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PubMed2003

Phylogeography of Cavernularia hultenii: evidence of slow genetic drift in a widely disjunct lichen.

Population structure and history is poorly known in most lichenized ascomycetes. Many species display large-scale infraspecific disjunctions, which have been explained alternately by range fragmentation in species of high age and widespread long-distance dispersal. Using the lichen Cavernularia hultenii, which is widely disjunct across North America and Europe, Pleistocene and Holocene population history was inferred. The internal transcribed spacer (ITS) and part of the the intergenic spacer (IGS) region of the nuclear ribosomal DNA were sequenced in 300 individuals representing 62 populations across the range of the species. While four ancestral haplotypes are found in all areas, none of the observed tip haplotypes is present in more than one of the three part ranges. Although this is evidence for a past fragmentation event, nested clade analysis (NCA) remains equivocal in the choice between allopatric fragmentation and long-distance dispersal. Mismatch distributions indicate exponential population growth, probably during postglacial invasion of C. hultenii into formerly glaciated areas of western North America. The presence of one southern and at least one northern glacial refugium in South Central Alaska is inferred. Evidence for another refugium in the Queen Charlotte Islands or Alexander Archipelago is inconclusive because of sparse sampling. However, a range expansion was not confirmed unambiguously by NCA. The limited power of NCA to infer past range fragmentations and expansions is due apparently to the shallow haplotype network and widespread ancestral haplotypes. This can be explained by slow genetic drift causing incomplete removal of ancestral haplotypes from the postfragmentation and postexpansion areas.

Base Sequence↗

PubMed2002

Laurasian migration explains Gondwanan disjunctions: evidence from Malpighiaceae.

Explanations for biogeographic disjunctions involving South America and Africa typically invoke vicariance of western Gondwanan biotas or long distance dispersal. These hypotheses are problematical because many groups originated and diversified well after the last known connection between Africa and South America (approximately 105 million years ago), and it is unlikely that "sweepstakes" dispersal accounts for many of these disjunctions. Phylogenetic analyses of the angiosperm clade Malpighiaceae, combined with fossil evidence and molecular divergence-time estimates, suggest an alternative hypothesis to account for such distributions. We propose that Malpighiaceae originated in northern South America, and that members of several clades repeatedly migrated into North America and subsequently moved via North Atlantic land connections into the Old World during episodes starting in the Eocene, when climates supported tropical forests. This Laurasian migration route may explain many other extant lineages that exhibit western Gondwanan distributions.

Arabidopsis Proteins↗

PubMed1975

The disjunction property implies the numerical existence property.

Any recursively enumerable extension of intuitionistic arithmetic which obeys the disjunction property obeys the numerical existence property. Any recursively enumerable extension of intuitionistic arithmetic proves its own disjunction property if and only if it proves its own inconsistency.

Journal Article↗

PubMed1988

Meiotic disjunction of homologs in Saccharomyces cerevisiae is directed by pairing and recombination of the chromosome arms but not by pairing of the centromeres.

We explored the behavior of meiotic chromosomes in Saccharomyces cerevisiae by examining the effects of chromosomal rearrangements on the pattern of disjunction and recombination of chromosome III during meiosis. The segregation of deletion chromosomes lacking part or all (telocentric) of one arm was analyzed in the presence of one or two copies of a normal chromosome III. In strains containing one normal and any one deletion chromosome, the two chromosomes disjoined in most meioses. In strains with one normal chromosome and both a left and right arm telocentric chromosome, the two telocentrics preferentially disjoined from the normal chromosome. Homology on one arm was sufficient to direct chromosome disjunction, and two chromosomes could be directed to disjoin from a third. In strains containing one deletion chromosome and two normal chromosomes, the two normal chromosomes preferentially disjoined, but in 4-7% of the tetrads the normal chromosomes cosegregated, disjoining from the deletion chromosome. Recombination between the two normal chromosomes or between the deletion chromosome and a normal chromosome increased the probability that these chromosomes would disjoin, although cosegregation of recombinants was observed. Finally, we observed that a derivative of chromosome III in which the centromeric region was deleted and CEN5 was integrated at another site on the chromosome disjoined from a normal chromosome III with fidelity. These studies demonstrate that it is not pairing of the centromeres, but pairing and recombination along the arms of the homologs, that directs meiotic chromosome segregation.

Centromere↗

PubMed1977

Centromeric effect on the degree of nonrandom disjunction in the female Drosophila melanogaster.

