The Development and Meiotic Behavior of Asymmetrical Isochromosomes in Wheat

To determine which segments of a chromosome arm are responsible for the initiation of chiasmate pairing in meiosis, a series of novel isochromosomes was developed in hexaploid wheat (Triticum aestivum L.). These isochromosomes are deficient for different terminal segments in the two arms. It is proposed to call them ``asymmetrical.' Meiotic metaphase I pairing of these asymmetrical isochromosomes was observed in plants with various doses of normal and deficient arms. The two arms of an asymmetrical isochromosome were bound by a chiasma in only two of the 1134 pollen mother cells analyzed. Pairing was between arms of identical length whenever such were available; otherwise, there was no pairing. However, two arms deficient for the same segment paired with a frequency similar to that of normal arms, indicating that the deficient arms retained normal capacity for pairing. Pairing of arms of different length was prevented not by the deficiency itself, but rather, by the heterozygosity for the deficiency. Whether two arms were connected via a centromere in an isochromosome or were present in two different chromosomes had no effect on pairing. This demonstrates that in the absence of homology in the distal regions of chromosome arms, even if relatively short, very long homologous segments may remain unrecognized in meiosis and will not be involved in chiasmate pairing.     

Formation of supernumerary euchromatic short arm isochromosomes: parent and cell stage of origin in new cases and review of the literature.

In order to get insight in the formation of isochromosomes we analysed different supernumerary euchromatic short arm isochromosomes for the parent and cell stage of origin. After cytogenetic detection and confirmation by fluorescence-in-situ hybridization we performed short tandem repeat typing in a child with i(9p), three with i(12p) and three with i(18p). The extra chromosomes were monocentric in each case, the i(9p) and i(12p) constitutions were found in mosaic with normal cell lines. Our results and those of other groups indicate a strong role of maternal meiosis in isochromosome formation: in one i(8p), 4 out of 5 i(9p), 7 out of 12 i(12p) and 18 out of 23 i(18p) families a maternal meiotic nondisjunction had occurred prior to the centromere misdivision. For chromosome 18, the majority of isochromosomes originated from a maternal meiosis II error (16/18). For the other tetrasomic constitutions the isochromosomes could be delineated from paternal as well as from maternal origin, the short tandem repeat typing patterns being consistent with meiotic or mitotic cell stages of formation. Thus, independently of the chromosomal origin, in the majority of cases with additional euchromatic isochromosomes maternal meiosis nondisjunction is the initial step followed by centromeric misdivision. Postzygotic nondisjunction as suggested previously due to mosaics observed in tetrasomies 9p and 12p seems to be of minor importance. The observed origin of isochromosomes 18 corresponds to that of trisomy 18, where the majority of cases can be delineated from maternal meiosis II errors.     

Centric fission consequences in man

The authors summarise the consequences of centric fission in man as follows: classical (monocentric) isochromosomes; usually either for p or q, exceptionally for both arms; stable telocentrics for either one or both arms; isochromosome for one arm, stable telocentric for the other; isochromosome for one arm concurring with translocation of the telocentric for the other; telocentric/isochromosome mosaicism for the same arm; stable telocentric for a part of one arm, the remaining of the chromosome forming a smaller element (obviously this rearrangement requires an additional break outside the centromere), and whole-arm translocations. These events are discussed in the light of current notions about centromere structure and function.     

Cytological identification of an isotetrasomic in rice and its application to centromere mapping

The aneuploid with isochromosome or telochromosome is ideal material for exploring the position of centromere in lingkage map.For obtaining these aneuploids in rice,the primary trisomics from triplo-1 to triplo-12 and the aneuploids derived from a triploid of indica rice variety Zhongxiao 3037 were carefully investigated.From the offsprings of triplo-10,a primary trisomic of chromosome 10 of the variety,an isotetrasomic “triplo-10-1” was obtained.Cytological investigation revealed that a pair of extra isochromosomes of triplo-10-1 were come from the short arm of chromosome 10.In the offsprings of the isotetrasomic,a secondary trisomic “triplo-10-2”,in which the extra-chromosome was an isochromosome derived from the short arm of chromosome 10,was identified.With the isotetrasomic,secondary trisomic,primary trisomic and diploid of variety Zhongxiao 3037,different molecular markers were used for exploring the position of the centromere of chromosome 10.Based on the DNA dosage effect,it was verified that the molecular markers G1125,G333 and L169 were Located on the short arm,G1084 and other 16 available molecular markers were on the long arm of chromosome 10.So the centromere of chromosome 10 was located somewhere between G1125 and G1084 according to the RFLP linkage map given by Kurata et al[1].The distance from G1125 to G1084 was about 3.2cM.     

