Chromosome Cycle Research Articles

Early stages of spindle formation and independence of chromosome and microtubule cycles in Haemanthus endosperm.

We analyzed transformation of the interphase microtubular cytoskeleton into the prophase spindle and followed the pattern of spindle axis determination. Microtubules in endosperm of the higher plant Haemanthus (Scadoxus) were stained by the immunogold and immunogold silver-enhanced methods. Basic structural units involved in spindle morphogenesis were "microtubule converging centers." We emphasized the importance of relative independence of chromosomal and microtubular cycles, and the influence of these cycles on the progress of mitosis. Cells with moderately desynchronized cycles were functional, but extreme desynchronization led to aberrant mitosis. There were three distinct phases of spindle development. The first one comprised interphase and early to mid-prophase. During this phase, the interphase microtubule meshwork radiating from the nuclear surface into the cytoplasm rearranged and formed a dense microtubule cage around the nucleus. The second phase comprised mid to late prophase, and resulted in the formation of normal (bipolar) or transitory aberrant (apolar or multipolar) prophase spindles. The third phase comprised late prophase with prometaphase. The onset of prometaphase was accompanied by a rapid association of microtubule converging centers with kinetochores. In this stage aberrant spindles transformed invariably into bipolar ones. Lateral association of a few bipolar kinetochore fibers at early prometaphase established the core of the bipolar spindle and its alignment. We concluded that (1) spindle formation is a largely independent microtubular process modified by the chromosomal/kinetochore cycle; and (2) the initial polarity of the spindle is established by microtubule converging centers, which are a functional substitute of the centrosome/MTOC. We believe that the dynamics of microtubule converging centers is an expression of microtubule self-organization driven by motor proteins as proposed by Mitchison [1992: Philos. Trans. R. Soc. Lond. B. 336:99].

Genetic mapping of Hessian fly avirulence gene <i>vH6</i> using bulked segregant analysis

The Hessian fly, Mayetiola destructor (Say), an important insect pest of wheat, Triticum aestivum L., has a gene-for-gene relationship with wheat: single genes in the insect condition avirulence to specific resistance genes in wheat. We report the discovery of the first molecular genetic marker that is tightly linked to a Hessian fly avirulence gene. This dominant DNA polymorphism (OPG15-1) was identified using bulked segregant analysis and arbitrary primers in polymerase chain reactions. Bulked segregant analysis was modified to accommodate the anomalous chromosome cycle of the Hessian fly. It was used to identify DNA polymorphisms linked to the gene (vH6) that confers avirulence to the resistance gene H6 in wheat. OPG15-1 was cloned and sequenced, and a pair of site-specific primers were designed that converted it into a codominant single-stranded conformational polymorphism. Both OPG15-1 and vH6 were shown to be X-linked, and the genetic distance between the two loci was 2.5 +/- 2.5 cM. In situ hybridization to polytene chromosomes of larval salivary glands indicated that OPG15-1 resides near the centromere of Hessian fly chromosome X1.

Isolation and chromosomal localization of a germ line-specific highly repetitive DNA family in Acricotopus lucidus (Diptera, Chironomidae).

In the chironomid Acricotopus lucidus, parts of the genome, the germ line-limited chromosomes, are eliminated from the future soma cells during early cleavage divisions. A highly repetitive, germ line-specific DNA sequence family was isolated, cloned and sequenced. The monomers of the tandemly repeated sequences range in size from 175 to 184 bp. Analysis of sequence variation allowed the further classification of the germ line-restricted repetitive DNA into two related subfamilies, A and B. Fluorescence in situ hybridization to gonial metaphases demonstrated that the sequence family is highly specific for the paracentromeric heterochromatin of the germ line-limited chromosomes. Restriction analysis of genomic soma DNA of A. lucidus revealed another tandem repetitive DNA sequence family with monomers of about 175 bp in length. These DNA elements are found only in the centromeric regions of all soma chromosomes and one exceptional germ line-limited chromosome by in situ hybridization to polytene soma chromosomes and gonial metaphase chromosomes. The sequences described here may be involved in recognition, distinction and behavior of soma and germ line-limited chromosomes during the complex chromosome cycle in A. lucidus and may be useful for the genetic and cytological analysis of the processes of elimination of the germ line-limited chromosomes in the soma and germ line.