From crosses of females possessing a heteromorphic X-chromosome bivalent, FR1/+, the shorter crossover products were recovered on the average more frequently than the longer reciprocals as predicted by Novitski's (1951) hypothesis of nonrandom disjunction (NRD). The present study stemmed from an unexpected result of these crosses. Evidence for a centromeric effect on NRD was obtained, suggested by a negative correlation between the degree of NRD, c, and the distance between the region of exchange and the centromere as inferred from SET's (single exchange tetrads). Studies on sex chromosome systems other than FR1 confirmed these results. An analogous centromeric effect on preferential segregation had been clearly demonstrated in maize (Kikudome 1958, 1959; Rhoades and Dempsey 1966). However, prior to the present investigation, no such effect of the centromere on NRD in Drosophila had been described, although reanalysis of part of the data of Novitski (1951) and Novitski and Sandler (1956) suggests some evidence of a seriation of increasing c values extending from the most distal region of the chromosome toward the centromere. A suggestion that the effect in Drosophila may be related in some way to the time required for chiasma terminalization, i.e., those terminalizing earlier (distally located crossovers) permitting more random disjunction of the chromatids from the asymmetric dyad and those terminalizing later, progressively less random, is considered and rejected since in general the expected pattern of c values for the various double exchange tetrads (DET's) is inconsistent with that prediction and provides evidence suggesting the possibility of reversals, in part, of c values obtained for SET's.

Animals↗

PubMed1994

Non-disjunction of chromosome 21 in maternal meiosis I: evidence for a maternal age-dependent mechanism involving reduced recombination.

Over 300 cases of trisomy 21 were analyzed to characterize the causes of maternal non-disjunction and to evaluate the basis for maternal age-dependent trisomy 21. We confirmed the observation that recombination along 21q is reduced among non-disjoined chromosomes 21 and further demonstrated that the alterations in recombination are restricted to meiosis I origin. Analysis of the crossover distribution indicates that reduction in recombination is not due simply to failure of pairing and/or absence of recombination in a proportion of cases. Instead, we observed an increase in both zero- and one-exchange events among trisomy 21-generating meioses suggesting that an overall reduction in recombination may be the underlying cause of non-disjunction. Lastly, we observed an age-related reduction in recombination among the meiosis I cases, with older women having less recombination along 21q than younger women. Thus, reduced genetic recombination may be responsible, at least in part, for the association between advancing maternal age and trisomy 21.

Adult↗

PubMed1992

Hyperthermia specifically inhibits bivalent chromosome disjunction in maturing mouse oocytes.

The effect of hyperthermia on mammalian oocyte maturation was studied by allowing preovulatory mouse oocytes to mature spontaneously for 17 h in vitro under controlled temperature conditions. At the end of culture, oocytes were screened for their maturation stage and for chromosome morphology and number. Mild hyperthermic conditions (38.5-40.0 degrees C) during maturation specifically disturbed the process of bivalent chromosome disjunction, but not other maturation steps, by blocking oocytes at the metaphase I stage and preventing cells from entering subsequent maturation steps. Some oocytes that had reached metaphase II under hyperthermic conditions had chromosome imbalance. Oocytes matured at 40.0 degrees C displayed chromosome morphological abnormalities, including altered sister chromatid separation and nucleus/nuclei formation, at a frequency significantly higher than oocytes matured at 37.0-39.0 degrees C. When incubation temperature was raised above 40.0 degrees C, increasing fractions of oocytes were inhibited from entering initial maturation steps. We conclude that hyperthermia during mammalian oocyte maturation specifically damages the process of bivalent chromosome disjunction and induces the appearance of chromosome structural defects and imbalance in unfertilized eggs.

Animals↗

PubMed1996

New method of endoscopic pterygomaxillary disjunction for a Le Fort Type I osteotomy.

Endoscopic pterygomaxillary (PM) disjunction in a Le Fort type I osteotomy is presented, and the versatility of this new technique is discussed on the basis of our own experience. We have reexamined the anatomy of the posterior part of the maxilla, focusing in particular on the neurovascular bundle, to attempt to perform the endoscopic PM disjunction through the maxillary sinus along a new osteotomy line. We found that the use of an endoscope provided a magnified field of vision and a view of the objects being manipulated on a television monitor, thereby reducing the necessity of working blind. We have applied this technique in 10 clinical cases, and in all cases the Le Fort type I osteotomy was accomplished safely and with minimal bleeding. The operative time using an endoscope was about 15 minutes to 20 minutes on each side, and the blood loss during this manipulation was less than 50 mL. The application of the endoscope should widen in craniofacial surgical field when its advantages are recognized and new operative approaches for using it are developed.

Adolescent↗

PubMed2026

A CCNA1 Missense Variant Associated With Chromatid Non-Disjunction in Abnormal-Headed Sperm and Male Infertility.