Isochromosome X mosaicism in a child with Kabuki syndrome phenotype: A rare cytogenetic association

Isochromosome is a structurally unbalanced chromosome consisting of two short arms or two long arms, which are derived by abnormal centromere division or sister-chromatid exchange. Most autosomal isochromosomes are unusual, while those involving sex chromosomes are common. Kabuki syndrome (KS, OMIM 147920) is a multiple malformation/mental retardation syndrome of unknown etiology. A conventional cytogenetic study on lymphocytes from a 4-year-old girl with physical features suggestive of KS was found to have mosaicism for isochromosome for the long arm of the X. Although most manifestations present in this patient have been described before, this report is a rare association of clinical and cytogenetic findings in this syndrome. A genome-wide analysis and a larger number of patient groups studied could improve our understanding of the genetic basis of KS.     

Rad51 suppresses gross chromosomal rearrangement at centromere in Schizosaccharomyces pombe

Centromere that plays a pivotal role in chromosome segregation is composed of repetitive elements in many eukaryotes. Although chromosomal regions containing repeats are the hotspots of rearrangements, little is known about the stability of centromere repeats. Here, by using a minichromosome that has a complete set of centromere sequences, we have developed a fission yeast system to detect gross chromosomal rearrangements (GCRs) that occur spontaneously. Southern and comprehensive genome hybridization analyses of rearranged chromosomes show two types of GCRs: translocation between homologous chromosomes and formation of isochromosomes in which a chromosome arm is replaced by a copy of the other. Remarkably, all the examined isochromosomes contain the breakpoint in centromere repeats, showing that isochromosomes are produced by centromere rearrangement. Mutations in the Rad3 checkpoint kinase increase both types of GCRs. In contrast, the deletion of Rad51 recombinase preferentially elevates isochromosome formation. Chromatin immunoprecipitation analysis shows that Rad51 localizes at centromere around S phase. These data suggest that Rad51 suppresses rearrangements of centromere repeats that result in isochromosome formation.     

Recombination in an isochromosome preferentially occurs between cis isochromatids

An isochromosome has identical arms attached to the same centromere. At the pachytene stage of meiosis, it has four isochromatids and recombination can occur either between cis isochromatids (attached to the same half-centromere) or trans isochromatids (attached to different half-centromeres). Normally such recombination cannot be detected because all four chromatids are homogenetic (arose from misdivision of a centromere to which genetically identical sister chromatids were attached). We isolated an isochromosome of wheat that is heterogenetic for the distal 64% of the long arm. The heterogenetic isochromosome was recovered from the progeny of a cross between Triticum aestivum cv. Chinese Spring containing an isochromosome for the long arm of chromosome 5B (i5BL) and a disomic substitution line of Triticum turgidum ssp. dicoccoides chromosome 5B in Chinese Spring wheat. New recombinants were produced when the two arms of i5BLrec paired at metaphase I of meiosis. Only trans isochromatid exchanges led to some homozygous loci in i5BLrec, whereas exchanges between cis isochromatids resulted in heterozygosity at all loci similar to the parental type. There was an average frequency of 0.87 chiasmata per pollen mother cell for the heterogenetic i5BL, which will result in 0.44 cis and 0.44 trans isochromatid exchanges, assuming that both are occurring at the same frequency. The average crossover frequency based on recombination between trans isochromatid exchange detected by restriction fragment length polymorphism analysis in 98 plants was 0.29. This observed value is significantly lower (P<0.01) than the value of 0.44 as expected from chiasmata counts. Our study provides the first experimental evidence that crossovers preferentially occur between cis isochromatids rather than trans isochromatids.     

Meiotic behaviour of single-arm tetrasomic combinations of isochromosomes,telocentrics and normal chromosomes in rye (Secale cereale L.)

The isochromosome studied was derived from the short arm of the satellite chromosome of rye (Secale cereale, 2n=14); the telocentrics represent both the short and long arms of the same chromosome. Three different combinations, tetrasomic for the short arm, have been composed and studied: I: 2 isochromosomes (short arm) + 2 telocentrics (long arm) + 6 normal pairs. II: 1 isochromosome + 2 telocentrics (short arm) + 2 telocentrics (long arm) + 6 normal pairs. III: 1 isochromosome + 1 telocentric (short arm) + 1 normal satellite chromosome + 1 telocentric (long arm) + 6 normal pairs. — Over 20,000 cells were analysed. Simple mathematical models describing the frequencies of the different types of MI configurations in terms of frequency of chiasmata in the different pairing combinations of the polysomic arms, and of the frequency of multivalent pairing of this arm, were developed. They were used to derive estimates for chiasma frequencies and multivalent pairing frequencies in the different chromosome constitutions from the observations on configuration frequencies. Variation between plants and within plants was studied, and it was concluded that much of the within plant heterogeneity was due to regulatory variation expressed independently in different chromosomal segments. There was also a significant genetic component. Analysis of the reasons for the models to fail under certain conditions led to suggestions for extension of the models.     

