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PubMed1988

Orientation and segregation of Robertsonian trivalents in Dichroplus pratensis (Acrididae).

Pairing behavior, metaphase I orientation, and anaphase I segregation of centric fusion trivalents were studied in 26 single, 15 double, and 2 triple male fusion heterozygotes of the polymorphic South American melanopline grasshopper Dichroplus pratensis. They represent the seven different fusions and their combinations already described in different populations of the species. Our analysis showed the following: (1) pairing behavior is very regular in all trivalents; (2) frequencies of linear orientation was very low irrespective of the trivalent involved; (3) reorientation seems to occur frequently since frequencies of abnormal segregation and aneuploid second division cells were invariably lower than those of nonconvergent orientation; (4) aneuploidy and abnormal sperm production increases with increasing number of fusions; (5) chiasma frequency and localisation is relevant to trivalent orientation since trivalents with nonconvergent orientations showed proximal and interstitial chiasmata more frequently than convergently oriented ones. The results are in agreement with the hypothesis that these polymorphisms are old and stable, and confirm that for the maintenance of a balanced polymorphism, if this polymorphism is adaptive because of its consequences on recombination, position effects, etc., changes tending to stabilise trivalent orientation and segregation are central.

Anaphase↗
PubMed1978

Chromosome age and segregation during sporulation of Bacillus megaterium.

The effect of chromosome age on segregation during sporulation was investigated. Vegetative cells of Bacillus megaterium were labeled with [Me-3H]thymine and then were grown at 30 degrees C in nonradioactive medium for various times before being allowed to sporulate. The ratio of the amount of label in sporal DNA to that in sporangial DNA, obtained after minor correction for the sporulation frequency, remained essentially constant as the postlabeling growth period was increased from one to seven generations. The spores were preferentially located at the older poles of sporangia, i.e. the poles formed by divisions occurring prior to those forming the sporangia. Therefore, it seems that old (labeled) chromosomes segregate randomly with respect to both the morphological and genealogical polarities of sporangia. Examination of total cell lysates by dye-buoyant density gradient centrifugation revealed the presence of covalently closed circular DNA from cells grown at 37 degrees C, but none was obtained from cells grown at 30 degrees C. Thus, possible interference by large amounts of extrachromosomal DNA in the determination of the chromosomal segregation pattern is unlikely.

Bacillus megaterium↗
PubMed2000

The pattern of sex chromosome kinetochore phosphorylation during nonrandom segregation in a flea beetle.

In the flea beetle species, Alagoasa bicolor, males have two sex chromosomes, X and Y, each of which is larger than the rest of the genome combined. These large sex chromosomes do not pair at meiosis I, and are therefore not joined at metaphase I. Nevertheless, they always segregate from each other at anaphase I. As prometaphase I progresses, the unpaired X and Y undergo reorientation from a parallel to a linear configuration. Using 3F3/2, an antibody that detects the level of phosphorylation of a kinetochore protein or proteins, we have determined that this reorientation is not accompanied by a change in the level of phosphorylation of the kinetochores of either X or Y. This implies that: i) either the reorientation does not involve the loss or gain of kinetochore microtubules, or ii) if such loss or gain occurs, it does not effect a change in the tension placed on the nonrandomly segregating kinetochores, or iii) the sex chromosomes, as in some other species, have lost the ability to sense kinetochore tension changes. Evolution of nonrandom segregation may necessitate the inability of the participating chromosomes to affect the metaphase checkpoint.

Animals↗
PubMed2004

Granular size segregation in underwater sand ripples.

We report an experimental study of a binary sand bed under an oscillating water flow. The formation and evolution of ripples is observed. The appearance of a granular segregation is shown to strongly depend on the sand bed preparation. The initial wavelength of the mixture is measured. In the final steady state, a segregation in volume is observed instead of a segregation at the surface as reported before. The correlation between this phenomenon and the fluid flow is emphasised. Finally, different "exotic" patterns and their geophysical implications are presented.

Geological Phenomena↗
PubMed1998

Kinetochores and the checkpoint mechanism that monitors for defects in the chromosome segregation machinery.