BACKGROUND: Macrozoospermia is a rare form of teratozoospermia characterized by tetraploids, large-headed spermatozoa with multiple flagella, usually caused by bi-allelic AURKC mutations. The etiology of atypical phenotypes with a lower proportion of large headed spermatozoa and single flagella however often remains unresolved. OBJECTIVE: To investigate the genetic cause of severe sperm-head abnormalities with moderate macrozoospermia without multiflagellated spermatozoa in a patient with repeated ICSI failure. An infertile male with three failed ICSI attempts underwent semen analysis, revealing complete teratozoospermia, including 25% macrocephalic spermatozoa. METHODS: Multi-probe FISH targeting chromosomes 13, 18, 21, X, Y assessed chromosomal segregation. Whole-exome sequencing (WES) was performed to identify a candidate variant associated with meiotic abnormalities. RESULTS: FISH analysis revealed a high proportion of spermatozoa with n (23) chromosomes and 2c DNA content, consistent with sister chromatid non-disjunction during meiosis II. WES identified a homozygous missense variation in CCNA1, coding for a protein described to be essential for meiotic progression and chromatin remodeling in male germ cells. DISCUSSION: The variant affects a highly conserved residue within a functional domain and is predicted to be deleterious. This study establishes the first clinical association between CCNA1 mutations and chromatid non-disjunction in human spermatogenesis. It highlights the limitations of current morphology-based diagnostic thresholds and supports cytogenetic and genomic assessment for severe teratozoospermia (especially head abnormalities) and ART failure. CONCLUSION: Expanding genetic screening panels to include CCNA1 may improve diagnostic precision and clinical management in atypical macrozoospermia cases.

ART failure↗

PubMed1978

2:2 and 3:1 meiotic disjunctions in a carrier of a reciprocal 10/14 translocation.

The offspring of a female with a reciprocal whole-arm translocation between a chromosome No. 10 and a chromosome No. 14 is described. She gave birth to three cytogenetically different children: one with a normal, one with an abnormal but balanced, and one with an unbalanced karyotype. In these three cases, 2:2 disjunctions must have occurred during maternal meiosis. She also had a trisomy 14 abortion, which is assumed to have been caused by a 3:1 meiotic disjunction.

Chromosomes, Human, 13-15↗

PubMed2005

Parental origin and cell stage of non-disjunction of double trisomy in spontaneous abortion.

Using polymorphic analysis of microsatellites, we investigated the parental origin and mechanism of double trisomies seen in cases of spontaneous abortion. We obtained chorionic villi from spontaneous abortions, and peripheral blood from females who experienced abortion and their spouses. Chromosomal analysis of 170 cases revealed four cases with double trisomy. The karyotypes of these cases are 48,XX,+16,+22, 48,XXY,+18, 48,XX,+15,+21 and 48,XX,+2,+5. In the present study, the incidence of double trisomy was 2.4% of spontaneous abortions. Polymorphic analysis of microsatellites indicated that extra chromosomes were all of maternal origin in the four cases of double trisomy. The predominance of maternal origin in cases of double trisomy is similar to cases of single trisomy. The result also indicated that both extra chromosomes in two cases occurred by non-disjunction at the first meiotic division, and extra chromosomes in the other two cases occurred by non-disjunction at the first mitotic division. The mean maternal age in cases of double trisomy was significantly higher than that in cases of single trisomy. These findings suggest the possibility that abnormal separation of two or more chromosomes may occur simultaneously in oogonia, and that this phenomenon may increase in relation to the increase in age of women.

Aborted Fetus↗

PubMed2002

Neural basis of disjunctive eye movements.

New evidence has challenged a widely accepted interpretation of Hering's law of equal innervation, which states that disjunctive saccades are produced by the linear addition of conjugate and vergence innervation commands produced by independent oculomotor subsystems. We hypothesize, instead, that saccades are produced by a monocular premotor control network. A model, based on this hypothesis and consistent with known brain-stem anatomy, simulates realistic disjunctive saccades including initial and late slow vergence movements.

Animals↗

PubMed1976

Non-disjunction of an unusual X chromosome.

Because of multiple abnormalities in her children, a young mother was investigated and shown to have a 47,XXX chromosome constitution. Additional C group chromosomes without visible centromeric constrictions were found in a number of cells from the peripheral blood, and using C and Q banding techniques these chromosomes were identified as X chromosomes. Analysis of the banding karyotypes of 300 cells revealed that the acentric X chromosomes had the ability to replicate and that this replication was associated with non-disjunction leading to aneuploid cells. Even though cultured skin cells did not have acentric or extra chromosomes in addition to the triple-X, examination of buccal mucosa cells for the presence of X-bodies suggested that the phenomenon of non-disjunction was present in the epithelial cells of the patient. In addition to the X without a visible centromeric constriction, either acentric D or E chromosomes were found. The data suggest that a functional defect in the cells per se is responsible for the appearance of the acentric chromosomes.