47,X,idic(Y),inv dup(Y): a non-mosaic case of a phenotypically normal boy with two different Y isochromosomes and neocentromere formation

A de novo aberrant karyotype with 47 chromosomes including 2 different-sized markers was identified during prenatal diagnosis. Fluorescence in situ hybridization (FISH) with a Y painting probe tagged both marker chromosomes which were supposed to be isochromosomes of the short and the long arm, respectively. A normal boy was born in time who shows normal physical and mental development. To characterize both Y markers in detail, we postnatally FISH-mapped a panel of Y chromosomal probes including SHOX (PAR1), TSPY, DYZ3 (Y centromere), UTY, XKRY, CDY, RBMY, DAZ, DYZ1 (Yq12 heterochromatin), SYBL1 (PAR2), and the human telomeric sequence (TTAGGG)(n). The smaller Y marker turned out to be an isochromosome containing an inverted duplication of the entire short arm, the original Y centromere, and parts of the proximal long arm, including AZFa. The bigger Y marker was an isochromosome of the rest of the Y long arm. Despite a clearly visible primary constriction within one of the DAPI- and DYZ1-positive heterochromatic regions, hybridization of DYZ3 detected no Y-specific alphoid sequences in that constriction. Because of its stable mitotic distribution, a de novo formation of a neocentromere has to be assumed.     

Tetrasomy 18p de novo: Identification by FISH with conventional and microdissection probes and analysis of parental origin and formation by short sequence repeat typing

We report a de novo supernumerary isochromosome 18p in a child with tetrasomy 18p, analyzed by a straightforward combination of cytogenetic and molecular cytogenetic methods. The diagnostic procedure consisted of standard banding techniques and fluorescence in situ hybridization (FISH) with centromere and library DNA probes for chromosome 18, and 18p-specific FISH probes prepared by chromosome microdissection and in vitro amplification. The maternal origin as well as the most probable cell stages of formation of the supernumerary isochromosome were determined by typing of short sequence repeats (SSRs). The pattern of allelic distribution suggests a nondisjunction during meiosis followed by a centromeric misdivision in an early postzygotic mitosis as the most probable mode of isochromosome 18p formation. The combination of the applied methods represents a powerful tool to investigate the nature and the origin of de novo marker chromosomes. Received: 28 August 1995 / Revised: 3 November 1995; 20 December 1995     

将大赖草种质转移给普通小麦的研究——Ⅶ.一个普通小麦-大赖草等臂染色体异附加系的选育与鉴定

通过花粉母细胞减数分裂中期Ⅰ染色体配对构型分析、Giemsa C-分带,从普通小麦-大赖草第7条染色体二体异附加系自交后代中选育并鉴定出了93G51-9和93G52-8 2个等臂染色体异附加系。该异附加系在花粉母细胞减数分裂中期Ⅰ,其等臂染色体自身两臂配对频率高,染色体易发生断裂,且又携带有较抗赤霉病的基因,是向小麦转移大赖草赤霉病抗性基因的有用中间材料。     

Cytological and molecular analysis of centromere misdivision in maize.

B chromosome derivatives suffering from breaks within their centromere were examined cytologically and molecularly. We showed by high resolution FISH that misdivision of the centromere of a univalent chromosome can occur during meiosis. The breaks divide the centromere repeat sequence cluster. A telocentric chromosome formed by misdivision was found to have the addition of telomeric repeats to the broken centromere. A ring chromosome formed after misdivision occurred by fusion of the broken centromere to the telomere. Pulsed-field electrophoresis analyses were performed on the telocentric and ring chromosomes to identify fragments that hybridize to both the telomeric repeat and the B-specific centromeric repeat. We conclude that healing of broken maize centromeres can be achieved through the mechanisms of addition or fusion of telomeric repeat sequences to the broken centromere.     