Microtubule Dependence of Chromosome Cycles inXenopus laevisBlastomeres under the Influence of a DNA Synthesis Inhibitor, Aphidicolin

The spindle-assembly checkpoint of the cell cycle develops inXenopus laevisembryos at the midblastula transition (MBT). Our previous experiments using animal-cap blastomeres indicate that the checkpoint is regulated by a mechanism that depends on age, but not on the nucleocytoplasmic (N/C) ratio (Clute and Masui, 1995). In the present study, the time of appearance of the spindle-assembly checkpoint is examined in animal-cap blastomeres whose N/C ratio is reduced by treatment with aphidicolin. Animal-cap blastomeres treated with aphidicolin from the 2-cell stage cleave more slowly after 4th cleavage, in a dose-dependent manner, but cleavage and chromosome cycles continue up to the 11th to 13th cleavage and then arrest. Blastomeres treated with aphidicolin have a reduced DNA content and N/C ratio compared to control blastomeres of the same age. Nevertheless, nocodazole-sensitive chromosome cycles appear at the same time as in control blastomeres, at 3 to 5 hr after 5th cleavage, regardless of the N/C ratio. The arrest in interphase caused by treating blastula stage animal caps with aphidicolin can be reversed by treatment with caffeine. The caffeine-induced mitosis becomes sensitive to nocodazole after the MBT, but not before. Therefore, the same mechanism which stabilizes maturation-promoting factor activity in the absence of a mitotic spindle also operates after the MBT in blastomeres that are treated with aphidicolin, if mitosis is induced by caffeine. This mechanism may involve the translation of a maternal mRNA at the time of the MBT, as suggested previously.

Nuclear Components with Microtubule-Organizing Properties in Multicellular Eukaryotes: Functional and Evolutionary Considerations

The nucleus and the microtubular cytoskeleton of eukaryotic cells appear to be structurally and functionally interrelated. Together they constitute a "cell body". One of the most important components of this body is a primary microtubule-organizing center (MTOC-I) located on or near the nuclear surface and composed of material that, in addition to constitutive centrosomal material, also comprises some nuclear matrix components. The MTOC-I shares a continuity with the mitotic spindle and, in animal cells, with the centrosome also. Secondary microtubule-organizing centers (MTOC-IIs) are a special feature of walled plant cells and are found at the plasma membrane where they organize arrays of cortical MTs that are essential for ordered cell wall synthesis and hence for cellular morphogenesis. MTOC-IIs are held to be similar in origin to the MTOC-I, but their material has been translocated to the cell periphery, perhaps by MTs organized and radiating from the MTOC-I. Many intranuclear, matrix-related components have been identified to participate in MT organization during mitosis and cytokinesis; some of them also seem to be related to the condensation and decondensation of chromatin during the mitotic chromosome cycle.

Chromosomal abnormalities in HPV-16-immortalized oral epithelial cells

Human papilloma virus (HPV) type 16 has an established association with anogenital carcinoma, and to some extent with human oral squamous cell carcinoma. We hypothesize that HPV type 16 is capable of inducing chromosomal and cell cycle changes in cultured oral epithelial cells. Normal human oral epithelia] cells were immortalized with recombinant retrovirus containing the E6/E7 open reading frames of HPV type 16. These cells have been in culture for more than 350 passages and over 4 years. Flow cytometry demonstrated an average of 42% nuclear aneuploidy in HPV 16-immortalized cells; 16% in normal controls (probably tetrasomy). Cytogenetic analysis demonstrated significant progression of chromosomal abnormalities. Cells at early passage (p10) showed trisomy 20, with no other major changes. At passage 18, trisomy 1q and monosomy 13 were seen in addition to trisomy 20. At passage 61 there were two distinct cell populations ('a' and 'b'), with multiple chromosomal changes including trisomy 5q,14,20 in one line and 7p,9q,llq in the other. Both populations had monosomy 3p, with monosomy 8p in one population and monosomy 13 in the other. At passage 136, the cells were essentially identical to population 'b' of passage 61. At this passage, mutation of the p53 gene was detected at codon 273 of exon 8, with G to T conversion (Arg to Leu). This was absent in the normal cells from which this line was developed. Passage 262 contained the two major cell populations, each with a sub-group with additional chromosomal changes such as 10p monosomy. Cells from passages 217 and 305 were injected into nude mice a year apart. Both failed to produce tumors, as did normal cells. In conclusion, we present an HPV type 16-immortalized oral epithelial cell line (IHGK) with extensive and progressive chromosomal abnormalities, invasive growth in culture and yet no tumor formation in nude mice. We suggest that the question as to whether HPV alone can induce transformation is still open.

