The karyotype of threeCryptochironomus species of West Siberia,USSR

Karyotype characteristics C. ussouriensis and two species of C. gr. defectus from the main and special lobe cells of the salivary gland are presented in detail in this paper for the first time. C. ussouriensis has two main chromosome markers, nucleolus in chromosome II and large Balbiani ring adjacent to the centromeric band in chromosome I. Balbiani ring is developed only in the main lobe cells and is absent in the special lobe cells. Each of two chromosomes of C. gr. defectus (C. obreptans) has a nucleolus and a Balbiani ring. They are well developed in the main lobe cells but in the special lobe cells the only nucleolus in chromosome II are functioning. The karyotype of C. gr. defectus (karyotype 1) is characterized with a large number of functionally active regions: nucleoli, BRs and puffs. Two BRs (in region 10 of chromosome I and in region 8 of chromosome III) functioning in the main lobe cells are absent in the special lobe cells. Since the pattern of nucleoli, BRs and puffs are of great importance in the karyotype characteristic of the different Cryptochironomus species, a comparative study of chromosomes from the different salivary gland lobes is needed.     

Split decisions: coordinating cytokinesis in yeast

Cytokinesis in eukaryotes involves the regulated assembly and contraction of a ring comprising filamentous (F)-actin and myosin II. Assembly of the contractile ring occurs through the accumulation of cortical cues at the specified division plane, followed by recruitment of F-actin, myosin II and accessory proteins involved in generating the mature ring. Ring contraction is temporally regulated to occur only after chromosome segregation and, in yeast, it is controlled by a conserved signaling cascade that becomes active only after Cdk1-Cyclin-B inactivation. In this article (which is part of the Cytokinesis series), we discuss recent studies that have begun to clarify both the spatial and the temporal order of ring assembly and that have illuminated the signals that trigger ring contraction in yeast. These studies add to the growing knowledge of the processes that control eukaryotic cell division.     

A perikinetochoric ring defined by MCAK and Aurora-B as a novel centromere domain

Mitotic Centromere-Associated Kinesin (MCAK) is a member of the kinesin-13 subfamily of kinesin-related proteins. In mitosis, this microtubule-depolymerising kinesin seems to be implicated in chromosome segregation and in the correction of improper kinetochore-microtubule interactions, and its activity is regulated by the Aurora-B kinase. However, there are no published data on its behaviour and function during mammalian meiosis. We have analysed by immunofluorescence in squashed mouse spermatocytes, the distribution and possible function of MCAK, together with Aurora-B, during both meiotic divisions. Our results demonstrate that MCAK and Aurora-B colocalise at the inner domain of metaphase I centromeres. Thus, MCAK shows a “cone”-like three-dimensional distribution beneath and surrounding the closely associated sister kinetochores. During the second meiotic division, MCAK and Aurora-B also colocalise at the inner centromere domain as a band that joins sister kinetochores, but only during prometaphase II in unattached chromosomes. During chromosome congression to the metaphase II plate, MCAK relocalises and appears as a ring below each sister kinetochore. Aurora-B also relocalises to appear as a ring surrounding and beneath kinetochores but during late metaphase II. Our results demonstrate that the redistribution of MCAK at prometaphase II/metaphase II centromeres depends on tension across the centromere and/or on the interaction of microtubules with kinetochores. We propose that the perikinetochoric rings of MCAK and Aurora-B define a novel transient centromere domain at least in mouse chromosomes during meiosis. We discuss the possible functions of MCAK at the inner centromere domain and at the perikinetochoric ring during both meiotic divisions.     

The location of repeated DNA sequences in the chromosomes of Chironomus tentans

Polytene chromosomes of Chironomus tentans were hybridized in situ with in vivo labelled nuclear and chromosomal RNA. Nuclear RNA formed hybrids preferentially in five distinct regions considered to contain clustered, repeated DNA sequences. These are the two nucleolar organizer regions, Balbiani ring 1 and 2, and the 5 S RNA genes in region 2A of chromosome II, which together comprised almost 70% of the total number of grains over the complement. The remaining grains were diffusely distributed over the chromosomes. There was a significant difference in the distribution of grains when RNA from different chromosomes was used for hybridization. Chromosome I RNA hybridized preferentially with chromosome I, and chromosome II+III RNA preferentially with chromosome II+III. Some regions within the chromosomes hybridized significantly more chromosomal RNA than other regions. A considerable cross-hybridization of RNA from one particular type of chromosome with the other chromosomes was also found. It is concluded that repeated DNA sequences which hybridize with heterogeneous chromosomal RNA in C. tentans are widely dispersed in the genome. Some of these sequences have a delimited localization, others are dispersed, and some sequences which are transcribed in one particular chromosome are present also in the other chromosomes.     

