (26.0 +/- 1.9%) exhibiting minimal geographical overlap in the southeast. The eastern clade of Nerita atramentosa is also widespread in northern New Zealand (43 specimens, 10 localities), but no significant genetic differentiation is explained by the Tasman Sea, a 2000-km-wide oceanic barrier. Spatial genetic structure was not detected within either clade, consistent with the species' dispersive planktotrophic phase lasting for 5-6 months. Digital analysis of operculum colouration revealed substantial differences between eastern (tan) and western (black) specimens. Genetic analysis and visual inspection of 88 Australian specimens revealed a completely nonrandom association between mtDNA data and operculum colouration. Independent examination of a further 7822 specimens from 14 sites in southern Australia revealed both colour morphs at all localities, but reinforced the phylogeographical data by indicating a marked turnover in colour morph abundance associated with a palaeogeographical barrier: Wilsons Promontory. This sharp biogeographical disjunction is in marked contrast to the species' high dispersal abilities. The genetic similarity of Nerita morio (Easter Island) and the eastern Australian + New Zealand lineage (1.1 +/- 0.3%) provides further evidence of long-distance dispersal in southern Nerita. Phylogenetic relationships of nine species (four genera) of Neritidae, an almost exclusively tropical gastropod family, are consistent with the hypothesis that southern temperate black nerites comprise a monophyletic radiation.

Analysis of Variance↗

PubMed1986

Paternal non-disjunction in a 46,XY/47,XXY individual with a fragile 17p12 in the mother.

In a family where the mother carried a fragile site at 17p12, RFLP-analysis with the X-specific probe L1.28 showed that the 46,XY/47,XXY mosaicism detected in her Klinefelter son was due to a non-disjunctional event in paternal meiosis I, followed by a secondary loss of an X-chromosome by a mitotic non-disjunction. Thus, an association between the primary meiotic non-disjunction and the presence of the fragile site could be excluded.

Adolescent↗

PubMed1980

Non-disjunction in trisomy 21: study of chromosomal heteromorphisms in 110 families.

QM variants on chromosome 21 and silver staining of NOR regions were applied in the study of non-disjunction in 110 families from different parts of Denmark. In 76% of the families the study was informative. Paternal failures were observed in 11% on Funen as compared 23.5% on Zealand. In one family, crossing-over on the short arms of chromosome 21 in the mother and mitotic non-disjunction of chromosome 21 was observed. Maternal first meiotic error predominates in both high maternal and low maternal age. Also in paternal non-disjunction failures of first meiotic division predominate. Two maternally and one paternally originated cases of de novo translocations were observed.

Adolescent↗

PubMed1996

Centric heterochromatin and the efficiency of achiasmate disjunction in Drosophila female meiosis.

The chromosomal requirements for achiasmate (nonexchange) homolog disjunction in Drosophila female meiosis I have been identified with the use of a series of molecularly defined minichromosome deletion derivatives. Efficient disjunction requires 1000 kilobases of overlap in the centric heterochromatin and is not affected by homologous euchromatin or overall size differences. Disjunction efficiency decreases linearly as heterochromatic overlap is reduced from 1000 to 430 kilobases of overlap. Further observations, including rescue experiments with nod kinesin-like protein transgenes, demonstrate that heterochromatin does not act solely to promote chromosome movement or spindle attachment. Thus, it is proposed that centric heterochromatin contains multiple pairing elements that act additively to initiate or maintain the proper alignment of achiasmate chromosomes in meiosis I. How heterochromatin could act to promote chromosome pairing is discussed here.

Animals↗

PubMedDate not supplied

Disjunctive optokinetic nystagmus in a naturally esotropic macaque monkey: interaction between nasotemporal asymmetries of versional eye movement and convergence.