Topoisomerase I activity in irradiated Chinese hamster cells: relation to chromosomal aberrations and cell cycle progression

Topoisomerase I activity in irradiated Chinese hamster cells: relation to chromosomal aberrations and cell cycle progression

Regulation of the Appearance of Division Asynchrony and Microtubule-Dependent Chromosome Cycles in Xenopus laevis Embryos

Divisions of animal-cap blastomeres dissociated from Xenopus laevis embryos are synchronous mostly up to 12th cleavage or the 13th cell cycle, but become asynchronous afterward, during the midblastula transition (MBT), and at the same time, chromosome cycles become microtubule-dependent and are arrested in mitosis if treated with nocodazole. To investigate causes for these changes in cell-cycle control, we observed division synchrony in animal-cap blastomeres dissociated from embryos whose nucleocytoplasmic ratio (N/C) had been altered by constriction of zygotes or by delaying nucleation into zygote halves and compared their mitotic indices in the presence and absence of nocodazole. Thus, we found that asynchronous divisions always commenced when N/C reached the value of 128 to 256 times that of an animal blastomere of the 32-cell embryo, corresponding to the 12th and 13th cycles of a normal embryo, while the number of synchronous cycles became variable, ranging from 9 to 14, depending on the initial N/C. Treatment with α-amanitin or cycloheximide did not alter the number of synchronous cycles. However, the time at which the mitotic index of nocodazole-treated blastomeres first exceeded that of control remained constant, at 3 to 5 hr after 5th cleavage, regardless of the initial N/C. Thus, chromosome cycles of blastomeres first became sensitive to nocodazole at a variable N/C, ranging from 8 to 1024 times that of an animal blastomere of the 32-cell embryo. The timing of the appearance of nocodazole sensitivity was unaffected by α-amanitin treatment, whereas it was markedly delayed following cycloheximide treatment. These results suggest that the commencement of division asynchrony is N/C-dependent, whereas the development of microtubule-dependent cell cycles is age-dependent, most likely being programmed by the translation of stored mRNAs.

Is there a unique form of chromatin at the Saccharomyces cerevisiae centromeres?

Chromosome transmission in S. cerevisiae requires the activities of many structural and regulatory proteins required for the replication, repair, recombination and segregation of chromosomal DNA, and co-ordination of the chromosome cycle with progression through cell cycle. An important structural domain on each chromosome is the kinetochore (centromere DNA and associated proteins), which provides the site of attachment of chromosomes to the spindle microtubules. Stoler et al. have recently reported the cloning of an essential gene CSE4, mutations in which cause chromosome nondisjunction of a marked chromosome bearing a centromere DNA mutation. The cse4-1 mutation causes cells to arrest in the G2/M phase of the cell cycle with a 2N DNA content in a RAD9 checkpoint-independent manner. The carboxyl terminus of Cse4p and the human centromere-localized protein CENP-A have a high degree of homology to the C-terminal domain of histone H3. Since both CENP-A and Cse4p also have biochemical properties similar to histones H3 and H4, it is tempting to speculate that these histone H3-like proteins are components of specialized nucleosomes, a class of which may be unique to the centromeres.

Lineage-specific alteration in cell cycle structure in early Tubifex embryos.

Embryos of the freshwater oligochaete Tubifex exhibit asynchrony in division timing as early as the second cleavage; this cleavage asynchrony becomes pronounced as development proceeds. The present study was undertaken to elucidate the composition and duration of the cell cycles of early Tubifex embryos, with special reference to their cell lineages. No significant variations in lengths of cleavage cycles were found among early embryos. In all blastomeres up to the eighth cleavage cycle, the M phase was followed directly by a 30 min S phase, which suggested that early embryos lack G1 phase. The durations of the M phase did not change during this period of development, but did differ between cell lines. The M phase in the A and B cell lines lasted for about 130 min, while the M phase in the C and D cell lines lasted for about 95 min. An examination of chromosome cycles showed that this difference in M phase durations resulted from a longer stay by the A/B cell lines in prometaphase. Only G2 phase lengthened during early development. After several rounds of G2 phase extension, three classes of G2 phase duration were established: the most extended G2 phase (∼6 h) in the first quartette of micromeres (cells 1 a-1 d), the shortest G2 phase (∼1.58 h) in teloblasts, and an intermediate G2 phase (∼2.4 h) in the progeny of macromeres (i.e. endodermal cells). Experiments with syncytial blastomeres showed that the timing of entry into the M phase, hence the duration of the G2 phase, was affected by cytoplasmic compositions. The shortest G2 phase correlated closely with the presence of yolk-free cytoplasm called pole plasm.

