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PubMed2000

Phylogenetic and biogeographic relationships of eastern Asian and eastern North American disjunct Suillus species (fungi) as inferred from nuclear ribosomal RNA ITS sequences.

Species of Suillus produce fleshy, pored mushrooms. They are important symbiotic (ectomycorrhizal) partners of many coniferous trees. The genus includes several putative eastern Asian and eastern North American disjunct species, i.e., the S. americanus-S. sibiricus and S. decipiens-S. spraguei complexes. Phylogenetic relationships among the groups were determined to further understand the biogeographic pattern. Analyses were based on 40 sequences of the ITS region of the nuclear ribosomal RNA tandem repeats, representing 18 distinct species/populations. Our phylogenetic analyses suggested that: (1) Chinese and United States' (U.S.) S. spraguei plus S. decipiens form a strongly supported monophyletic group, with North American S. decipiens and Chinese S. spraguei being sister taxa; (2) S. americanus, Asian and U.S. S. sibiricus, plus S. umbonatus form a clade supported by a high bootstrap value; and (3) little ITS sequence divergence exists within the latter group compared to the S. decipiens-S. spraguei clade. Phylogenetic patterns revealed by this study imply a close phylogenetic relationship between eastern Asian and eastern North American disjunct population/species of Suillus. These fungi display relatively high host fidelity (at least to the host subgenus level), suggesting potential coevolutionary/comigratory trends.

Agaricales↗

PubMed1999

The Disjunction Effect and Reason-Based Choice in Games.

This paper reports an experiment that extends previous find ings of the disjunction effect, sometimes described as a violation of Savage's sure-thing principle. Evidence of the disjunction ef fect is observed using elicited beliefs about others' actions (rather than controlled beliefs) in a prisoners' dilemma studied by Shafir and Tversky (1992) as well as in an asymmetric version of it and in a nondilemma game with a unique equilibrium. Debiasing tech niques as well as implications for these extensions are discussed. Copyright 1999 Academic Press.

Journal Article↗

PubMed2001

The Disjunction Effect: Does It Exist for Two-Step Gambles?

One of the basic axioms of the rational theory of decision under uncertainty is Savage's (1954) Sure Thing Principle. It states that if Prospect x is preferred to Prospect y knowing that Event A occurred, and if x is preferred to y knowing that A did not occur, then x should also be preferred to y when it is not known whether A occurred. Tversky and Shafir (1992) claim to have demonstrated a violation of this principle in two-step gambles, which is termed a disjunction effect. The present article evaluates the replicability of the disjunction effect for two-step gambles. The findings show that people do not violate the sure thing principle in repeated gambles. The validity of alleged violations in other paradigms is discussed. Copyright 2001 Academic Press.

Journal Article↗

PubMed1977

Meiosis I non-disjunction as the main cause of trisomy 21.

The relative roles of Meiosis I and Meiosis II non-disjunctions in the causation of trisomy 21 have been assessed by analysing the distribution of polymorphic phenotypes of the chromosomes 21 in a group of individuals with Down's syndrome. The data suggest that the majority of cases of trisomy 21 are due to meiosis I non-disjunctions.

Chromosomes, Human, 21-22 and Y↗

PubMed1981

Adjacent 2 meiotic disjunction. report of a case resulting from a familial 13q;15q balanced reciprocal translocation and review of the literature.

An abnormal short-lived female infant with almost complete trisomy 13 (pter leads to q32 or 33) and partial monosomy 15 (pter leads to q14 or 15) resulting from an adjacent 2 meiotic disjunction of a paternal reciprocal translocation is described. Cases with monosomy of chromosome 15 material are reviewed. It appears likely that monosomy of an interstitial long arm segment, approximating to 15q21 leads to 24, imparts the lethality associated with the full monosomic condition. Adjacent 2 disjunction in man has been further characterised by reviewing the literature.

Abnormalities, Multiple↗

PubMed1986

Direct estimation of the non-disjunction rate at first meiotic division in the human male. Preliminary results.