Whether we consider the division of the simplest unicellular organisms into two daughter cells or the generation of haploid gametes by the most complex eukaryotes, no two processes secure the continuance of life more than the proper replication and segregation of the genetic material. The cell cycle, marked in part by the periodic rise and fall of cyclin-dependent kinase (CDK) activities, is the means by which these two processes are separated. DNA damage and mistakes in chromosome segregation are costly, so nature has further devised elaborate checkpoint mechanisms that halt cell cycle progression, allowing time for repairs or corrections. In this article, we review the mitotic checkpoint mechanism that responds to defects in the chromosome segregation machinery and arrests cells in mitosis prior to anaphase onset. At opposite ends of this pathway are the kinetochore, where many checkpoint proteins reside, and the anaphase-promoting complex (APC), the metaphase-to-interphase transition regulator. Throughout this review we focus on budding yeast but reference parallel processes found in other organisms.

Animals↗
PubMed2000

Segregated signal averaging of sympathetic baroreflex responses in humans.

The goal of this study was to merge the methods currently used to assess beat-by-beat changes in muscle sympathetic nerve activity with a signal-averaging approach and overcome the inherent subjectivity and time-consuming nature of manual analysis of baroreflex-mediated sympathetic responses in humans. This is a retrospective study using data obtained during two prior studies [J. R. Halliwill, J. A. Taylor, and D. L. Eckberg. J. Physiol. (Lond.) 495: 279-288, 1996; C. T. Minson, J. R. Halliwill, T. Young, and M. J. Joyner. FASEB J. 13: A1044, 1999]. Beat-by-beat arterial pressure (Finapres device) and muscle sympathetic nerve activity (microneurography) were recorded in seven healthy, nonsmoking, normotensive subjects (2 men, 5 women) between the ages of 23 and 32 yr during arterial pressure changes induced by bolus injections of nitroprusside and phenylephrine. The muscle sympathetic nerve activity-diastolic pressure relationship was analyzed by both the traditional manual detection method and a novel segregated signal-averaging method. The results show the two analysis approaches are highly correlated across subjects (r = 0.914, P < 0. 05) and are in close agreement [slope for manual detection -6.17 +/- 0.91 (SE) vs. slope for segregated signal averaging -5.98 +/- 0.83 total integrated activity. beat(-1). mmHg(-1); P = 0.60]. However, a considerable time savings is seen with the new method (min vs. h). Segregated signal averaging as developed here provides a valid alternative to "by-hand" analysis of beat-by-beat changes in muscle sympathetic nerve activity that occur during dynamic baroreflex-mediated changes in sympathetic outflow. This approach provides an objective, rapid method to analyze nerve recordings.

Adult↗
PubMed2002

Segregation of ON and OFF retinogeniculate connectivity directed by patterned spontaneous activity.

In many parts of the developing nervous system, the early patterns of connectivity are refined by processes that require neuronal activity. These processes are thought to involve Hebbian mechanisms that lead to strengthening and maintenance of inputs that display correlated pre- and postsynaptic activity and elimination of inputs that fire asynchronously. Here we investigated the role of patterned spontaneous retinal activity and Hebbian synaptic mechanisms on segregation of ON and OFF retinal afferents in the dorsal lateral geniculate nucleus (dLGN) of the developing ferret visual system. We recorded extracellularly the spontaneous spike activity of neighboring pairs of ganglion cells and found that OFF cells have significantly higher mean firing rates than ON cells. Spiking is best correlated between cells of the same sign (ON, ON; OFF, OFF) compared with cells of opposite sign (ON, OFF). We then constructed a simple Hebbian model of retinogeniculate synaptic development based on a correlational framework. Using our recorded activity patterns, together with previous calcium-imaging data, we show that endogenous retinal activity, coupled with Hebbian mechanisms of synaptic development, can drive the segregation of ON and OFF retinal inputs to the dLGN. Segregation occurs robustly when heterosynaptic competition is present within time windows of 50-500 ms. In addition, our results suggest that the initial patterns of connectivity (biases in convergence of inputs) and the strength of inhibition in the network each play a crucial role in determining whether ON or OFF inputs dominate at maturity.

