Genetic Control of Mitosis: Mutation ff3Causes Premature Beginning of Telophase Chromatin Reorganization in Drosophila melanogaster

The effect of cell cycle mutation ff3 on chromosome segregation was studied on fixed cells of neural ganglia of Drosophila melanogasterlarvae. The cell distributions by diameter of interphase nuclei and by distance between sister chromatid sets were compared at anaphase and telophase. In the control wild-type strain Lausenne, the cell distribution by distance between sister chromatids in anaphase was similar to their distribution by nuclear size. The mean distance between segregating chromatids at anaphase (l _av) coincided with the mean diameter of interphase nuclei (d _av) and was 8.3 m. Cells passed to telophase when chromatids were at least 10 m apart. The mutant ff3 strain differed from the control strain Lausenne in cell distribution by interphase nuclear diameter and distance between sister chromatids in anaphase; the mean nuclear diameter and mean distance between segregating chromatids similarly increased to 9.3 m. A specific feature of mitosis in mutant strain ff3 was a premature beginning of telophase chromatin reorganization. This caused the occurrence of cells with abnormally short (less then the interphase nuclear diameter) distance between sister chromatid sets in telophase but not in anaphase, as if these cells had passed from anaphase to telophase prematurely, during the chromatid movement toward poles in anaphase A.     

The effects of X-rays on the chromosomes of locust embryos

The variation in yield and the dose-response for chromatid aberration types following x-irradiation of Schistocerca gregaria embryo cells is described. Marked variations in yield are found for all aberration types during the G₂ and latter part of S stages of interphase. Only gaps appear to follow similar curves, other aberration types having unique patterns of response. The dose exponents for the various chromatid aberration types are similar to, but lower than, those reported for other organisms. Chromatid “breaks” appear to have a dose exponent greater than 1.0 — a fact which is in conformation with the exchange hypothesis. The chromosome radiosensitivity of this organism is similar to that reported for other organisms.     

The role of interphase nitrogen transport in the dynamic measurement of the overall volumetric mass transfer cefficient in air-sparged systems

It has been shown both theoretically and experimentally that interphase nitrogen transport may have a significant influence on the rate of interphase oxygen transport, and thereby also on the value of the volumetric mass transfer coefficient of oxygen, k_la, determined in mechanically agitated bubble fermentors using the variants of dynamic method presented in the literature. The experiments were carried out in 1M KCI solution at five stirrer frequencies and two gas inlet levels. The gas interchanges were performed either without interrupting the aeration and agitation of the charge (A) or with the aeration and agitation of the charge turned on at the same time (B). The applied variants of the interchange were N₂→ O₂→, O₂→ N₂, N₂→ air, air→ N₂, O→ O₂, and O→ air. In the two last variants the oxygen dissolved in the charge was removed by reacting with sulfite ions. The k_la values calculated by allowing for the nitrogen transport for procedure A were approximately equal to the values obtained by disregarding the nitrogen transport, whereas those for procedure B were higher (up to 40%), than the values obtained disregarding the nitrogen transport.     

The effects of X-rays on the chromosomes of locust embryos

Irradiation of Schistocerca gregaria embryo cells during the S and G₂ stages of interphase produces the same general classes of chromatid aberration as have been seen in organisms, such as Vicia faba, which have been used widely in radiocytological work. In addition, however, a series of intrachange aberrations has been found which is novel. In order to account for these Revell's exchange hypothesis has been extended to include the involvement of isolocus primary lesions. The exchange-type hypothesis is preferred because it can more readily accomodate the whole spectrum of aberrations found.     

Random homologous pairing and incomplete sister chromatid alignment are common in angiosperm interphase nuclei

The chromosome arrangement in interphase nuclei is of growing interest, e.g., the spatial vicinity of homologous sequences is decisive for efficient repair of DNA damage by homologous recombination, and close alignment of sister chromatids is considered as a prerequisite for their bipolar orientation and subsequent segregation during nuclear division. To study the degree of homologous pairing and of sister chromatid alignment in plants, we applied fluorescent in situ hybridisation with specific bacterial artificial chromosome inserts to interphase nuclei. Previously we found in Arabidopsis thaliana and in A. lyrata positional homologous pairing at random, and, except for centromere regions, sister chromatids were frequently not aligned. To test whether these features are typical for higher plants or depend on genome size, chromosome organisation and/or phylogenetic affiliation, we investigated distinct individual loci in other species. The positional pairing of these loci was mainly random. The highest frequency of sister alignment (in >93% of homologues) was found for centromeres, some rDNA and a few other high copy loci. Apparently, somatic homologous pairing is not a typical feature of angiosperms, and sister chromatid aligment is not obligatory along chromosome arms. Thus, the high frequency of chromatid exchanges at homologous positions after mutagen treatment needs another explanation than regular somatic pairing of homologues (possibly an active search of damaged sites for homology). For sister chromatid exchanges a continuous sister chromatid alignment is not required. For correct segregation, permanent alignment of sister centromeres is sufficient.     