Chromosomal analysis of 100 second metaphases from 19 men attending an infertility clinic for various reasons was carried out to estimate the rate of non-disjunction occurring at first meiotic division. Second metaphases were selected on the basis of the good spread of their chromosomes. Karyotypes were performed using the relative length of the chromosomes and the centromeric index. Among aneuploid cells, only those containing a hyperhaploid complement (24) were regarded as informative. Of the 100 MII cells, two were hyperhaploid. The frequency of aneuploid MII cells following non-disjunction at first meiotic division is compared to the rate of aneuploid spermatozoa observed after using fertilization of zona-free golden hamster eggs.

Adult↗

PubMed1982

Trisomy 21: origin of non-disjunction.

The Q-band heteromorphisms of chromosome 21 were used in a sample of 48 families with a Down's syndrome child to evaluate the origin of non-disjunction. The parental origin and the meiotic error were determined in 27 families, and in eight families only partial information was obtained. Paternal and maternal origin of non-disjunction was in a 1:3 ratio. Failures were five times more frequent in first than in second meiotic division in both sexes. The mean parental age and environmental factors in relation to the origin of the anomaly are discussed. Our results are compared with those obtained previously in similar studies by other authors.

Chromosomes, Human, 21-22 and Y↗

PubMed1995

Immunocytology of chiasmata and chromosomal disjunction at mouse meiosis.

Immunocytological and in situ hybridization evidence supports the hypothesis that at meiosis of chiasmate organisms, chromosomal disjunction and reductional segregation of sister centromeres are integrated with synaptonemal complex functions. The Mr 125,000 synaptic protein, Syn1, present between cores of paired homologous chromosomes during pachytene of meiotic prophase, is lost from synaptonemal complexes coordinately with homolog separation at diplotene. Separation is constrained by exchanges between non-sister chromatids, the chiasmata. We show that the Mr 30,000 chromosomal core protein, Cor1, associated with sister chromatid pairs, remains an axial component of post-pachytene chromosomes until metaphase I. We demonstrate that at this time the chromatin loops are still attached to their cores. A reciprocal exchange event between two homologous non-sister chromatids is therefore immobilized by anchorage of sister chromatids to their respective cores. Cores thus contribute to the sister chromatid cohesiveness required for maintenance of chiasmata and proper chromosomal disjunction. Cor1 protein accumulates in juxtaposition to pairs of sister centromeres during metaphase I. Presumably, independent movement of sister centromeres at anaphase I is restricted by Cor1 anchorage. That reductional separation of sister centromeres is mediated by Cor1, is supported by the dissociation of Cor1 from separating sister centromeres at anaphase II and by its absence from mitotic anaphases.

Animals↗

PubMed2001

Numerical chromosome abnormalities in the spermatozoa of the fathers of children with trisomy 21 of paternal origin: generalised tendency to meiotic non-disjunction.

The purpose of this study was the evaluation of aneuploidy frequencies in the spermatozoa of two fathers (DP-4 and DP-5) who had children with Down syndrome (DS) of paternal origin and in whom a previous sperm analysis by fluoresence in situ hybridisation (FISH) had suggested a generalised tendency to meiotic non-disjunction. Sperm samples were simultaneously hybridised with FISH probes for chromosomes 4, 13 and 22. Disomy frequencies for each of the chromosomes and diploidy frequencies were compared with data obtained from nine control donors. Both DS fathers had a statistically significant increase in the frequency of disomy for chromosomes 13 and 22. DP-5 also had an increased frequency of diploid spermatozoa. Our data suggest that the two DS fathers have a generalised susceptibility to meiotic non-disjunction and that acrocentric chromosomes seem to be more sensitive to such disturbance in the meiotic process.

Child↗

PubMed1979

Relationships between satellite association and the occurrence of non-disjunction in man.

