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PubMed1989

Hypersegregation in U.S. metropolitan areas: black and Hispanic segregation along five dimensions.

Residential segregation has traditionally been measured by using the index of dissimilarity and, more recently, the P* exposure index. These indices, however, measure only two of five potential dimensions of segregation and, by themselves, understate the degree of black segregation in U.S. society. Compared with Hispanics, not only are blacks more segregated on any single dimension of residential segregation, they are also likely to be segregated on all five dimensions simultaneously, which never occurs for Hispanics. Moreover, in a significant subset of large urban areas, blacks experience extreme segregation on all dimensions, a pattern we call hypersegregation. This finding is upheld and reinforced by a multivariate analysis. We conclude that blacks occupy a unique and distinctly disadvantaged position in the U.S. urban environment.

Black or African American↗

PubMed1988

Variations for susceptibilities to ultraviolet induced cellular inactivation and gene segregation among protoplast fusion hybrids of Candida albicans.

Hybrids of the naturally diploid, asexual and opportunistically pathogenic yeast, Candida albicans, can be obtained artificially by protoplast fusion. Evidence is presented that gene conversion and reciprocal recombination contribute to ultraviolet (UV)-induced segregations of heterozygous markers from both diploid and hybrid strains, and that hybrids also segregate through induced chromosome loss. Heterozygous diploid strains independently derived from the same wild-type diploid stock were alike in post-UV survival and segregational responses, and the organization of a four gene linkage group identified in diploids from the segregant products of reciprocal recombinations was transmitted intact to all hybrids from fusions between diploids of isogenic or nonisogenic backgrounds. However, hybrids arising independently from a given fusion cross differed significantly from each other in post-UV survival, absolute ability to segregate some parental markers, frequency of gene segregation, and proclivities for each of the three mechanisms of gene segregation. The bearings of the genetic backgrounds of parental strains and of growth temperatures during hybrid formation on each of these variables are described. The findings emphasize that awareness of the intrinsic heterogeneities of fusion hybrids is essential for reliable application of the protoplast fusion procedure to genetic analysis of C. albicans.

Candida albicans↗

PubMed1996

Growth and microbial flora of nonmedicated, segregated, early weaned pigs from a commercial swine operation.

OBJECTIVE: To determine whether segregated, early weaned pigs have better growth performance and different microbial flora than those pigs raised on-site. DESIGN: Prospective, observational study. ANIMALS: Pigs from a commercial operation that were known to be infected with several common swine pathogens. PROCEDURE: Pigs (7 to 10 days old) were weaned and segregated from the farm of origin and compared with littermate control pigs (14 to 17 days old) that were weaned and raised on-site. Pig weight was measured and microbial flora were isolated at 14-day intervals for 84 days, beginning when the pigs were 7 to 10 days old. RESULTS: At 50 days of age, the segregated, early weaned pigs had a mean weight of 23.7 kg, compared with a mean weight of 12.5 kg for control pigs. Pasteurella multocida was isolated from fewer segregated, early weaned pigs than from controls. Signs of Mycoplasma hyopneumoniae infection were detected in control pigs but not in segregated early weaned pigs. Clinical, serologic, or bacteriologic signs of early postnatal vertical transmission of Actinobacillus pleuropneumoniae were not detected in either group. CLINICAL IMPLICATION: Vertical transmission of M hyopneumoniae was prevented by weaning pigs at 7 to 10 days of age and segregating them off-site, without the use of medication. Although medicated controls were not compared, results from this herd revealed that use of antibiotics is not the most important factor for disease control in segregated, early weaning programs. Minimizing antibiotic use in disease-control protocols reduces costs as well as removes the need for extra-label drugs.

Actinobacillus pleuropneumoniae↗

PubMed2004

Preferential alternate segregation in the common t(11;22)(q23;q11) reciprocal translocation: sperm FISH analysis in two brothers.

Segregation behaviour studies in t(11;22) carriers have reported controversial results. Whereas some authors have detected a preponderance of 3:1 products, no evidence of such prevalence was found by others. This study reports a fluorescence in-situ hybridization (FISH) segregation analysis on decondensed spermatozoa in two brothers, carriers of the same t(11;22)(q23;q11) rearrangement. Data revealed a similar meiotic segregation pattern in both carriers, 2:2 Alternate segregation being the most frequent (42.94 and 45%), while 3:1 genotypes were the least frequent in both patients, with percentages around 10%. The production of three chiasmata, based on the presence of G-light bands along the translocated segments and the presence of recombination sites at 11q and 22q distal regions, are proposed as the cause of a preponderance of the Alternate segregation. Interchromosomal effects involving chromosomes 13, 18, 21, X and Y were also evaluated. An increased frequency of sex chromosome disomies was detected in one patient. Reviewing the literature, a relationship between this phenomenon and the involvement of acrocentric chromosomes in the reorganization is suggested. FISH segregation and interchromosomal effects studies in spermatozoa are encouraged to gather information to establish the best approach for preimplantational genetic diagnosis in reorganization carriers.