A schedule to demonstrate radiation-induced sister chromatid exchanges in human lymphocytes

Summary The reciprocal interchange between the chromatids of a chromosome, termed sister chromatid exchange (SCE), is considered to be one of the most sensitive and accurate cytogenetic parameters and respond to toxic chemicals at very low doses. But the response of SCE to ionizing radiation is very poor. Human lymphocytes fail to give SCE response when irradiated at G₀. Probably the primary lesions induced at G₀ do not remain available long enough to find expression as SCEs. Based on this assumption a schedule was developed using caffeine to demonstrate radiation induced SCEs. Following this schedule a dose-dependent increase in the frequency of radiation induced SCEs has been observed.     

DISTRIBUTION OF TRITIUM-LABELED DNA AMONG CHROMOSOMES DURING MEIOSIS : I. Spermatogenesis in the Grasshopper

Thymidine-H³ of high specific activity was used to study the distribution of labeled chromatids during meiotic divisions in spermatocytes of a species of grasshopper (Orthoptera). The distribution is regularly semiconservative as has been shown previously for mitosis, i.e., all chromatids are labeled after incorporation of thymidine-H³ into DNA at premeiotic interphase. If incorporation occurs at the interphase preceding this one, the chromosomes arrive at meiotic divisions with the equivalent of one chromatid of each homologue labeled. Chromatid exchanges occur at a frequency which is very nearly that predicted on the assumption that each chiasma represents an exchange between homologous chromatids. However, the exchanges are randomly distributed among chromosomes in a size group, whereas chiasmata are not. A quantitative analysis of the frequency and pattern of exchanges indicates that most of these result from breakage and reciprocal exchange between homologous chromatids. Sister chromatid exchanges are much less frequent and may be limited to premeiotic stages.     

The stage of chromosome duplication in the cell cycle as revealed by X-ray breakage and 3H-thymidine labeling

By means of combined experiments of X-irradiation and ³H-thymidine labeling of the chromosomes which are in the phase of synthesis, and the subsequent analysis at metaphase on the autoradiographs of the chromosomal damage induced during interphase, it was shown that in somatic cells from a quasi-diploid Chinese hamster line cultured in vitro the chromosomes change their response to radiation from single (chromosome type aberrations) to double (chromatid type aberrations) in late G₁. These results are interpreted to indicate that the chromosome splits into two chromatids in G₁, before DNA replication. — By extending the observations at the second metaphase after irradiation, it was also seen that cells irradiated while in G₂ or late S when they reach the second post-irradiation mitosis still exhibit, beside chromosome type aberrations, many chromatid exchanges, some of which are labeled. Two hypotheses are suggested to account for this unexpected reappearance of chromatid aberrations at the second post-irradiation division. The first hypothesis is that they arise from half-chromatid aberrations. The second hypothesis, which derives from a new interpretation of the mechanisms of production of chromosome aberrations recently forwarded by Evans, is that they arise from gaps or achromatic lesions which undergo, as the cells go through the next cycle, a two-step repair process culminating in the production of aberrations.This work was supported in part by grant No. RH-00304 from the Division of Radiological Health, Bureau of State Services, Public Health Service, U.S.A.     

Heterochromatin and chromosomal aberration in microtus agrestis: role of chromosomal association

Mitomycin C (MC) -induced chromatid aberrations among the chromosomes of Microtus agrestis are preferentially localized in the constitutive heterochromatic regions, i.e., major part of the sex chromosomes and the centromeric regions of the autosomes. In the sex chromosomes, intrachanges predominate, while interchanges between the two X chromosomes are very rare. This pattern of distribution of different types of aberrations is interpreted as due to the individual chromocentres that are formed by the two X chromosomes in the interphase.     

Roles of the sister chromatid cohesion apparatus in gene expression,development, and human syndromes

The sister chromatid cohesion apparatus mediates physical pairing of duplicated chromosomes. This pairing is essential for appropriate distribution of chromosomes into the daughter cells upon cell division. Recent evidence shows that the cohesion apparatus, which is a significant structural component of chromosomes during interphase, also affects gene expression and development. The Cornelia de Lange (CdLS) and Roberts/SC phocomelia (RBS/SC) genetic syndromes in humans are caused by mutations affecting components of the cohesion apparatus. Studies in Drosophila suggest that effects on gene expression are most likely responsible for developmental alterations in CdLS. Effects on chromatid cohesion are apparent in RBS/SC syndrome, but data from yeast and Drosophila point to the likelihood that changes in expression of genes located in heterochromatin could contribute to the developmental deficits.     