Effect of staurosporine on MOLT‐4 cell progression through G2 and on cytokinesis

Staurosporine (SSP) is an inhibitor of a variety of protein kinases with an especially high affinity towards protein kinase C. Whereas SSP has been shown to halt the cell cycle progression of various normal, nontransformed cell types in G1, most virus transformed or tumor cells are unaffected in G1 but arrest in G2 phase. SSP has also been observed to increase the appearance of cells with higher DNA content, suggestive of endoreduplication, in cultures of tumor cells. Using multivariate flow cytometry (DNA content vs. expression of cyclin B, nuclear p120 protein, or protein reactive with Ki-67 antibody) which makes it possible to discriminate cells with identical DNA content but at different phases of the cycle, we have studied the cell cycle progression of human lymphocytic leukemic MOLT-4 cells in the presence of 0.1 microM SSP. MOLT-4 cells did not arrest in G1 or G2 phase in the presence of the inhibitor. Rather, they failed to undergo cytokinesis, entering G1 phase at higher DNA ploidy (tetraploidy; G1T), and then progressed through ST (rereplication) into G2T and MT. The rates of entrance to G2 and G2T were essentially identical, indicating that the rates of cell progression through S and ST as well as through G2 and G2T, respectively, were similar. Cells entrance to mitosis and mitotic chromatin condensation were also similar at the diploid and tetraploid DNA content level and were unaffected by 0.1 microM SSP. No evidence of growth imbalance (altered protein or RNA to DNA ratio) was observed in the case of tetraploid cells. The data show that, in the case of MOLT-4 cells, all events associated with the chromosome or DNA cycle were unaffected by SSP; the only target of the inhibitor appears to be kinase(s) controlling cytokinesis.

Dithiothreitol prevents membrane fusion but not centrosome or microtubule organization during the first cell cycles in sea urchins.

Dithiothreitol (DTT), a disulfide reducing agent, inhibits the fusion of male and female pronuclei within the activated cytoplasm of sea urchin eggs. The migrations of the pronuclei are not affected by DTT, indicating that microtubule function is not impaired. Centrosomal antigens are detected in the sperm aster and in all subsequent microtubule-based configurations. Nuclear membranes never fuse and the chromatin of male and female pronuclei never mix in the DTT-treated cells. During prophase, when nuclear envelopes break down to undergo mitosis, both sets of chromosomes undergo condensation cycles independent from each other. Both pronuclei initially stain for centrosomal material and surrounding microtubules. With time, the female's centrosomal material as well as the microtubules disappear while the male forms a bipolar spindle. Interestingly, one pole of the paternal mitotic apparatus communicates with the separate maternal chromatin, forming a half spindle which moves the egg-derived chromatin towards its pole. At the time for cell division, the individual karyomeres are not able to fuse their nuclear membranes to reconstitute the blastomere nuclei. When DTT is applied at prometaphase of the first cell cycle, the chromosome cycle continues until next metaphase. Centrosomes also continue their cycle and undergo somewhat atypical splitting during the time for second telophase. Division furrows are initiated but aborted. These results support the hypothesis that disulfide groups are required for membrane fusion of the pronuclei, for membrane fusion of the karyomeres, and for the completion of the division furrow to achieve successful cell division.

Cleavage Asynchrony in the Tubifex Embryo: Involvement of Cytoplasmic and Nucleus-Associated Factors

The smaller AB-cell of the 2-cell embryo of Tubifex divides 40 min later than the larger CD-cell. The present study was undertaken to elucidate those factors that are responsible for this cleavage asynchrony. An examination of chromosome cycles showed that the AB-cell entered M-phase 5 min later and spent about 35 min longer in the prometaphase portion of M-phase than did the CD-cell. Even if cultured in isolation, the length of M-phase of AB isolates continued to be comparable to that of intact AB-cells. When AB and CD nuclei were brought into a common cytoplasm, they entered M-phase simultaneously. The durations of M-phase portions from prometaphase through telophase were not altered even in a common cytoplasm, as long as the mitotic figures of AB and CD nuclei were separated from each other. Only when the mitotic spindles of both nuclei were united at at least one pole was the length of prometaphase of the AB nucleus significantly shortened. Whole-mount immunocytochemistry of microtubules revealed that asters were involved in mitotic spindle assembly in the CD-cell but not in the AB-cell. The fully formed spindle of the AB-cell was anastral and barrel-shaped, while that of the CD-cell possessed asters at its poles. The morphology of the spindle was not altered by changes in the cytoplasmic environments; that is, the spindle of AB nucleus was found to be anastral even in the presence of a CD-cell cytoplasm. These results suggest that unlike the case of other animals so far studied, cleavage asynchrony in the Tubifex embryo is brought about not only by cytoplasmic components but also by nucleus-associated factors.