Adult↗

PubMed2002

Two separate DNA sequences within oriC participate in accurate chromosome segregation in Bacillus subtilis.

Current views of bacterial chromosome segregation vary in respect of the likely presence or absence of an active segregation mechanism involving a mitotic-like apparatus. Furthermore, little is known about cis-acting elements for chromosome segregation in bacteria. In this report, we show that two separate DNA regions, a 3' coding region of dnaA and the AT-rich sequence between dnaA and dnaN (the initial opening site of duplex DNA during replication), are necessary for efficient segregation of the chromosome in Bacillus subtilis. When a plasmid replicon was integrated into argG, far from oriC, on the chromosome and then the oriC function was disrupted, the oriC-deleted mutant formed anucleate cells at 5% possibly because of defects in chromosome segregation. However, when the two DNA sequences were added near oriN, frequency of anucleate cells decreased to 1%. In these cells, the origin (argG) regions were localized near cell poles, whereas they were randomly distributed in cells without the two DNA sequences. These results suggest that the two DNA sequences in and downstream of the dnaA gene participate in correct positioning of the replication origin region within the cell and that this function is associated with accurate chromosome segregation in B. subtilis.

Bacillus subtilis↗

PubMed2003

Segregation of the Escherichia coli chromosome terminus.

We studied the segregation of the replication terminus of the Escherichia coli chromosome by time-lapse and still photomicroscopy. The replicated termini lie together at the cell centre. They rapidly segregate away from each other immediately before cell division. At fast growth rate, the copies move progressively and quickly toward the centres of the new-born cells. At slow growth rate, the termini usually remain near the inner cell pole and migrate to the cell centre in the middle of the cell cycle. A terminus domain of about 160kb, roughly centred on the dif recombination site, segregated as a unit at cell division. Sequences outside this domain segregated before division, giving two separate foci in predivision cells. Resolution of chromosome dimers via the terminus dif site requires the XerC recombinase and an activity of the FtsK protein that is thought to align the dif sequences at the cell centre. We found that anchoring of the termini at the cell centre and proper segregation at cell division occurred normally in the absence of recombination via the XerC recombinase. Anchoring and proper segregation were, however, frequently disrupted when the C-terminal domain of FtsK was truncated.

Base Sequence↗

PubMed2006

Host-parasite coevolution and selection on sex through the effects of segregation.

The advantage of producing novel variation to keep apace of coevolving species has been invoked as a major explanation for the evolution and maintenance of sex (the Red Queen hypothesis). Recent theoretical investigations of the Red Queen hypothesis have focused on the effects of recombination in haploid species, finding that species interactions rarely favor the evolution of sex unless selection is strong. Yet by focusing on haploids, these studies have ignored a potential advantage of sex in diploids: generating novel combinations of alleles at a particular locus through segregation. Here we investigate models of host-parasite coevolution in diploid species to determine whether the advantages of segregation might rescue the Red Queen hypothesis as a more general explanation for the evolution of sex. We find that the effects of segregation can favor the evolution of sex but only under some models of infection and some parameter combinations, almost always requiring inbreeding. In all other cases, the effects of segregation on selected loci favor reductions in the frequency of sex. In cases where segregation and recombination act in opposite directions, we found that the effects of segregation dominate as an evolutionary force acting on sex in diploids.

Animals↗

PubMed2000

Mitochondrial DNA segregation in the developing embryo.