Algorithms↗
PubMed2006

Loss of binocular responses and reduced retinal convergence during the period of retinogeniculate axon segregation.

In the developing mammalian visual system, axon terminals from the two eyes overlap in the dorsal lateral geniculate nucleus (LGN) but then undergo a period of refinement and segregate to form distinct eye-specific domains. We report on the changes in synaptic transmission that occur in rodent LGN during the period of retinogeniculate axon segregation by using anterograde labeling techniques in conjunction with an in vitro preparation where large segments of each optic nerve are preserved. Anterograde labeling of retinal projections in early postnatal day (P) rats with cholera toxin beta subunit indicated an age-related recession in uncrossed retinal projections. Between P2 and P5 uncrossed projections occupied as much as 50% of the LGN and overlapped substantially with crossed projections. Between the first and second postnatal week uncrossed projections receded, so by P14 they assumed an adultlike profile occupying 15-20% of LGN and showed little or no overlap with crossed projections. The postsynaptic responses of LGN cells evoked by the separate stimulation of each optic nerve indicated that before P14, many relay cells were binocularly innervated and received at least four to six inputs from each eye. However, these features of retinogeniculate connectivity were transient and their attrition occurred in concert with a retraction of retinal arbors into nonoverlapping, eye-specific regions. By P18 cells were monocularly innervated and received input from one to three retinal ganglion cells. These results provide a better understanding of the underlying changes in synaptic circuitry that occur during the anatomical segregation of retinal inputs into eye-specific territories.

Animals↗
PubMed2000

Texture segregation in the human visual cortex: A functional MRI study.

The segregation of visual scenes based on contour information is a fundamental process of early vision. Contours can be defined by simple cues, such as luminance, as well as by more complex cues, such as texture. Single-cell recording studies in monkeys suggest that the neural processing of complex contours starts as early as primary visual cortex. Additionally, lesion studies in monkeys indicate an important contribution of higher order areas to these processes. Using functional MRI, we have investigated the level at which neural correlates of texture segregation can be found in the human visual cortex. Activity evoked by line textures, with and without texture-defined boundaries, was compared in five healthy subjects. Areas V1, V2/VP, V4, TEO, and V3A were activated by both kinds of line textures as compared with blank presentations. Textures with boundaries forming a checkerboard pattern, relative to uniform textures, evoked significantly more activity in areas V4, TEO, less reliably in V3A, but not in V1 or V2/VP. These results provide evidence that higher order areas with large receptive fields play an important role in the segregation of visual scenes based on texture-defined boundaries.

Adult↗
PubMed2002

A novel family-specific translocation t(2;20)(p24.1;q13.1) associated with recurrent abortions: molecular characterization and segregation analysis in male meiosis.

In the present study, we present a novel reciprocal translocation t(2;20)(p24.1;q13.1) and its segregation in a three generation family. The rate of miscarriages (50%) in pregnancies from male translocation carriers could be explained by unbalanced translocation-bearing spermatozoa found with a frequency of approximately 55% in the entire sperm population of a t(2;20)(p24.1;q13.1) carrier. These imbalanced spermatozoa mainly present as 2, der(20) and der(2), 20 missegregated (approximately 46%) while adjacent 2 and 3:1 segregation patterns account for approximately 5% and 4% of imbalances, respectively. While the translocation is associated clearly with an increased risk of early abortions (7/12) in both male and female carriers, no malformed livebirths were observed. Our results suggest complete embryonic lethality of imbalanced offspring. With respect to a high rate of segregation to 2, der(20) and to der(2), 20 imbalanced spermatozoa in male translocation carriers and with respect to known cases of partial trisomy 2p and 20q we consider that their corresponding monosomies result in fetal loss. This is the first study reporting multiple abortions associated with partial monosomy 20q13.1-->qter and 2pter-->p24.1 and the first report on the frequency of chromosomal imbalances in gametes of a male t(2;20)(p24.1;q13.1) heterozygote.