Ultrastructure of meiotic pairing in B chromosomes of Crepis capillaris

The meiotic pairing behaviour of four B isochromosomes of Crepis capillaris was studied by synaptonemal complex (SC) surface spreading of pollen mother cells. The four B chromosomes form a tightly associated group, separate from the standard chromosomes, throughout zygotene and pachytene. All four B chromosomes are also folded around their axis of symmetry, the centromere, and the eight homologous arms are closely aligned from the earliest prophase I stages. A high frequency of multivalent pairing of the four B chromosomes is observed at pachytene, in excess of 90%, mirroring the situation observed at metaphase I but exceeding the frequency expected (76.2%) on the assumption of random pairing among the eight B isochromosome arms with a single distal pairing initiation site per arm. The higher than expected frequency of multivalents is due to the occurrence of multiple pairing initiations along the B isochromosome arms, resulting in high frequencies of pairing partner switches. Pairing of the standard chromosome set is frequently incomplete in the presence of four B chromosomes, and abnormalities of SC structure such as thickening and splitting of axes and lateral elements are also frequently seen. Similarly, B chromosomes show partial pairing failure, the extent of which is correlated with pairing failure in the standard chromosome set. The B chromosomes themselves also show abnormalities of SC structure. Both standard and B chromosomes show non-homologous foldback pairing of regions that have failed to pair homologously.by D. Schweizer     

Two dicentric Y isochromosomes,one without the Yqh heterochromatic segment

Summary Two women with primary amenorrhoea and few other stigmata of Turner's syndrome were found to be chromosome mosaics: 45,X/46,X,idic(Y). In Case 1, the dicentric isochromosome Y was found to have a long-arm breakpoint of formation. This structure was interpreted as containing two Y short arms and centromeres separated by a region derived from the proximal Y long arm. One of the centromeres in the Case 1 —idic(Y) was suppressed in 80% of cells in blood, and in these cells it appeared as a regular Y-shaped chromosome. In Case 2 the idic(Y) was derived by a short-arm breakpoint of formation. In all the dicentrics of this case with one primary constriction (functional monocentrics) there was a single Cd band. In the 10% of dicentrics with two primary constrictions, there were two Cd bands. It is argued that the instability of sex isochromosomes is due to this functional dicentricity in some cells. These cases are compared with 42 other Y isochromosomes with various short- and long-arm breakpoints of formation. It is suggested that some of the nonheterochromatic, nonfluorescent Y chromosomes previously reported may be explained as dicentric i(Y) with proximal long-arm breakpoints of formation and one suppressed centromere.     

Meiotic loss of the B chromosomes of maize is influenced by the B univalent co-orientation and the TR-1 knob constitution of the A chromosomes

The suppression of meiotic loss when the maize B chromosomes are unpaired is genetically determined. Two genotypes were selected in 1B x 0B crosses: the H line where the B transmission rate is Mendelian (50%) and the L line where the B is present in only about 40% of the progeny. Using the ZmBs probe located at the centromere and at the distal portion of the B chromosome in FISH, we found that the centromeric and telomeric ends of the B univalent co-orient at metaphase I. This feature seems to promote proper centromere orientation causing the lack of meiotic loss of the unpaired B. The co-orientation was observed in both lines, however in the L line the B univalents were not always properly oriented, showing amphitelic orientation in about 25% of the metaphase I cells. We also studied plants of the H and L lines with FISH to test the possible relation between the knob constitution and B loss. It has been found that the plants of both lines are similarly variable for the 180-bp knob repeat, but they differ in the TR-1 350-bp repeat, the L line having more TR-1 knobs. The use of a 45S rDNA probe which labels chromosome 6, allowed us to determine that this chromosome shows the main variability between the two lines: the L line has TR-1 in both arms, showing a large TR-1 knob on the long arm. The H line has only one, generally located on the short arm besides the NOR.     

Mosaicism for an ectopic NOR at 8pter and a complex rearrangement of chromosome 8 in a patient with severe psychomotor retardation

We describe a 3-year-old girl with severe delays in mental and motor skills, a history of generalized seizures, and subtle dysmorphic features. Conventional cytogenetics revealed a mosaic karyotype. A de novo ectopic NOR at the telomeric region of the short arm of one chromosome 8 (8ps) was found in 90% of lymphocyte and in 98% of fibroblast metaphases. A small NOR-bearing marker chromosome and a large derivative chromosome 8 without short arm satellites (der(8)) were present in the remaining cells. FISH with a probe specific for centromeres 14 and 22 labeled both the telomeric region of 8ps and the small marker centromere. Der(8) included an inverted duplication of 8p and a rearranged duplication of 8q but lacked a second centromere. A subtelomeric probe for 8p revealed a cryptic deletion in 8ps and der(8). Thus, the karyotype represents a combination of submicroscopic partial monosomy 8pter and mosaic trisomy 8.     

