Sequence of centromere separation: separation in a quasi-stable mouse-human somatic cell hybrid

Unrepaired DNA damage facilitates elimination of uniparental chromosomes in interspecific hybrid cells

Elimination of uniparental chromosomes occurs frequently in interspecific hybrid cells. For example, human chromosomes are always eliminated during clone formation when human cells are fused with mouse cells. However, the underlying mechanisms are still elusive. Here, we show that the elimination of human chromosomes in human-mouse hybrid cells is accompanied by continued cell division at the presence of DNA damage on human chromosomes. Deficiency in DNA damage repair on human chromosomes occurs after cell fusion. Furthermore, increasing the level of DNA damage on human chromosomes by irradiation accelerates human chromosome loss in hybrid cells. Our results indicate that the elimination of human chromosomes in human-mouse hybrid cells results from unrepaired DNA damage on human chromosomes. We therefore provide a novel mechanism underlying chromosome instability which may facilitate the understanding of carcinogenesis.

Imaging genome abnormalities in cancer research

Increasing attention is focusing on chromosomal and genome structure in cancer research due to the fact that genomic instability plays a principal role in cancer initiation, progression and response to chemotherapeutic agents. The integrity of the genome (including structural, behavioral and functional aspects) of normal and cancer cells can be monitored with direct visualization by using a variety of cutting edge molecular cytogenetic technologies that are now available in the field of cancer research. Examples are presented in this review by grouping these methodologies into four categories visualizing different yet closely related major levels of genome structures. An integrated discussion is also presented on several ongoing projects involving the illustration of mitotic and meiotic chromatin loops; the identification of defective mitotic figures (DMF), a new type of chromosomal aberration capable of monitoring condensation defects in cancer; the establishment of a method that uses Non-Clonal Chromosomal Aberrations (NCCAs) as an index to monitor genomic instability; and the characterization of apoptosis related chromosomal fragmentations caused by drug treatments.

Parental genome separation and asynchronous centromere division in interspecific F1 hybrids in Lathyrus

Chromosomes were studied in root-tip metaphase cells of several F1 interspecific Lathyrus hybrids including: L. hirsutus x L. cassius (H x C), L. cassius x L. hirsutus (C x H), L. cassius x L. odoratus (C x O), and their parents, all with 2n = 2x = 14. Two types of morphologically distinct centromeres were identified in the hybrids on the basis of the degree of contraction of the primary constriction. At least 12 well-defined centromeres were seen in all cells of L. hirsutus, L. cassius and L. odoratus, and about 80% of cells had 14. The hybrids were more variable than the species. H x O contained between six and 14 well-defined centromeres, while cells of H x C, C x H and C x O all had seven well-defined and seven weakly defined centromeres. These were used as markers to plot their spatial disposition in two dimensions on metaphase spreads. In H x C, C x H and C x O the two types of centromeres showed a significant tendency to occupy two spatially distinct and concentrically arranged domains on the metaphase plate (P < 0.005). Owing to shortage of material subsequent work was restricted to H x C and C x H. Six or seven chromosomes of one parental genome were selectively labelled by in situ hybridization using biotinylated total genomic DNA from either parent as a probe. Moreover, there was a very strong correlation between centromere type and genomic origin (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

The radial positions of metaphase chromosomes may be a consequence of the relative strength of their interaction with the spindle and their size

Microtubule oriented forces acting on chromosomes on spindles in mitosis and meiosis will produce a radial component of force in the plane of the metaphase plate. The strength of this vector will depend on the angle at which the microtubule meets the plate. Radial forces will tend to segregate chromosomes to peripheral or central positions, depending on their size, and also on the strength of the activity of individual centromeres. In prometaphase, forces pushing chromosomes from the poles will tend to force them to the periphery of the metaphase plate, as seen in radial metaphases. Tension towards the poles at late metaphase will pull smaller chromosomes and those with more powerfully active kinetochores towards the centre of the plate. If the two genomes in a hybrid cell have different centromeric activities, their chromosomes will be segregated. Microtubule assembly and disassembly, and motor proteins such as the kinesins and dynein which haul organelles along microtubules, can provide forces in both directions.

Somatic hybridization between potato and Nicotiana plumbaginifolia : 1. Spontaneous biparental chromosome elimination and production of asymmetric hybrids

Electrofusion was carried out between mesophyll protoplasts from the transformed diploid S. tuberosum clone 413 (2n=2x=24) which contains various genetic markers (hormone autotrophy, opine synthesis, kanamycin resistance, β-glucuronidase activity) and mesophyll protoplasts of a diploid wild-type clone of N. plumbaginifolia (2n=2x=20). Hybrid calli were obtained after continuous culture on selection medium containing kanamycin. Parental chromosome numbers, determined at 2 months after fusion, revealed hybrid-specific differences between the individual calli. On the basis of these differences three categories of hybrids were distinguished. Category I hybrids contained between 8 and 24 potato chromosomes and more than 20 N. plumbaginifolia chromosomes; category II hybrids had between 1 and 20 N. plumbaginifolia chromosomes and more than 24 potato chromosomes; category III hybrids contained diploid or subdiploid numbers of chromosomes from both parents. The hybrids were evenly distributed over the three categories. After a 1-year culture of 24 representative hybrid callus lines on selection medium the karyotype of 10 hybrids remained stable, whereas 8 hybrids showed polyploidization of the genome of one parent, together with no or minor changes of the chromosome numbers of the other parent. Six hybrids showed slight changes in the hybrid karyotype. The elimination of chromosomes of a particular parent was not correlated to their metaphase location. The processes of spontaneous biparental chromosome elimination leading to the production of asymmetric hybrids of different categories are discussed.

The centromere-kinetochore complex: a repeat subunit model

The three-dimensional structure of the kinetochore and the DNA/protein composition of the centromere-kinetochore region was investigated using two novel techniques, caffeine-induced detachment of unreplicated kinetochores and stretching of kinetochores by hypotonic and/or shear forces generated in a cytocentrifuge. Kinetochore detachment was confirmed by EM and immunostaining with CREST autoantibodies. Electron microscopic analyses of serial sections demonstrated that detached kinetochores represented fragments derived from whole kinetochores. This was especially evident for the seven large kinetochores in the male Indian muntjac that gave rise to 80-100 fragments upon detachment. The kinetochore fragments, all of which interacted with spindle microtubules and progressed through the entire repertoire of mitotic movements, provide evidence for a subunit organization within the kinetochore. Further support for a repeat subunit model was obtained by stretching or uncoiling the metaphase centromere-kinetochore complex by hypotonic treatments. When immunostained with CREST autoantibodies and subsequently processed for in situ hybridization using synthetic centromere probes, stretched kinetochores displayed a linear array of fluorescent subunits arranged in a repetitive pattern along a centromeric DNA fiber. In addition to CREST antigens, each repetitive subunit was found to bind tubulin and contain cytoplasmic dynein, a microtubule motor localized in the zone of the corona. Collectively, the data suggest that the kinetochore, a plate-like structure seen by EM on many eukaryotic chromosomes is formed by the folding of a linear DNA fiber consisting of tandemly repeated subunits interspersed by DNA linkers. This model, unlike any previously proposed, can account for the structural and evolutional diversity of the kinetochore and its relationship to the centromere of eukaryotic chromosomes of many species.