OBJECTIVE: To assess the interaction between versional and vergence eye movements in normal and strabismic monkeys. METHODS: Horizontal optokinetic nystagmus (OKN) and vergence were measured using the magnetic scleral search coil technique in a normal adult monkey and a strabismic monkey who had naturally occurring early-onset esotropia. Mean eye velocity and vergence angles were calculated during the slow phases of OKN. RESULTS: The strabismic monkey had a nasotemporal asymmetry of OKN favoring nasally directed motion in each eye. During monocular optokinetic stimulation, mean eye velocities were substantially greater for the adducting as compared to the abducting eye. The velocity of the abducting eye was between 55 and 80% of the velocity of the adducting eye (p < 0.01). As a consequence of the disjunctive movements, the eyes converged an average of 4 +/- 2.8 degrees during OKN. Saccadic analysis documented normal lateral rectus function in each eye. Neither an OKN asymmetry nor disjunctive OKN was observed in the normal monkey. CONCLUSION: Disjunctive OKN in the esotropic monkey suggests that the cerebral maldevelopment responsible for nasally biased OKN also contributes to nasal biases in vergence pathways.

Animals↗

PubMed1982

The use of Robertsonian translocations in the mouse for studies on non-disjunction.

In the mouse, gametes with gross chromosome duplications and deficiencies can complement each other to give viable zygotes (with some notable exceptions involving particular chromosomes). These complementation-type offspring can be recognised in intercrosses between translocation heterozygotes in which one parent is homozygous for a recessive genetic marker not carried by the other. This system has beeN used by Lyon and colleagues (1976) to study non-disjunction in heterozygotes for tobacco mouse and laboratory-derived Robertsonian translocations. Although non-disjunction is frequent in the former group, still higher frequencies are needed for a workable test system in which wild type mice are treated and mated to a tester stock generating many aneuploid gametes. Possible approaches include (1) use of semidominant markers, (2) marking both arms, (3) combining two or three independent Robertsonians in the tester stock, (4) use of compounds of Robertsonians wih monobrachial homology, since these give very high frequencies of non-disjunction, (5) generation of a compound of three Robertsonians with tribrachial homology, which should produce aneuploid gametes only. This last seems the most promising approach, if the compound proves fertile, and would be analogous to the isochromosome system of Drosophila.

Aneuploidy↗

PubMed2001

Non-disjunction and chromosome loss in gamma-irradiated human lymphocytes: a fluorescence in situ hybridization analysis using centromere-specific probes.

Centromere-specific DNA probes for chromosomes 4, 7 and 18 were used to simultaneously analyze chromosome loss, non-disjunction, breaks within the labeled region, and nucleoplasmic bridges induced by gamma rays in binucleated human lymphocytes. The doses used were 0, 1, 2 and 4 Gy, and approximately 1000 cells were scored per dose. Micronucleus frequency increased in a linear-quadratic fashion. For chromosome loss, significant increases were observed at 2 and 4 Gy, whereas for non-disjunction significant increases were observed at 1 Gy; thus non-disjunction allowed us to detect the effects of radiation at a lower dose than chromosome loss. The use of centromere-specific probes allowed discrimination between the clastogenic and aneugenic effects of ionizing radiation. The analysis of chromosome loss, not taking fragmented signals into account, ensures the detection of an aneugenic effect, which was not possible using pancentromeric probes. The frequency of chromosome breakage within the labeled regions was higher in nuclei than in micronuclei, suggesting an increase in the engulfment of chromosomal material by nuclei as a consequence of the presence of cytochalasin B in the cultures. Chromatin filaments connecting main nuclei, the so-called nucleoplasmic bridges, were observed in irradiated samples, and are a manifestation of rearranged chromosomes producing anaphase bridges.

Centromere↗

PubMed1992

[Frequency of non-disjunction and loss of sex chromosomes in oogenesis in the Drosophila melanogaster mutant I(1) ts 403 with a defect in the heat-shock protein system in anoxia and at high temperature].