Cytogenetic studies on <Emphasis Type="Italic">Metasequoia glyptostroboides</Emphasis>, a living fossil species

The chromosome morphology and meiotic pairing behavior in the pollen mother cells (PMCs) of Metasequoia glyptostroboides were investigated. The results showed that: (1) The chromosome number of the PMCs was 2n=22. (2) The PMCs developed in the successive manner, and the nucleoids in the dynamic development were similar to those of the other gymnosperms. (3) At prophase, most of the chromosomes were unable to be identified distinctively because the chromosomes were long and tangled together. The chromosome segments were paired non-synchronously. At pachytene, the interstitial or terminal regions of some bivalents did not form synapsis and the paired chromosomes showed difference in sizes, indicating that there were structure differences between the homologous chromosomes. (4) At diakinesis, the ring bivalents showed complicated configurations due to the differences in location and number of chiasmata. In addition, there were cross-linked bivalents. (5) At metaphase I, the chromosome configuration of each cell was 8.2II ⁰ + 1.1II + 1.3II ⁺ + 0.8I. Most of the chromosomes were ring bivalents, but some were cross-linked bivalents, rod bivalents, or univalents. (6) 15\% PMCs at anaphase I and 22\% PMCs at anaphase II presented chromosome bridges, chromosome fragments, micronuclei, and lagging chromosomes. Twenty seven percent microspores finally moved into one to three micronuclei. Twenty five percent pollens were abortive. The results indicated that the observed individual of M. glyptostroboideswas probably a parpcentric inversion heterozygote, and there were structural and behavioral differences between the homologous chromosomes. The chromosomal aberration of M. glyptostroboidesmay play an important role in the evolution of this relict species, which is known as a living fossil. Further evidence is needed to test whether the differences between homologous chromosomes were due to hybridization.     

Primordial linkage of β2-microglobulin to the MHC

β2-Microglobulin (β2M) is believed to have arisen in a basal jawed vertebrate (gnathostome) and is the essential L chain that associates with most MHC class I molecules. It contains a distinctive molecular structure called a constant-1 Ig superfamily domain, which is shared with other adaptive immune molecules including MHC class I and class II. Despite its structural similarity to class I and class II and its conserved function, β2M is encoded outside the MHC in all examined species from bony fish to mammals, but it is assumed to have translocated from its original location within the MHC early in gnathostome evolution. We screened a nurse shark bacterial artificial chromosome library and isolated clones containing β2M genes. A gene present in the MHC of all other vertebrates (ring3) was found in the bacterial artificial chromosome clone, and the close linkage of ring3 and β2M to MHC class I and class II genes was determined by single-strand conformational polymorphism and allele-specific PCR. This study satisfies the long-held conjecture that β2M was linked to the primordial MHC (Ur MHC); furthermore, the apparent stability of the shark genome may yield other genes predicted to have had a primordial association with the MHC specifically and with immunity in general.     

来自山羊草的杀配子染色体(Aegilops cylindryca 2C)对小黑麦减数分裂及花粉粒有丝分裂过程的影响

用来自柱穗山羊草的杀配子染色体2C,诱导六倍体、八倍体小黑麦染色体的断裂,观察杂种F1的减数分裂行为,在PMCI及PMCII后期观察到了大量的落后染色体、染色体断片、环状染色体及桥,在二分了解子及四分孢子中有为数甚多的、大小不一的微核,有的还形成多分孢子。在对F1花粉粒的有丝分裂观察中,未见分裂异常。由此推断杀配子染以体诱导染色体断裂可能不发生在配子形成的有丝分裂过程。这与巳有的报道不同。     

A functional centromere lacking CentO sequences in a newly formed ring chromosome in rice