It has been reported that there is an increased incidence of Down's syndrome among the children of parents who have been exposed to ionizing radiations for radiodiagnostic or radiotherapeutic reasons. Work with Drosophila, mice and human lymphocytes has shown that irradiation with X- or gamma-rays induces aneuploidy, presumably by non-disjunction. It has been suggested that in man the frequency of satellite association (s.a.) of acrocentric chromosomes may be involved in the causation of chromosomal non-disjunction. In the present work the effects of radiation on s.a. have, therefore, been investigated. The frequency of s.a. between acrocentric chromosomes was determined after the exposure of human blood from normal and chromosomally abnormal individuals to various small doses of Co-60 gamma-rays. The criteria of Zang and Back were used for the evaluation of s.a. complexes. No effects of radiation on the frequency of s.a. were apparent within the dose range investigated. The same result was obtained when s.a. was evaluated using the silver-staining technique in which physical connections between the associating satellites may be observed and the association complexes evaluated directly. The effects of other radiation sources have also been investigated.

Aneuploidy↗

PubMed1987

Meiotic non-disjunction induced by fission neutrons relative to X-rays observed in mouse secondary spermatocytes. II. Dose-effect relationships after treatment of pachytene cells.

(C57B1/Cne X C3H/Cne)F1 male mice were irradiated with single acute doses of 0.4 MeV neutrons (from 0.11 to 0.72 Gy) or 250 kV X-rays (from 0.25 to 3 Gy) and sacrificed 5 days later. Chromosome preparations of secondary spermatocytes, irradiated at the stage of pachytene, were analysed and the incidence of hyper-haploidies and chromosome fragments was recorded. Data on numerical aberrations were fitted by highly significant linear relationships for both types of radiation. A relative biological effectiveness (RBE) value of 5.65 was estimated by the ratio between the slopes of the two regression lines. The same linear fitting was applied to frequencies of cells with fragments, even if in this case other types of functions could not be excluded. An RBE value was estimated in the same way as for numerical aberrations and yielded a comparable figure of 5.23. A significant correlation was also found between the incidence of numerical and structural aberrations, which points to the chromosome itself as the prevalent target for radiation-induced non-disjunction (ND). In addition, the highly significant linearity of the dose-effect relationship observed for the induction of aneuploidies suggests, as the simplest hypothesis, a single-hit mechanism of radiation action, possibly through pre-non-disjunctional damage to the centromeric region, rather than an indirect induction of segregational difficulties after primarily induced chromatid interchanges.

Aneuploidy↗

PubMed1991

Disjunctive eye movement evoked by microstimulation in an extrastriate cortical area of the cat.

Slow disjunctive eye movement similar to ocular convergence was evoked by microstimulation in parts of the lateral suprasylvian area (LSA) in alert cats. A tungsten-in-glass microelectrode was used for stimulation, and eye movement was monitored using the magnetic search coil method. The velocity-versus-amplitude relationship of disjunctive eye movement evoked by microstimulation was comparable to that of ocular convergence evoked by presenting a visual target. It is suggested that the LSA plays a role in controlling convergence eye movement.

Animals↗

PubMed2000

Illusory inferences from a disjunction of conditionals: a new mental models account.

(Johnson-Laird, P.N., & Savary, F. (1999, Illusory inferences: a novel class of erroneous deductions. Cognition, 71, 191-229.) have recently presented a mental models account, based on the so-called principle of truth, for the occurrence of inferences that are compelling but invalid. This article presents an alternative account of the illusory inferences resulting from a disjunction of conditionals. In accordance with our modified theory of mental models of the conditional, we show that the way individuals represent conditionals leads them to misinterpret the locus of the disjunction and prevents them from drawing conclusions from a false conditional, thus accounting for the compelling character of the illusory inference.

Adult↗

PubMed2000

Disjunction of homologous chromosomes in meiosis I depends on proteolytic cleavage of the meiotic cohesin Rec8 by separin.