Mitochondrial (mt)DNA is strictly maternally inherited in mammals; new mutations thus segregate along maternal lineages without the benefit of homologous recombination with mtDNA of paternal origin. Despite the high mtDNA copy number (approximately 100000 or more) in mature oocytes, and despite the relatively small number of cell divisions during oogenesis, mtDNA sequence variants segregate rapidly between generations. This paradoxical behaviour has been ascribed to the presence of a mtDNA 'bottleneck' in oogenesis or early embryogenesis. The nature and size of this bottleneck have been the subject of much controversy. This review argues that segregation of mtDNA sequence variants in the female germline occurs primarily during mitosis in the oocyte precursor population. Segregation is rapid because the precursor cells (primordial germ cells and oogonia) contain a relatively small number of mtDNA templates (the bottleneck) and because the replication of mtDNA is under relaxed control. For the most part, the process appears similar in mice segregating polymorphic sequence variants and in human pedigrees segregating pathogenic point mutations. In particular, there is no evidence for selection against high levels of pathogenic mtDNA point mutations in oogenesis, in early embryonic development, or in fetal development, thus suggesting that efficient respiratory chain function is not critical until post-natal life. These results have important practical implications for clinical genetics.

Chromosome Segregation↗

PubMedDate not supplied

Distorted segregation resulting from pea chromosome reconstructions with alien segments from Pisum fulvum.

Pea (Pisum sativum L.) satellited chromosome reconstructions were analyzed by cytologic markers to identify segregation distortion events. The presence of modified chromosomes was evaluated on the basis of additional rDNA genes, an extra and a longer satellite, all derived from chromosome 5 and chromosome 7 from P. fulvum Sibth. & Sm. The segregation of modified satellited chromosome 5 was monitored through fluorescent in situ hybridization with rDNA probe; it fitted the expected 1:2:1 ratio after self-pollination of a heterozygous genotype for modified chromosome 5. In different genotypes, which were heterozygous for both modified chromosomes 5 and 7, the combined segregation of these chromosomes showed the occurrence of seven karyotype classes instead of the expected nine. The classes with modified chromosome 7 and without modified chromosome 5, whether heterozygous or homozygous, were absent. The hypothesis of gamete selection was rejected since the expected segregation ratio of 5:3:1 was significant by chi-square test. Based on the other hypothesis of postzygotic selection, the segregation ratio did not show a significant deviation from the expected 9:3:1 ratio, thereby indicating that embryo abortion caused the segregation distortion (SD). The hypothesis of the SD system involving two loci carried by the alien satellites of modified chromosomes 5 and 7 is discussed in relation to the evolution of the P. fulvum genome.

Chromosome Segregation↗

PubMedDate not supplied

Evidence of variation in segregation patterns within a Cedrus population.

We used horizontal starch-gel electrophoresis for a genetic analysis of isozymes within one French Cedrus atlantica stand. Eleven to 29 megagametophytes per tree from 186 trees were assayed. Among the 33 enzyme systems tested, 15 correctly resolved and 8 appeared variable in at least one zone of activity: ACP, GOT, IDH, LAP, MDH, MNR, PGI, and SKDH. They were coded by at least 12 polymorphic loci which were described and tested for Mendelian segregation and linkage. Segregation patterns and linkage relationships were variable in the population. We detected homogeneous segregation distortion for loci Idh, Acp-c, and Got-a over the whole set of segregating progeny. We also found segregation distortions in a significant proportion of progeny for loci Got-b, Mdh-c, Pgi-b, and SKDH: The Acp-c and Got-b loci were linked with an overall map distance of 17 cM, but distance varied drastically among progeny. Both segregation distortions and heterogeneity of recombination frequencies indicate the occurrence of a genetic load in this population.

Chromosome Segregation↗

PubMed2006

Actin homolog MreB and RNA polymerase interact and are both required for chromosome segregation in Escherichia coli.

The actin-like MreB cytoskeletal protein and RNA polymerase (RNAP) have both been suggested to provide the force for chromosome segregation. Here, we identify MreB and RNAP as in vivo interaction partners. The interaction was confirmed using in vitro purified components. We also present convincing evidence that MreB and RNAP are both required for chromosome segregation in Escherichia coli. MreB is required for origin and bulk DNA segregation, whereas RNAP is required for bulk DNA, terminus, and possibly also for origin segregation. Furthermore, flow cytometric analyses show that MreB depletion and inactivation of RNAP confer virtually identical and highly unusual chromosome segregation defects. Thus, our results raise the possibility that the MreB-RNAP interaction is functionally important for chromosome segregation.

Actins↗

PubMed2007

Left-right dynein motor implicated in selective chromatid segregation in mouse cells.