An awned rice(Oryza sativa) plant carrying a tiny extra chromosome was discovered among the progeny of a telotrisomic line 2nt4L. Fluorescence in situ hybridization(FISH) using chromosome specific BAC clones revealed that this extra chromosome was a ring chromosome derived from part of the long arm of chromosome 4. So the aneuploidy plant was accordingly named as 2nt4L ring. We did not detect any Cent O FISH signals on the ring chromosome, and found only the centromeric probe Centromeric Retrotransposon of Rice(CRR) was co-localized with the centromere-specific histone CENH3 as revealed by sequential FISH after immunodetection. The extra ring chromosome exhibited a unique segregation pattern during meiosis, including no pairing between the ring chromosome and normal chromosome 4during prophase I and pre-separation of sister chromatids at anaphase I.     

Mechanisms of small ring formation suggested by the molecular characterization of two small accessory ring chromosomes derived from chromosome 4.

Molecular cloning of a microdissected small accessary ring chromosome 4 from a moderately retarded and dysmorphic patient has been performed to identify the origin of the ring chromosome. FISH was performed with cosmids identified with the cloned, microdissected products and with other markers from chromosome 4. The present study clearly demonstrates that the small ring in this patient originates from three discontinuous regions of chromosome 4: 4p13 or 14, the centromere, and 4q31. It is suggested that the origin of the ring chromosome is a ring involving the entire chromosome 4, which has then been involved in breakage and fusion events, as a consequence of DNA replication generating interlocked rings. A second severely retarded and dysmorphic patient also had a small accessary ring derived from chromosome 4. FISH studies of this ring are consistent with an origin from a contiguous region including the centromere to band 4q12. It is apparent that there are at least two mechanisms for the formation of small ring chromosomes. This adds a further complication in any attempt to ascertain common phenotypes between patients known to have morphologically similar markers derived from the same chromosome.     

Characterization of the first supernumerary tricentric ring chromosome 1 mosaicism by conventional and molecular cytogenetic techniques

We report on the conventional cytogenetic and fluorescence in situ hybridization (FISH) results obtained for a 3.5-year-old girl with developmental and language delay and a supernumerary ring chromosome mosaicism in 8% of T-lymphocytes analyzed. Using different conventional and molecular cytogenetic techniques as YAC hybridization and comparative genomic hybridization, we could show that the extra tricentric ring chromosome consists of three heterochromatic blocks with inserted euchromatic material. Additionally, chromosome microdissection followed by FISH analysis demonstrated that the small tricentric ring chromosome consisted of material from the pericentromeric region of chromosome 1q21. Thus, the patient has a mosaic of normal cells and cells with partial pentasomy of the pericentromeric region of chromosome 1. So far, 19 cases with single supernumerary marker chromosome 1 have been published, but no tricentric ring chromosome 1 is, to our knowledge, reviewed in the literature. In this study, we compare the clinical features of our patient with cytogenetically comparable cases described in the literature. We introduce a hypothesis for the formation of a tricentric ring chromosome: starting with a monocentric ring, sister chromatid exchange leading to the formation of a tetracentric ring, which underwent intrastrand recombination generating the tricentric ring.     

Tandem duplication chromosome 21 in the offspring of a ring chromosome 21 carrier

A direct tandem duplication chromosome 21 was found in a boy with Down's syndrome. The proband's mother and grandmother both carried a ring chromosome 21. The observed duplication chromosome in the child may be explained either by recombination between the maternal ring and the mother's normal chromosome 21 or by break of a double-sized ring chromosome 21.     

Chromosome 18 replaced by two ring chromosomes of chromosome 18 origin

We here describe the first example of the replacement of an autosome by two ring chromosomes originating from the missing chromosome, presented in a patient with a single chromosome 18 and two additional ring chromosomes. Detailed fluorescence in situ hybridization (FISH) analysis revealed the chromosome 18 origin of both ring chromosomes and characterized the small and the large ring chromosome as derivatives of the short and long arm of chromosome 18, respectively. The loss of subtelomeric regions of the short and the long arm of chromosome 18 in the ring chromosomes was confirmed by FISH studies. Molecular studies showed the exclusive presence of the paternal alleles for microsatellite markers located distal to the short and long arm loci D18S843 and D18S474, respectively. This indicates the maternal origin of both rings and provides evidence for substantial deletions of the distal parts of both arms of chromosome 18 in the ring chromosomes. The dysmorphic features of the patient can be explained by these deletions in both chromosome arms, as the clinical findings partly overlap with observations in 18p- and 18q-syndrome and are similar to some cases of ring chromosome 18. Centromere misdivision is suggested as one mechanism involved in the formation of the ring chromosomes.     