It has been proposed but never proven that cohesion between sister chromatids distal to chiasmata is responsible for holding homologous chromosomes together while spindles attempt to pull them toward opposite poles during metaphase of meiosis I. Meanwhile, the mechanism by which disjunction of homologs is triggered at the onset of anaphase I has remained a complete mystery. In yeast, cohesion between sister chromatid arms during meiosis depends on a meiosis-specific cohesin subunit called Rec8, whose mitotic equivalent, Sccl, is cleaved at the metaphase to anaphase transition by an endopeptidase called separin. We show here that cleavage of Rec8 by separin at one of two different sites is necessary for the resolution of chiasmata and the disjunction of homologous chromosomes during meiosis.

Amino Acid Sequence↗

PubMed1998

An anaphase calcium signal controls chromosome disjunction in early sea urchin embryos.

A transient increase in intracellular calcium concentration [Ca2+]i occurs throughout the cell as sea urchin embryos enter anaphase of the first cell cycle. The transient just precedes chromatid disjunction and spindle elongation. Microinjection of calcium chelators or heparin, an InsP3 receptor antagonist, blocks chromosome separation. Photorelease of calcium or InsP3 can reverse the block. Nuclear reformation is merely delayed by calcium antagonists at concentrations that block chromatid separation. Thus, the calcium signal triggers the separation of chromatids, while calcium-independent pathways can bring about the alterations in microtubule dynamics and nuclear events associated with anaphase progression. That calcium triggers chromosome disjunction alone is unexpected. It helps explain previous conflicting results and allows the prediction that calcium plays a similar role at anaphase in other cell types.

Anaphase↗

PubMed1998

Loss of heterozygosity at the dilute-short ear (Myo5a-Bmp5) region of the mouse: mitotic recombination or double non-disjunction?

The occurrence of homozygous-viable dilute-short ear (Myo5a-Bmp5) double mutants in mouse specific locus mutation experiments has generally been assumed to be the result of double non-disjunction such that the mutant inherits two copies of chromosome 9 carrying the recessive alleles from the test-stock. A homozygous viable Myo5a-Bmp5 double mutant was recovered recently in our laboratory. We were able to genetically analyse both the Myo5a-Bmp5 region and proximal and distal markers in the original mutant as well as in offspring of the original mutant. Our results indicate the mutational event to be due to mitotic recombination and not double non-disjunction.

Alleles↗

PubMed2002

The elusive matching bias effect in the disjunctive selection task.

When reasoning with conditional statements (i.e., if [not] p then [not] q), for example when solving Wason's selection task, subjects tend to display matching bias: Options which match the entities named in the rule tend to be selected irrespective of whether this is logically appropriate. Recently, there have been suggestions that the underlying causes of matching bias reflect a general phenomenon that applies to many types of logical rule, not just conditionals. A study is reported in which performance is investigated for selection tasks with categorical or disjunctive rules. Although matching bias was clearly present for categorical rules, inverted matching bias was identified for disjunctive rules, calling into question the generality of the phenomenon and its explanations. In addition, performance at one task was not correlated with performance at the other, calling into question recent cognitive capacity accounts of selection task performance.

Adolescent↗

PubMed2001

Distinguishing logic from association in the solution of an invisible displacement task by children (Homo sapiens) and dogs (Canis familiaris): using negation of disjunction.

Prior research on the ability to solve the Piagetian invisible displacement task has focused on prerequisite representational capacity. This study examines the additional prerequisite of deduction. As in other tasks (e.g., conservation and transitivity), it is difficult to distinguish between behavior that reflects logical inference from behavior that reflects associative generalization. Using the role of negation in logic whereby negative feedback about one belief increases the certainty of another (e.g., a disjunctive syllogism), task-naive dogs (Canis familiaris; n=19) and 4- to 6-year-old children (Homo sapiens; n=24) were given a task wherein a desirable object was shown to have disappeared from a container after it had passed behind 3 separate screens. As predicted, children (as per logic of negated disjunction) tended to increase their speed of checking the 3rd screen after failing to find the object behind the first 2 screens, whereas dogs (as per associative extinction) tended to significantly decrease their speed of checking the 3rd screen after failing to find the object behind the first 2 screens.

Animals↗