Adult↗

PubMed1980

Maternal age and origin of non-disjunction in trisomy 21.

The role of maternal age in chromosomal non-disjunction was investigated by studying 51 families in whom the origin of the meiotic anomaly had been identified. Results of this study were compared with previously published data. This comparison did not show any difference in mean maternal age, nor in distribution of maternal ages when the origin of non-disjunction was maternal, or paternal, or occurred at the first or second meiotic division. These results make questionable the role of maternal age in the birth of Down's syndrome children.

Chromosome Banding↗

PubMed1991

Molecular studies of non-disjunction in trisomy 16.

The origin of the additional chromosome in 26 trisomy 16 spontaneous abortions was studied using DNA probes for chromosome 16, including a probe for centromeric alpha sequences. We were able to determine the parent and meiotic stage of origin of trisomy in 22 cases, with all being attributable to maternal meiosis I non-disjunction. Furthermore, in each of the remaining four cases the results were compatible with this origin. Thus, it is likely that the high incidence of trisomy 16 results from an abnormal process acting at maternal meiosis I which more frequently involves chromosome 16 than other similar sized chromosomes. In studies of recombination, we found little evidence for an association between reduced or absent recombination and chromosome 16 non-disjunction; however, we were unable to rule out an effect of hyperrecombination.

Cells, Cultured↗

PubMed1995

Double non-disjunction in maternal meiosis II giving rise to a fetus with 48,XXX,+21.

We describe a prenatally detected case of double trisomy involving chromosome 21 and the X chromosome (48,XXX,+21) along with determination of the segregation errors responsible for the double aneuploidy. The patient was ascertained as a result of an abnormal maternal serum analyte screen showing an increased risk for fetal Down's syndrome. Following determination of the abnormal karyotype, pregnancy termination was elected. Microsatellite polymorphisms and cytogenetic heteromorphisms were used to determine that both aneuploidies arose as a result of non-disjunction in maternal meiosis II. These results support hypotheses that a segregation defect at a cellular level may cause non-disjunction involving more than one chromosome.

Abortion, Induced↗

PubMed1978

Problems in estimating non-disjunction rate from the F-bodies of two-headed human spermatozoa.

The F-bodies of two-headed human spermatozoa offer a unique potential for estimating non-disjunction rate of the Y chromosome. But there is an unexplained quantitative discrepancy: a gross deficiency of two-headed spermatozoa with no F-bodies, relative to the numbers with one F-body in each head. Any estimation of non-disjunction rate would be overshadowed by this and other major factors. Two-headed spermatozoa cannot be regarded as a random aggregation of haploid heads in pairs. DNA measurements confirmed that each head of 25 normal-sized two-headed spermatozoa was haploid. The distribution of F-body classes is independent of whether sperm tails are single or double. Donors with an exceptionally high incidence of two-headed spermatozoa exist.

Genetics, Medical↗

PubMed2006

Cyclin-B1-mediated inhibition of excess separase is required for timely chromosome disjunction.

Separase, the cysteine protease that cleaves cohesin and thereby triggers chromosome disjunction, is inhibited by both securin- and phosphorylation-dependent cyclin B1 binding. Using a novel phosphorylation-specific antibody, we show that mitotic-specific phosphorylation of human separase on S1126 is required to establish, but not maintain, cyclin B1 binding. Cells expressing a non-phosphorylatable S1126A mutant maintain cohesion early in mitosis, aligning their chromosomes. Cohesion is then synchronously lost 5 minutes ahead of schedule, without degrading securin or cyclin B1. This premature chromatid disjunction requires the catalytic activity of separase, indicating that it is dependent on cohesin cleavage. Single chromatids then attempt to realign but the lack of tension results in unstable kinetochore-microtubule interactions and Aurora-B-dependent spindle checkpoint activation. Separase mutants that cannot bind cyclin B1 but are phosphorylated on S1126 phenocopy separase S1126A, indicating that cyclin B1 binding, rather than phosphorylation, is the key inhibitory event. Significantly, by overexpressing separase S1126A, we have simultaneously overridden the two known inhibitory mechanisms. First, by elevating separase levels above securin, securin-mediated inhibition is alleviated. Second, by preventing phosphorylation, cyclin-B1-mediated inhibition is also alleviated. Surprisingly, however, cohesion is maintained during the early stages of mitosis, indicating the existence of another mechanism that either inhibits separase or protects its substrate during early mitosis.

Animals↗