Anillin localization suggests distinct mechanisms of division plane specification in mouse oogenic meiosis I and II

Anillin is a conserved cytokinetic ring protein implicated in actomyosin cytoskeletal organization and cytoskeletal-membrane linkage. Here we explored anillin localization in the highly asymmetric divisions of the mouse oocyte that lead to the extrusion of two polar bodies. The purposes of polar body extrusion are to reduce the chromosome complement within the egg to haploid, and to retain the majority of the egg cytoplasm for embryonic development. Anillin's proposed roles in cytokinetic ring organization suggest that it plays important roles in achieving this asymmetric division. We report that during meiotic maturation, anillin mRNA is expressed and protein levels steadily rise. In meiosis I, anillin localizes to a cortical cap overlying metaphase I spindles, and a broad ring over anaphase spindles that are perpendicular to the cortex. Anillin is excluded from the cortex of the prospective first polar body, and highly enriched in the cytokinetic ring that severs the polar body from the oocyte. In meiosis II, anillin is enriched in a cortical stripe precisely coincident with and overlying the meiotic spindle midzone. These results suggest a model in which this cortical structure contributes to spindle re-alignment in meiosis II. Thus, localization of anillin as a conserved cytokinetic ring marker illustrates that the geometry of the cytokinetic ring is distinct between the two oogenic meiotic cytokineses in mammals.     

Ring chromosomes in barley (Hordeum vulgare L.)

A plant with 2n = 14 + 1 ring chromosomes was obtained in the progeny of a primary trisomie for chromosome 7 of a two-rowed cultivar, Shin Ebisu 16. The morphological characteristics of the trisomic plants with an extra ring chromosome were similar to the primary trisomic for chromosome 7 (Semierect), which suggests that it originated from this chromosome. The ring chromosomes were not completely stable in mitotic cells because of abnormal behavior. Chromosome complements varied in different plants and in different roots within a plant. Root tip cells and spikes with 2n = 14 and 14 + 2 ring chromosomes were observed on plants with 14 + 1 ring chromosomes. Breakage-fusion-bridge cycle was inferred. The ring chromosome was associated with two normal homologues forming a trivalent in 17.6% sporocytes at metaphase I. The transmission of the extra ring chromosome was 23.1% in the progeny of the plant with 14 + 1 ring chromosomes. Trivalent formation may have been much higher at early prophase stages which were difficult to analyze in barley; only 4 of 120 sporocytes analyzed showed an isolated ring at pachytene. The ring chromosome moved to one pole without separation in 24.7% of the sporocytes at AI, and divided in 27.1% sporocytes giving rise to 8-8 separation. Only 10% of the sporocytes showed bridge formation at AI.     

Interstitial deletion and ring chromosome derived from 19q. Proximal 19q trisomy phenotype.

A small supernumerary ring chromosome has been found in a boy with overweight, dysmorphic facies and mental retardation. His mother had an interstitial deletion of the long arm of chromosome 19 and the same ring chromosome. By means of fluorescence in situ hybridization the ring chromosome was shown to be derived from the deleted chromosome, after the occurrence of two breaks: one in the centromere region, the other in the q-arm of chromosome 19.     

Meiotic studies in Acanonicus hahni (Stål) (Coreidae,Heteroptera) I. Behaviour of univalents in desynaptic individuals

Male meiosis was studied in a population of Acanonicus hahni (Stål), and nine of the sixteen individuals analyzed showed desynapsis. The frequency of univalents varied from one to seven percent in eight of them, while in the ninth the percentage of cells with univalents was higher (12%). The univalents auto-orientate at metaphase I in the center of the ring formed by autosomal bivalents and divide equationally at anaphase I; at metaphase II they show touch-and-go pairing, and lie in the center of the ring of autosomes.A desynaptic origin of the univalents is proposed, and the arrangement of the chromosomes in the first and second metaphase plate in the normal and desynaptic individuals is compared and discussed. The meiotic characteristics of these desynaptic individuals are also compared with those described in other insects with holocentric and monocentric chromosomes. It is suggested that any achiasmatic chromosome, whether a univalent, m or sex chromosome, will induce the formation of a ring and with some or all of them lying in its centre.     