A study of cryptic terminal chromosome rearrangements in recurrent miscarriage couples detects unsuspected acrocentric pericentromeric abnormalities

Fifty chromosomally normal couples with three or more miscarriages were examined using fluorescent in situ hybridisation (FISH) and a library of subtelomere-specific probes together with alphoid repeats mapping to the acrocentric centromeres. Six abnormalities were found. Firstly, a cryptic reciprocal subtelomere translocation between the long arm of a chromosome 3 and the short arm of a chromosome 10. The other five cryptic abnormalities involved the acrocentric chromosome pericentromeric regions and in one case also Yp. Two patients had a rearranged chromosome 13, where the centromeric region was found to be derived from the short arm, centromere and proximal long arm of chromosome 15. Another two patients had a derived chromosome 22, where the centromere was replaced by two other centromeres, one derived from chromosome 14 and the other from either chromosome 13 or 21, while one patient had the subtelomere region of Yp translocated onto the short arm of a chromosome 21. These abnormalities may be the underlying cause of the recurrent miscarriages, because they may result in abnormal pairing configurations at meiosis leading to non-disjunction of whole chromosomes at metaphase I. The frequency of rearrangements seen in the recurrent miscarriage patient population was significantly different from that in the control group ( P=0.0096, Fisher's exact test) due to the acrocentric pericentromeric abnormalities.     

Isolation and mapping of 88 new RFLP markers on human chromosome 8.

To obtain new RFLP markers for construction of a high-resolution map of human chromosome 8, a cosmid library was constructed from a somatic hybrid cell that contained chromosome 8 as the only human component in mouse genomic background. Eighty-eight new RFLP markers were isolated and characterized, and 71 of them were sublocalized to chromosomal bands by fluorescent in situ hybridization (FISH). Of these, 36 were localized to the short arm, 34 to the long arm, and 1 to the centromeric region. Five markers defined VNTR loci. This work represents the first extensive isolation and physical mapping of RFLP markers on human chromosome 8. These new markers will serve as useful resources for linkage mapping of loci for inherited diseases and for efforts to identify a putative tumor suppressor gene(s) on chromosome 8.     

Molecular definition of breakpoints associated with human Xq isochromosomes: implications for mechanisms of formation.

To test the centromere misdivision model of isochromosome formation, we have defined the breakpoints of cytogenetically monocentric and dicentric Xq isochromosomes (i(Xq)) from Turner syndrome probands, using FISH with cosmids and YACs derived from a contig spanning proximal Xp. Seven different pericentromeric breakpoints were identified, with 10 of 11 of the i(Xq)s containing varying amounts of material from Xp. Only one of the eight cytogenetically monocentric i(Xq)s demonstrated a single alpha-satellite (DXZ1) signal, consistent with classical models involving centromere misdivision. The remaining seven were inconsistent with such a model and had breakpoints that spanned proximal Xp11.21: one was between DXZ1 and the most proximal marker, ZXDA; one occurred between the duplicated genes, ZXDA and ZXDB; two were approximately 2 Mb from DXZ1; two were adjacent to ALAS2 located 3.5 Mb from DXZ1; and the largest had a breakpoint just distal to DXS1013E, indicating the inclusion of 8 Mb of Xp DNA between centromeres. The three cytologically dicentric i(Xq)s had breakpoints distal to DXS423E in Xp11.22 and therefore contained > or = 12 Mb of DNA between centromeres. These data demonstrate that the majority of breakpoints resulting in i(Xq) formation are in band Xp11.2 and not in the centromere itself. Therefore, we hypothesize that the predominant mechanism of i(Xq) formation involves sequences in the proximal short arm that are prone to breakage and reunion events between sister chromatids or homologous X chromosomes.     

Characterization of a rare short arm heteromorphism of chromosome 22 in a girl with down-syndrome like facies

Chromosomal heteromorphisms are described as interindividual variation of chromosomes without phenotypic consequence. Chromosomal polymorphisms detected include most regions of heterochromatin of chromosomes 1, 9, 16 and Y and the short arms of all acrocentric chromosomes. Here, we report a girl with Down-syndrome such as facies and tremendously enlarged short arm of a chromosome 22. Fluorescence in situ hybridization (FISH) with a probe specific for all acrocentric short arms revealed that the enlargement p arms of the chromosome 22 in question contained exclusively heterochromatic material derived from an acrocentric short arm. Parental studies identified a maternal origin of this heteromorphism. Cryptic trisomy 21 of the Down-syndrome critical region was excluded by a corresponding FISH-probe. Here, we report, to the best of our knowledge, largest ever seen chromosome 22 short arm, being ~×1.5 larger than the normal long arm.