Timing the phases of the microtubule cycles involved in cytoplasmic and nuclear divisions in cells of undisturbed onion root meristems

The duration of the different phases of the microtubule and chromosome cycles were estimated in the native diploid cell populations of Allium cepa L root meristems proliferating undisturbed, under steady state conditions, at the physiological temperature of 15°C. The cycles were coupled by considering their fitting in relation to the short process of nuclear envelope breakdown. In the cycle related to cytoplasmic division, the preprophase band which predicts the future position of the phragmoplast made its appearance, as a wide band, 16 mm before the G 2 to prophase transition, ie it was only present during the final 5% of the total G 2 timing (5 h 30 mm). The band became narrow only 6 mm after prophase had started and it was present in this form for the remaining prophase time (2 h 24 mm). Its disappearance occurred strictly coinciding with nuclear envelope breakdown, at the end of prophase. No microtubules related to cytoplasmic division were apparent until 9 mm after telophase had initiated. The two initial stages of phragmoplast formation which followed occupied, respectively, 27 mm and 54.5 mm of the 2-h long telophase. On the other hand, the third and last stage in phragmoplast formation covered both the final 35 mm of mitosis and the 6 initial mm of the G 1 of the next interphase. A very short (less than 4 mm) stage of microtubular nucleation around the nuclear envelope took place immediately afterwards, before the cortical array of microtubules appeared. The microtubule cycle related to nuclear division started with the apparent activation of the future spindle poles 7.4 mm before prophase was over. The mitotic spindle developed in the 5.6 mm long prometaphase. The spindle functioned in metaphase for the 42 mm it lasted, half spindles being separated for the 37 mm anaphase occupied in these cells.

Development of Microtubule-Dependence of the Chromosome Cycle at the Midblastula Transition in Xenopus laevis Embryos: (Xenopus/cell cycle/chromosomes/microtubutes/midblastula transition).

Animal-cap cells isolated from Xenopus laevis morulae and blastulae are cultured for 2 to 6 hr in medium containing nocodazole, Colcemid or taxol, at concentrations completely inhibiting cell division. At 20°C, cells from each control embryo undergo synchronous cell cycles up to the 12th, with a period of 32 min, of which 60% represents the chromosome condensation (mitotic or M-) phase, and the average mitotic index remains near 50%. Cells treated with nocodazole, Colcemid or taxol before 12th cleavage undergo chromosome cycles with a similar period as controls, albeit without chromosome segregation, and the average mitotic index remains near 50%. From the 12th to 15th cycles, control cycles become asynchronous, their period gradually increases 2 to 3 times, and the mitotic index declines to 10%. In cells treated after 12th cleavage with taxol, the mitotic index declines, similarly to control cells. However, in nocodazole-treated cells, it increases steadily, and exceeds 70% at 2 hr of treatment, but gradually declines to 40% at 6 hr. Therefore, while inhibition of microtubule activities does not significantly alter the timing of chromosome condensation cycles during synchronous cleavage, inhibition of microtubule assembly can prolong M-phase during asynchronous cleavage after the midblastula transition.

Cloning and expression of Drosophila HP1 homologs from a mealybug, Planococcus citri.

The mealybug chromosome cycle is one of the most dramatic examples of genomic imprinting known. In embryos that are to become male the entire paternal chromosome set becomes heterochromatic and inactive at the blastoderm stage, while the maternal set remains active and euchromatic. HP1 is a protein from Drosophila melanogaster, which binds preferentially to heterochromatin on polytene chromosomes and is likely to be a modifier of position effect variegation. This paper describes the isolation and sequencing of two cDNA clones encoding HP1 homologs from the mealybug, Planococcus citri. The protein product of the cDNA clone that was closer to HP1 in sequence was expressed as a fusion protein in Escherichia coli, and polyclonal rat antibodies were raised against it. Immunohistochemistry to mealybug squash preparations showed that this protein was a male-specific nuclear protein, but that it was not specifically associated with the heterochromatic set of chromosomes.