Human chromokinesin KIF4A functions in chromosome condensation and segregation

Accurate chromosome alignment at metaphase and subsequent segregation of condensed chromosomes is a complex process involving elaborate and only partially characterized molecular machinery. Although several spindle associated molecular motors have been shown to be essential for mitotic function, only a few chromosome arm--associated motors have been described. Here, we show that human chromokinesin human HKIF4A (HKIF4A) is an essential chromosome-associated molecular motor involved in faithful chromosome segregation. HKIF4A localizes in the nucleoplasm during interphase and on condensed chromosome arms during mitosis. It accumulates in the mid-zone from late anaphase and localizes to the cytokinetic ring during cytokinesis. RNA interference--mediated depletion of HKIF4A in human cells results in defective prometaphase organization, chromosome mis-alignment at metaphase, spindle defects, and chromosome mis-segregation. HKIF4A interacts with the condensin I and II complexes and HKIF4A depletion results in chromosome hypercondensation, suggesting that HKIF4A is required for maintaining normal chromosome architecture. Our results provide functional evidence that human KIF4A is a novel component of the chromosome condensation and segregation machinery functioning in multiple steps of mitotic division.     

Cytogenetic studies on two F1 hybrids of autotetraploid rice varieties showing extremely high level of heterosis

Mechanisms of two F1 hybrids (D46A × DTP-4 and D46A × Dminghui63) of autotetraploid rice (2n = 4x = 48) showing extremely high pollen fertility 87.40% and 85.97%, respectively, seed set 82.00% and 79.00%, respectively and extremely high level of heterosis were analyzed cytologically. The chromosome pairing of D46A × DTP-4 and D46A × Dminghui63 was normal at metaphase I(MI), and had almost no I or III, with an average of 0.020I +14.36 II 6.44rod+7.91ring) +0.01III + 4.80 IV + 0.01VIII and 0.06 I + 17.67 II (11.01rod + 6.67ring)] + 0.06 III +3.10IV+0.01VI, respectively. The most frequent chromosome configurations were 10II+7IV and 12II+bIV. The bivalent frequency was less frequent in hybrids than that in restoring parents, and the same results were gained from univalents, trivalent and multivalents. However, the quadrivalent frequency was significantly higher in hybrids than that in restoring parents at MI. The other meiotic phases progressed normally, except for low percentages of PMCs with lagging chromosomes at AI and low percentages of PMCs with micronuclei at telophaseI (TI) and telophaseII (TII). PMCs with lagging chromosomes at AI and PMCs with micronuclei at TI and TII showed negative correlation between pollen fertility and seed set. Above 90% of the PMCs could form normal microspores, which resulted in the production of viable pollen grains, abnormal microspores were observed including penta-fission and hexa-fission. Based on these results we suggest that the two F1 hybrids had better behaviors of chromosome pairing and genetic stability than autotetraploid rice and other autotetraploid plants ever studied.     

Molecular characterization of a ring X chromosome in a male with short stature

We report the molecular characterization of a ring X chromosome that was transmitted from a mother to a male who has short stature and minor dysmorphic features. This represents only the second reported ring X chromosome in a male. The ring is derived from breakage within the Xp pseudoautosomal region (PAR) and just proximal to the Xq PAR. The total amount of deleted material is 700-900 kb DNA and includes six known transcribed genes. Interestingly, SHOX, a gene implicated in short stature, is not deleted from the ring chromosome. Possible pathogenetic explanations for the patient's clinical features include insufficient dosage of deleted genes, a position effect on SHOX expression, and cell death during development because of ring chromosome nondisjunction. The findings are also relevant to observations made of "complete" ring chromosomes.     

Moderate mental retardation and nonspecific dysmorphic syndrome associated with ring chromosome 9

Summary A ring chromosome 9 is reported in a 12-year-old, moderately mentally retarded boy. As in other ring chromosome 9 patients, the clinical stigmata are nonspecific and their expressivity is mild. The finding of two normal cells of a total of 121 cells examined favors the hypothesis of a postzygotic, mitotic origin of the ring chromosome formation.