Intraspecific hybridization and its bearing on chromosome evolution inVicia narbonensis (Fabaceae)

Nine accessions ofVicia narbonensis, considered to be the wild progenitor of faba bean (Vicia faba), were investigated to ascertain the nature and extent of intraspecific karyotypic polymorphism. The chromosome complements resolved into four distinct types (A, B, C, D), and the meiotic data of F₁ hybrids (A × B, B × C, A × C) revealed that alteration in chromosome morphology is the result of segmental interchanges. The interchange complexes indicate that the parents differ from each other by 1 to 2 interchanges. It is also evident that karyotype B, and not A as previously reported, is the normal karyotype of the species, and A and C are single homozygotes for unequal interchange. The comparative karyomorphology of the parents and the hybrids, and of two interchange heterozygotes of four chromosomes each in F₁ hybrids of A × C shows that the chromosomes involved in the single interchange homozygotes (A, C) are not common and the breaks in both interchanges occurred in short and long arms of the involved chromosomes. Identification of the interchanged chromosomes in the complements and the frequency of ring and chain quadrivalents in the heterozygotes enabled location of the breakpoints. The present results provide probably the first example indicating that interchange homozygosity (A) is not only firmly established but also has enabled the species to spread further by adapting to a wide range of habitats. — The genetic relationships between A and D are very different. All seven chromosome pairs in D could be distinguished from A, and for that matter, B and C as well. From the meiotic pairing properties it is also amply clear that genome D is well differentiated from A and possibly B, and C, and deserves special status.     

The cytogenetic effects of aflatoxin and gamma-rays on human leukocytes in vitro

Chromosomes from human leukocyte cultures in vitro were treated with γ-rays (200 R), aflatoxin (50 μg/ml, dissolved in dimethyl sulfoxide (DMSO)) and with a combination of both. At the time of treatment (48 h) cells were in all stages of interphase but G₁ cells were evidently predominant. All types of chromosome aberration were observed. Frequencies of chromosome-type aberrations were much higher than those of chromatid type after γ-ray treatment, but these types of chromosome aberration did not differ greatly when the cultures were treated with aflatoxin. Apparently the cytogenetic effect of aflatoxin was delayed longer than was that of irradiation. The present data also suggest the additive effect of γ-rays and aflatoxin in the combined treatment.     

Spontaneous chromosome mutations in Truxaline grasshoppers

Three distinct mutant conditions are described in single male individuals from three species of short horn grasshopper. Of these, one is an entire germ line mutant of Myrmeleotettix maculatus, heterozygous for a centric fusion between single M₄ and M₅ telocentric chromosomes. In contrast, the remaining two mutants are present in mosaic form. One is heterozygous for an L₁-M₄ interchange in Omocestus viridulus, the other tetrasomic for the M₄ chromosome in Chorthippus parallelus which in addition is characterised by the inclusion of a supernumerary heterochromatic segment on one S₈ homologue. Centric fusion in Myrmehotettix maculatus has neither disturbed the chiasma potential of the elements constituting the fusion multiple nor, has it apparently influenced the production of balanced gametes. The pattern of chiasma formation in the L₁-M₄ interchange multiple lends support to the contention that the process of chiasma formation originates near the distal end of chromosome arms in Omocestus viridulus. There is no interaction between the two mutant conditions of tetrasomy and the presence of supernumerary segments in Chorthippus parallelus. Moreover, because of the precocious nature of two of the four M₄ homologues there is little tendency to form multivalents. The two M₄ bivalents share a similar mean chiasma frequency.     

Genetic Control of Mitosis: Features of Anaphase Separation of Sister Chromatid Sets in Mutant Strain aar V158 in Drosophila melanogaster

The effect of mutation aar ^V158 on anaphase separation of chromatids was studied on fixed cells of neural ganglia of Drosophila melanogaster larvae. It was shown that mutation aar ^V158 causes three types of defective chromosome segregation manifested as (1) monopolar anaphase, (2) separation of chromatids to an abnormally short distance in anaphase, and (3) bridging and lagging of some chromatids or prolonged asynchronous separation of sister chromatid sets to the poles in anaphase. We believe that the former two types of defective segregation are caused by disturbed centrosome separation at the beginning of mitosis and the third type, by defects in chromatid separation during anaphase. During the two-year maintenance of the mutation in a heterozygous state, partial correction (adaptive modification) of the defects of type 1 and type 2 (but not type 3) occurred. The correction of type 1 and type 2 defects during adaptogenesis depended on the genotype: in heterozygotes and homozygotes, respectively type 1 and type 2 were preferentially corrected. The frequency of type 3 defects remained constant during the two-year period of maintenance of the mutation in a heterozygous state. However, in all variants of the experiment, their frequency decreased with increasing distance between the sister chromatid sets. In the cells that completed the previous division with abnormalities, the checkpoint system is supposed to effectively arrest the cell cycle in the subsequent division.     