Abortion, Habitual↗
PubMed2005

Segregation of chromosomes in sperm of Robertsonian translocation carriers.

Robertsonian translocations are the most frequent structural chromosomal abnormalities in humans and can affect fertility, with various degrees of sperm alterations in men; or the pregnancy outcome of the carriers. The studies on meiotic segregation of chromosomes in sperm of Robertsonian translocation males find a majority of normal or balanced spermatozoa for the chromosomes related to the translocation (mean 85.42%; range 60-96.60%). Furthermore, recent studies suggest an interchromosomal effect. Studies on spermatozoa from translocation carriers, and in mouse models help the comprehension of the meiotic segregation mechanisms. Results of meiotic segregation analysis in man could be integrated in genetic counselling especially when assisted reproductive technology is required.

Chromosomes, Human, Pair 13↗
PubMed2006

Segregation analysis of restless legs syndrome: possible evidence for a major gene in a family study using blinded diagnoses.

OBJECTIVE: The objective of this study was to ascertain the most likely inheritance pattern of restless legs syndrome (RLS) using segregation analysis. METHODS: Probands were RLS patients presenting to the Neurology and Sleep clinics of the Johns-Hopkins Bayview medical center with willing first and second degree relatives. Blinded diagnosis was made in those who exhibited the four diagnostic features of RLS. Analysis was performed on RLS as a dichotomous trait and considering age of onset models on 590 phenotyped subjects from 77 pedigrees. RESULTS: All non-genetic models were rejected considering RLS as a dichotomous trait. A single locus Mendelian dominant model with gender as a covariate had best fit with allele frequency of 0.077 and complete penetrance. RLS frequency in non-carrier subjects was estimated to be 0.14. Two underlying distributions of age of onset, with a possible dichotomy at 26.3 years, were identified. Contrary to the results for RLS as a dichotomous trait, age of onset models did not indicate single major gene inheritance. CONCLUSION: This segregation analysis suggests that the pattern of segregation is consistent with that of a single major locus, when RLS is treated as a dichotomous trait without considering age of onset. The high rate of phenocopies matches known population frequencies and taken with significant residual familial effects and the lack of evidence for a major gene controlling age of onset, indicates that non-genetic causes of RLS may exist and RLS is a complex disorder.

Aged↗
PubMed1976

Quantitative analysis of human chromosome segregation in man-mouse somatic cell hybrids.

The presumed random and independent process of human chromosome segregation in man-mouse somatic cell hybrids was studied. The results of chromosome analysis on 196 cells from 15 related hybrid strains have provided the first convincing evidence that segregation of human chromosomes can be nonindependent and often concordant. Different human chromosomes were not retained with equal frequency in these hybrid clones. Some were present in 80% of all the cells, whereas others appeared in less than 10% of the same cells. Linear regression analysis was used to test for correlation of the frequencies of all pair-wise combinations of human chromosomes present in these hybrid clones. Twenty-two of 136 possible correlations were statistically significant, indicating that concordant segregation of particular pairs of human chromosomes is a rather frequent occurrence.

Animals↗
PubMed1979

Pattern of segregation of chicken HPRT phenotype in Chinese hamster-chick red blood cell hybrids.

The pattern of segregation of hypoxanthine phosphoribosyltransferase (HPRT, E.C. 2.4.2.8) was determined in synchronized Chinese hamster-chick red blood cell hybrids. Three hybrid lines were synchronized at the G1-S boundary. Bromodeoxyuridine pulses were subsequently applied throughout the S phase, and the frequency of the segregant clones was determined. It was found that the segregation of the chicken-specific HPRT phenotype associated with the loss of a chromosome was potentiated by bromodeoxyuridine administered during the first hour following release of the block.

Animals↗
PubMed1987

Segregation after mitotic crossing-over in isodicentric X chromosomes.