Cell kinetic disturbances induced by treatment of human diploid fibroblasts with 5-azacytidine indicate a major role for DNA methylation in the regulation of the chromosome cycle

BrdU-Hoechst flow cytometry was used to investigate the effects of DNA hypomethylation, induced by treatment with 5-azacytidine (5AC), on cell proliferation. When human fibroblast-like cells derived from skin and amniotic fluid were exposed to 5AC during three successive cell cycles, their clone-forming ability was diminished after removal of the drug. Treated cells were rendered quiescent by culture with low serum in the absence of the drug. Upon serum stimulation, they showed a diminished fraction of proliferating cells, which exhibited a prolonged transit through the S and G2 phase of the cell cycle, and a permanent arrest within the G2 compartment. This pattern of disturbed cell proliferation may in part explain the changes in replication banding pattern reported in the literature. Cytogenetic analysis of 5AC-treated cells revealed numerous endomitoses and tetraploid metaphases indicating a disturbed chromosome cycle in association with these cell kinetic perturbations.

Mitotic chromosome transmission fidelity mutants in Saccharomyces cerevisiae.

We have isolated 136 independent mutations in haploid yeast strains that exhibit decreased chromosome transmission fidelity in mitosis. Eighty-five percent of the mutations are recessive and 15% are partially dominant. Complementation analysis between MATa and MAT alpha isolates identifies 11 chromosome transmission fidelity (CTF) complementation groups, the largest of which is identical to CHL1. For 49 independent mutations, no corresponding allele has been recovered in the opposite mating type. The initial screen monitored the stability of a centromere-linked color marker on a nonessential yeast chromosome fragment; the mitotic inheritance of natural yeast chromosome III is also affected by the ctf mutations. Of the 136 isolates identified, seven were inviable at 37 degrees and five were inviable at 11 degrees. In all cases tested, these temperature conditional lethalities cosegregated with the chromosome instability phenotype. Five additional complementation groups (ctf12 through ctf16) have been defined by complementation analysis of the mutations causing inviability at 37 degrees. Twenty-three of the 136 isolates exhibited growth defects at concentrations of benomyl permissive for the parent strain, and nine appeared to be tolerant of inhibitory levels of benomyl. All of the mutant strains showed normal sensitivity to ultraviolet and gamma-irradiation. Further characterization of these mutant strains will describe the functions of gene products crucial to the successful execution of processes required for aspects of the chromosome cycle that are important for chromosome transmission fidelity in mitosis.

A model of chromatin-dependent DNA replication sequences based on the decondensation units hypothesis

A model of chromatin-dependent DNA replication sequences was developed on the previously reported "decondensation units" hypothesis and its kinetic properties were examined by way of calculating various numerical indices using a Monte Carlo procedure. The model has much in common with the previous one but a fundamental difference is that the unit is assumed to consist of linearly arranged H-, D-, A- and S-zones each containing genes of different functional categories which are called H-, D-, A- and S-genes, respectively. The units are decondensed by the action of D-factors, i.e. decondensation factors, from H-zone to the end of S-zone and the genes in decondensed regions release signals to produce housekeeping enzymes, D-factors, A-factors and S-factors. These products are stored and at the same time degraded. A-factors activate replication origins in the decondensed regions and S-factors induce DNA synthesis at the activated origins. Replicated DNA is recondensed and gene activities are shut down in the recondensed chromatin. The factors are produced under the control of chromosome cycle and in turn affect chromosomes. Thus, dual control mechanism operates as Mazia and Prescott have argued. Biochemical and cytogenetic basis of this model was reviewed briefly and some results of simulation presented which include DNA synthesis rate vs. DNA content relationships. An outstanding characteristic of the model is the constancy of cellular state in A-subphase located in the late G1.

G-banding of germ line limited chromosomes in Acricotopus lucidus (Diptera, Chironomidae)

The germ line limited (K) chromosomes of Acricotopus lucidus (Diptera, Chironomidae) were stained for G-banding on gonial mitoses, along with the somatic (S) chromosomes. Nine different types of K chromosomes could be distinguished by the G-banding pattern and other cytological criteria. Various combinations of K chromosomes were found in the complements of different individuals and cells: some Ks were missing and others were present up to as many as five times. No two animals were completely alike in the composition of their gonial chromosome complement. Thus none of the different K types can be essential. These results are discussed in view of the complex chromosome cycle of the Orthocladiinae.