A unique property of Drosophila melanogaster l(1)ts403 strain with the defect in heat shock protein system (HSP) is high frequency of losses and non-disjunction of sex chromosomes induced by heat shock (HS) (37 degrees C, 1 h). This effect was shown in only 6-14-th stages of oocytes. Anoxia was not effective in induction of these mutations. Successive action of anoxia and HS decreased loss frequency and non-disjunction in comparison with the only action of HS. These findings agree with the data in literature indicating that HSP synthesis was increased in the l(1)ts403 mutant when first anoxia and then HS were administered, in contrast to the action of HS only. The role of HSP in the recovery of HS-induced disruptions (chromosomal proteins and meiotic division apparatus) which can lead to chromosome non-disjunction and losses is discussed.

Animals↗

PubMed1990

Adjacent-2 disjunction of a maternal t(9;22) leading to duplication 9pter----q22 and deficiency of 22pter----q11.2.

The proposita presented at birth with multiple congenital anomalies including craniofacial anomalies, bilateral cleft lip and palate, abnormalities of the urogenital system, talipes equinovarus, and the DiGeorge sequence. Cytogenetic investigation showed a 46,XX,-22,+der(9)t(9;22)(q22;q11.2) karyotype. The mother, maternal uncle, and maternal grandmother of the infant are carriers of a reciprocal balanced translocation involving chromosomes 9 and 22 at regions q22 and q11.2, respectively. The unbalanced karyotype seen in the proposita arose due to an adjacent-2 disjunction of the quadrivalent in the mother. Prenatal diagnosis of the second pregnancy of this woman showed a similar karyotype. Review of the literature shows that adjacent-2 disjunction may occur preferentially when certain chromosomes are involved in translocations.

Abnormalities, Multiple↗

PubMed1991

Separation of meiotic and mitotic effects of claret non-disjunctional on chromosome segregation in Drosophila.

The claret (ca) locus in Drosophila encodes a kinesin-related motor molecule that is required for proper distribution of chromosomes in meiosis in females and in the early mitotic divisions of the embryo. Here we demonstrate that a mutant allele of claret non-disjunctional (ca(nd)), non-claret disjunctional Dominant (ncdD), causes abnormalities in meiotic chromosome segregation, but is near wild-type with respect to early mitotic chromosome segregation. DNA sequence analysis of this mutant allele reveals two missense mutations compared with the predicted wild-type protein. One mutation lies in a proposed microtubule binding region of the motor domain and affects an amino acid residue that is conserved in all kinesin-related proteins reported to date. This region of the motor domain can be used to distinguish meiotic and mitotic motor function, defining an amino acid sequence criterion for classifying motors according to function. ncdD's mutant meiotic effect, but near wild-type mitotic effect, suggests that interactions of the ca motor protein with spindle microtubules differ in meiosis and mitosis.

Adenosine Triphosphatases↗

PubMed1999

Internal transcribed spacer sequence phylogeny of Crambe L. (Brassicaceae): molecular data reveal two Old World disjunctions.

Crambe L. (Brassicaceae) is an Old World genus with a disjunct distribution among four major centers of species diversity. A phylogenetic analysis of nucleotide sequences of the internal transcribed spacers (ITS) of the nuclear ribosomal repeat was conducted with 27 species of Crambe and 18 related genera. Cladistic analyses using weighted and unweighted parsimony support Crambe as a monophyletic genus with three major lineages. The first comprises those taxa endemic to the Macaronesian archipelagos. Taxa with a predominant Mediterranean distribution form the second assemblage, and a disjunction between east Africa (C. abyssinica) and the Mediterranean (C. hispanica) occurs in this clade. The third lineage includes all Eurosiberian-Asian taxa and C. kilimandscharica, a species from the highlands of east Africa. A basal biogeographic split between east Africa and Eurasia is present in the third clade. The patterns of relationships in the ITS tree are concordant with known climatic events in northern Africa and southwestern Asia since the middle Miocene. The ITS trees are congruent with the current sectional classification except for a few members of sections Crambe, Leptocrambe, and Orientecrambe (C. cordifolia, C. endentula, C. kilimandscharica, and C. kotschyana). Low levels of support in the basal branches do not allow resolution of which genera of the subtribes Raphaniae or Brassicinae are sister to Crambe. Both subtribes appear to be highly polyphyletic in the ITS trees.

Base Composition↗