Segregation after mitotic crossing-over in an isodicentric (idic) X chromosome with one active and one inactive centromere has given rise to two new cell lines, one in which the idic(Xpter) chromosome has two active centromeres (most of these chromosomes also have an inversion) and another in which neither centromere is active. The two X chromosomes are attached at the telomeres of their short arms. Similar segregation has given rise to two other cell lines with idic(Xq-) chromosomes. Other observations on segregation after mitotic crossing-over are reviewed. Unequal crossing-over has apparently played a major role in the evolution of various genes and heterochromatin. Retinoblastoma and Wilms tumor are in some cases associated with homozygosity of a chromosome segment resulting from mitotic crossing-over. Similarly, the high incidence of cancer in Bloom syndrome may be caused by mitotic crossing-over leading to homozygosity or amplification of oncogenes.

Amenorrhea↗
PubMed1988

Meiotic segregation of human sperm chromosomes in translocation heterozygotes: report of a t(9;10)(q34;q11) and a review of the literature.

Meiotic segregation products were studied in sperm from a man who was heterozygous for a reciprocal translocation, t(9;10)(q34;q11). A total of 171 sperm chromosome complements were studied by in vitro fertilization of hamster eggs. All possible 2:2 and 3:1 meiotic segregations were observed with the following frequencies: alternate, 41%; adjacent-1, 48%; adjacent-2, 5%; 3:1, 6%. Within alternate segregations, the number of normal sperm (35) was not significantly different from the number of sperm carrying a balanced form of the translocation (33), as expected. The proportion of sperm with an unbalanced form of the translocation was 60%. There was no evidence for an interchromosomal effect, since the frequencies of numerical (8%) and structural (15%) chromosomal abnormalities (both unrelated to the translocation) were within the normal range of control donors. The literature on a total of 10 translocation heterozygotes studied by sperm chromosome analysis was reviewed.

Adult↗
PubMed1995

Independent chromosome segregation and absence of interchromosomal effect at first meiotic division in male Chinese hamsters heterozygous for two reciprocal translocations.

Chromosome segregation and interchromosomal effects of reciprocal translocation at first meiotic division were investigated by chromosome analysis of meiotic cells from male Chinese hamsters heterozygous for two reciprocal translocations. We used six stocks heterozygous for two different translocations which were obtained by crosses between males and females homozygous or heterozygous for a different translocation. The frequency of second meiotic (MII) cells from each segregational class of one quadrivalent in heterozygotes for two reciprocal translocations agreed with that of the same quadrivalent in heterozygotes for the single translocation. This indicates that the two quadrivalents in heterozygotes for two reciprocal translocations segregate independently. The mean frequency of hyperhaploid MII cells from males heterozygous for two reciprocal translocations ranged from 7.1 to 15.4%. These cells were mostly derivatives from 3:1 disjunctions of the quadrivalents. The percentage of cells with extra chromosomes in translocation-unrelated groups was calculated at 0.10-0.25%. The frequency did not increase significantly as compared with that in karyotypically normal males, in which the mean frequency of hyperhaploid MII cells was 0.39%. These findings revealed that reciprocal translocations do not have interchromosomal effects on meiotic division in male Chinese hamsters.

Animals↗
PubMed1996

Sperm segregants from the murine Rb(11.14) heterozygote fertilizing in vivo and in vitro.

Segregants of the Rb(11.14) translocation in 17 heterozygous male mice were analyzed in G- and C-banded first-cleavage metaphases after in vivo (n = 440) and in vitro (n = 267) fertilization. Mating intervals of 3 and 14 d provided unaged and physiologically aged sperm for the oocytes, which were from chromosomally normal females. Significantly more normal than balanced sperm segregants were seen in both the in vivo fertilized (P < 0.01) and in vitro fertilized (P < 0.001) study groups. The distortion was elevated in vitro (P < 0.02), unaccompanied by a sex-ratio distortion, and had a segregant distribution that was independent of sperm age; thus it differs from that seen for other translocations. The findings argue for chromosome-specific effects of Robertsonian translocations on sperm function. A significantly (P < 0.05) increased hyperhaploidy rate (unrelated to the translocation) supports the sperm-aging hypothesis.

Animals↗