During cell division, copies of mouse chromosome 7 are segregated selectively or randomly to daughter cells depending on the cell type. The mechanism for differential segregation is unknown. Because mouse left-right dynein (LRD) gene mutations result in randomization of visceral organs' laterality, we hypothesized that LRD may also function in selective chromatid segregation. Indeed, upon knock-down by RNA interference methods, LRD depletion disrupts biased segregation. LRD messenger RNA presence or absence correlates with the observed segregation patterns. This work supports the claim that LRD functions in a mechanism for selective chromatid segregation.

Animals↗

PubMed2000

Bacillus subtilis SMC is required for proper arrangement of the chromosome and for efficient segregation of replication termini but not for bipolar movement of newly duplicated origin regions.

SMC protein is required for chromosome condensation and for the faithful segregation of daughter chromosomes in Bacillus subtilis. The visualization of specific sites on the chromosome showed that newly duplicated origin regions in growing cells of an smc mutant were able to segregate from each other but that the location of origin regions was frequently aberrant. In contrast, the segregation of replication termini was impaired in smc mutant cells. This analysis was extended to germinating spores of an smc mutant. The results showed that during germination, newly duplicated origins, but not termini, were able to separate from each other in the absence of SMC. Also, DAPI (4',6'-diamidino-2-phenylindole) staining revealed that chromosomes in germinating spores were able to undergo partial or complete replication but that the daughter chromosomes were blocked at a late stage in the segregation process. These findings were confirmed by time-lapse microscopy, which showed that after duplication in growing cells the origin regions underwent rapid movement toward opposite poles of the cell in the absence of SMC. This indicates that SMC is not a required component of the mitotic motor that initially drives origins apart after their duplication. It is also concluded that SMC is needed to maintain the proper layout of the chromosome in the cell and that it functions in the cell cycle after origin separation but prior to complete segregation or replication of daughter chromosomes. It is proposed here that chromosome segregation takes place in at least two steps: an SMC-independent step in which origins move apart and a subsequent SMC-dependent step in which newly duplicated chromosomes condense and are thereby drawn apart.

Bacillus subtilis↗

PubMed2004

Genetic recombination in Bacillus subtilis 168: contribution of Holliday junction processing functions in chromosome segregation.

Bacillus subtilis mutants classified within the epsilon (ruvA, DeltaruvB, DeltarecU, and recD) and eta (DeltarecG) epistatic groups, in an otherwise rec+ background, render cells impaired in chromosomal segregation. A less-pronounced segregation defect in DeltarecA and Deltasms (DeltaradA) cells was observed. The repair deficiency of addAB, DeltarecO, DeltarecR, recH, DeltarecS, and DeltasubA cells did not correlate with a chromosomal segregation defect. The sensitivity of epsilon epistatic group mutants to DNA-damaging agents correlates with ongoing DNA replication at the time of exposure to the agents. The Deltasms (DeltaradA) and DeltasubA mutations partially suppress the DNA repair defect in ruvA and recD cells and the segregation defect in ruvA and DeltarecG cells. The Deltasms (DeltaradA) and DeltasubA mutations partially suppress the DNA repair defect of DeltarecU cells but do not suppress the segregation defect in these cells. The DeltarecA mutation suppresses the segregation defect but does not suppress the DNA repair defect in DeltarecU cells. These results result suggest that (i) the RuvAB and RecG branch migrating DNA helicases, the RecU Holliday junction (HJ) resolvase, and RecD bias HJ resolution towards noncrossovers and that (ii) Sms (RadA) and SubA proteins might play a role in the stabilization and or processing of HJ intermediates.

Bacillus subtilis↗

PubMed2006

The condensin complex is essential for amitotic segregation of bulk chromosomes, but not nucleoli, in the ciliate Tetrahymena thermophila.

The macronucleus of the binucleate ciliate Tetrahymena thermophila contains fragmented and amplified chromosomes that do not have centromeres, eliminating the possibility of mitotic nuclear division. Instead, the macronucleus divides by amitosis with random segregation of these chromosomes without detectable chromatin condensation. This amitotic division provides a special opportunity for studying the roles of mitotic proteins in segregating acentric chromatin. The Smc4 protein is a core component of the condensin complex that plays a role in chromatin condensation and has also been associated with nucleolar segregation, DNA repair, and maintenance of the chromatin scaffold. Mutants of Tetrahymena SMC4 have remarkable characteristics during amitosis. They do not form microtubules inside the macronucleus as normal cells do, and there is little or no bulk DNA segregation during cell division. Nevertheless, segregation of nucleoli to daughter cells still occurs, indicating the independence of this process and bulk DNA segregation in ciliate amitosis.