A correlation between radiation sensitivity and initial chromatid breaks in cancer cell lines revealed by Calyculin A-induced premature condensation

Three human malignancy cell lines were irradiated with ⁶⁰Co γ-rays. Initial chromatid breaks were measured by using the chemically induced premature chromosome condensation technique. Survival curves of cells exposed to gamma rays was linear-quadratic while the efficiency of Calyculin A in inducing PCC of G₂ PCC was about five times more than G₁ PCC. A dose-dependent increase in radiation-induced chromatid/isochromatid breaks was observed in G₁ and G₂ phase PCC and a nearly positive linear correlation was found between cell survival and chromatin breaks. This study implies that low LET radiation-induced chromatid/isochromatid breaks can potentially be used to predict the radiosensitivity of tumor cells either in in vitro experimentation or in in vivo clinical radiotherapy.     

Chromosome behaviour in the Aloineae

At AI of meiosis dicentric bridges with acentric fragments and bridges with double side-arms, products of chromatid and sub-chromatid errors respectively, occur with varying frequency in the great majority of 186 plants of 167 taxa in the Aloineae. Correlations between the occurence of the two types of aberration and between the distribution of the sites of their formation and the distribution of chiasmata suggest that they are both caused by aberrant reunions at the sites of chiasma formation. Different types of reunion following four-strand sub-chromatid breakage account for the two findings that sub-chromatid errors are always accompanied by chromatid errors, but that the latter can occur alone.     

The interchange of 14C-kinetin and 14C-gibberellic acid between the bark and xylem of willow

Summary Considerable two-way interchange of label derived from ¹⁴C-GA₃ and ¹⁴C-kinetin has been shown to occur between the xylem and extra-cambial tissue of Salix viminalis L. Radial translocation of both growth substances from the transpiration stream to the sieve-tube sap has been demonstrated by the use of an aphid technique. This is the first record of the bark-xylem interchange of these two growth hormones.A high percentage of ¹⁴C-label is present in compounds other than GA₃ and kinetin 8 hours after application.Mechanisms of lateral transfer are discussed together with their possible physiological significance.This work was carried out during the tenure of an N.E.R.C. studentship by M.R.B.     

Types and frequencies of chromosomal aberrations induced by irradiation of different parts of interphase in Crepis capillaris

Quantitative and qualitative estimates of chromosomal damage in roots of Crepis capillaris were made in metaphase cells at many time intervals after irradiation with 200 or 400 rad of ⁶⁰Co gamma-rays. The results have confirmed the general pattern described for cells of other organisms, and have revealed in addition the following new facts. (1) The formation of aberrations of chromosome and chromatid type is not determined by the time of chromosome duplication alone. (2) The relative frequencies of different types of discontinuity form peaks with the following time succession: single gaps, chromatid breaks, isolocus breaks. (3) The location of peaks does not depend on the radiation dose, and shows no correlation which the time of synthesis. (4) Irradiation of G₂ induces a significant number of chromosome-type exchanges in Crepis. (5) Higher doses of radiation in G₂ favour the formation of chromatid over chromosome exchanges and of isochromatid breaks over chromosome breaks. A new interpretation of the production of certain types of aberration is discussed.     

UV-induced chromatid aberrations in cultured Chinese hamster cells after one, two, or three rounds of DNA replication

Synchronized G₁ or G₂ Chinese hamster cells were irradiated with UV light or X-rays and analyzed for chromosomal aberrations after one, two, or three replications. The cells were treated with Colcemid to induce polyploidy so that 2N, 4N, and 8N cells were scored. UV irradiation of G₁ cells induces mainly chromatid aberrations, whereas X-rays induce chromosome aberrations. After both types of radiation chromatid aberrations appear in the polyploid cells. These results can be interpreted as indicating that UV and X-rays induce lesions at the subchromatid level that cannot be expressed until one or two replications have occurred. Since UV can induce long-lived lesions, the UV data do not allow us to choose between mononemic and polynemic models of the chromosome. X-rays, however, are ionizing radiations that might not produce long-lived lesions. The X-ray data, therefore, are more easily interpretable in terms of lesions being induced in the subunits of a polynemic chromosome.