Adenosine Triphosphatases↗

PubMed2006

Paternal chromosome segregation during the first mitotic division determines Wolbachia-induced cytoplasmic incompatibility phenotype.

The most common Wolbachia-induced phenotype in insects is cytoplasmic incompatibility (CI), which occurs when sperm from infected males fertilize eggs from uninfected females. CI produces distinct phenotypes in three closely related haplo-diploid species of the genus Nasonia: mortality in N. longicornis and N. giraulti, and conversion to male development in N. vitripennis. We demonstrate that the majority of CI-induced mortality occurs during embryogenesis and that the pattern of paternal chromosome segregation during the first mitosis is a good predictor of CI phenotype. In N. giraulti and N. longicornis, the paternal chromosomes mis-segregate, producing abnormal nuclei connected by chromatin bridges. Consequently, these embryos arrest development with very few and abnormal nuclei. In contrast, the paternal genome in N. vitripennis is either not segregated or mis-segregates to one of the two daughter nuclei. Consequently, these embryos continue development utilizing the maternally derived haploid nuclei, resulting in male offspring. The latter class is the first documented example of asymmetric mitotic segregation of abnormal chromosomes. We conclude that in haplo-diploids, CI-induced embryonic lethality occurs only when abnormal paternal genome segregation affects both products of the first mitotic division. This is the first study to associate differences in types of CI with specific cytological defects.

Animals↗

PubMed2001

Coordinating the segregation of sister chromatids during the first meiotic division: evidence for sexual dimorphism.

Errors during the first meiotic division are common in our species, but virtually all occur during female meiosis. The reason why oogenesis is more error prone than spermatogenesis remains unknown. Normal segregation of homologous chromosomes at the first meiotic division (MI) requires coordinated behavior of the sister chromatids of each homolog. Failure of sister kinetochores to act cooperatively at MI, or precocious sister chromatid segregation (PSCS), has been postulated to be a major contributor to human nondisjunction. To investigate the factors that influence PSCS we utilized the XO mouse, since the chromatids of the single X chromosome frequently segregate at MI, and the propensity for PSCS is influenced by genetic background. Our studies demonstrate that the strain-specific differences in PSCS are due to the actions of an autosomal trans-acting factor or factors. Since components of the synaptonemal complex are thought to play a role in centromere cohesion and kinetochore orientation, we evaluated the behavior of the X chromosome at prophase to determine if this factor influenced the propensity of the chromosome for self-synapsis. We were unable to directly correlate synaptic differences with subsequent segregation behavior. However, unexpectedly, we uncovered a sexual dimorphism that may partially explain sex-specific differences in the fidelity of meiotic chromosome segregation. Specifically, in the male remnants of the synaptonemal complex remain associated with the centromeres until anaphase of the second meiotic division (MII), whereas in the female, all traces of synaptonemal complex (SC) protein components are lost from the chromosomes before the onset of the first meiotic division. This finding suggests a sex-specific difference in the components used to correctly segregate chromosomes during meiosis, and may provide a reason for the high error frequency during female meiosis.

Animals↗

PubMed2006

Analysis of one-sided marker segregation patterns resulting from mammalian gene targeting.

The double-strand break repair (DSBR) model is currently accepted as the paradigm for acts of double-strand break (DSB) repair that lead to crossing over between homologous sequences. The DSBR model predicts that asymmetric heteroduplex DNA (hDNA) will form on both sides of the DSB (two-sided events; 5:3/5:3 segregation). In contrast, in yeast and mammalian cells, a considerable fraction of recombinants are one sided: they display full conversion (6:2 segregation) or half-conversion (5:3 segregation) on one side of the DSB together with normal 4:4 segregation on the other side of the DSB. Two mechanisms have been proposed to account for these observations: (i) hDNA formation is restricted to one side of the DSB or the other, and (ii) recombination is initially two sided, but hDNA repair directed by Holliday junction cuts restores normal 4:4 segregation on that side of the DSB in which the mismatch is closest to the cut junction initiating repair. In this study, we exploited a well-characterized gene-targeting assay to test the predictions that these mechanisms make with respect to the frequency of recombinants displaying 4:4 marker segregation on one side of the DSB. Unexpectedly, the results do not support the predictions of either mechanism. We propose a derivation of mechanism (ii) in which the nicks arising from Holliday junction cleavage are not equivalent with respect to directing repair of adjacent hDNA, possibly as a result of asynchronous cleavage of the DSBR intermediate.

Animals↗