Showing papers on "Heterochromatin published in 1985"

C band heterochromatin and dna content in zea mays

Abstract: The number of mitotic chromosomal C-bands, the percent ofthe genome comprised of C-band heterochromatin, and genome size (4C DNA content) were determined for 22 North American inbred and open-pollinated lines of Zea mays. The number of C-bands ranged from 0 in Tama Knobless Flint to 18 in Zapolate Grande. The percent C-band heterochromatin ranged from 0% in Tama Knobless Flint to 16.9% in Tx601. Genome size varied over 23%: Gaspe Flint had the lowest DNA content (9.82 pg), and Zapolate Grande had thehighest (12*12 pg). Genome size and the amount of heterochromatin were significantly correlated. The corn lines were assigned to five maturity zones encompassing a south-to-north range from Mexico to Canada. Significant negative correlations were detected between the amount of C-band heterochromatin and maturity zones, and between DNA content and maturity zones among the lines. It is speculated that the simultaneous selection by man for earlier maturation and plant size may be related to the lower DNA content of corn varieties adapted to higher latitudes. Such selection for larger plants may have been achieved through selection for more cells, which could result from the shorter mitotic cycle time that correlates with reduced DNA amount.

Organization of a repetitive human 1.8 kb KpnI sequence localized in the heterochromatin of chromosome 15.

Abstract: We have isolated a repetitive 1.8 kb Kpnl DNA sequence which is amplified in the homogeneously staining regions of a human melanoma cell line. Under low stringency conditions this sequence (D15Z1) hybridized in situ to the centromeric heterochromatin of chromosomes 1, 9, 15p, 16, and distal Yq as well as to the the short arms of the other acrocentric chromosomes. Under conditions of high stringency, labelling was predominantly on the short arm of chromosome 15. D15Z1 was shown to be present at approximately 3,000 copies per haploid genome and organized in long tandem arrays showing restriction site heterogeneity. Sequences homologous to D15Z1 were highly enriched in the less dense shoulder region of a Ag+—Cs2SO4 gradient. Analysis of D15Z1 indicated that this sequence is composed of tandemly arranged imperfect repeats of the consensus 5′ AATGG 3′ similar to previously identified satellite III sequences. Digestion of D15Z1 with HinfI resulted in a series of restriction fragments making up a subset of the HinfI ladder components of satellites III and IV. These data suggest that D15Z1 represents a chromosome 15 specific domain of human satellites III or IV and that it makes up the major fraction of the heterochromatin of this chromosome. Possible relationships between this sequence and the cytochemical staining properties of human chromosomes with distamycin A/DAPI, D280/170, and antiserum to 5-methylcytosine are discussed.

Equilocality of heterochromatin distribution and heterochromatin heterogeneity in acridid grasshoppers

Abstract: Comparative fluorescence studies on the chromosome of ten species of acridid grasshoppers, with varying amounts and locations of C-band positive heterochromatin, indicate that the only regions to fluoresce differentially are those that C-band Within a given species there is a marked tendency for groups of chromosomes to accumulate heterochromatin with similar fluorescence behaviour at similar sites This applies to all three major categories of heterochromatin — centric, interstitial and telomeric Different sites within the same complement, however, tend to have different fluorescence properties In particular, centric C-bands within a given species are regularly distinguishable in their behaviour from telomeric C-bands Different species, on the other hand, may show distinct forms of differential fluorescence at equilocal sites These varying patterns of heterochromatin heterogeneity, both within and between species, indicate that whatever determines the differential response to fluorochromes has tended to operate both on an equilocal basis and in a concerted fashion This is reinforced by the fact that structural rearrangements that lead to the relocation of centric C-bands, either within or between species, may also be accompanied by a change in fluorescence behaviour

On biological functions mapping to the heterochromatin of Drosophila melanogaster.

Abstract: We examined the behavior of an autosomal recessive maternal-effect mutation, abnormal-oocyte (abo), that is located in the euchromatin of the left arm of chromosome 2. When homozygous in females, abo results in a marked reduction in the probability that an egg produced by a mutant mother will develop into an adult. However, this probability is increased if the fertilizing sperm delivers to the egg either a normal allele of the maternal-effect gene or a specific type of heterochromatin (called ABO) that is located in small regions of the X and Y chromosome constitutive heterochromatin as well as in some autosomal heterochromatin. These regions, moreover, all react to Hoechst 33258 fluorescent dye identically and specifically. The amelioration of the maternal effect produced by this heterochromatin differs temporally from that caused by the normal allele of the euchromatic gene: the heterochromatin reduces only precellular blastoderm mortality, whereas the normal allele of the euchromatic gene reduces only postblastoderm mortality. Thus, although the genome of the preblastoderm Drosophila embryo is apparently mostly silent, the ABO-containing heterochromatin functions at this early time. Finally, preliminary data indicate that abo is but one member of a cluster of linked genes, each of which interacts with its own normal allele and with a different, locus-specific, heterochromatic factor. From these observations, it appears that Drosophila heterochromatin contains developmentally important genetic elements, and that a functional concomitant of heterochromatic location is gene action at a developmental stage during which the activity of the euchromatic genome is as yet undetectable. Some general implications of these inferences are considered.

Genome evolution in pocket gophers (genus Thomomys). III. Fluorochrome-revealed heterochromatin heterogeneity.

Abstract: Heterochromatin is a dominant component of the genome in the bottae group of the pocket gopher genus Thomomys, having had a major role in the karyotypic evolution of member species. Heterochromatin characteristics of two subspecies of T. bottae and one of T. umbrinus were examined with fluorochrome dyes identifying presumptive GC- and AT-rich regions. In two karyotypic forms of T. b. fulvus and in T. umbrinus, chromatin that fluoresces brightly with chromomycin A3 is also C-band positive, although not all heterochromatin fluoresces. However, in T. b. bottae, only euchromatic regions fluoresce brightly with chromomycin. Fluorescence patterns produced with DAPI are the reverse of the chromomycin banding in all karyotypic forms. Heterochromatin in these taxa is thus highly differentiated, exhibiting heterogeneity in staining characteristics, and presumably in underlying DNA sequences, both across the genome within a given chromosomal complement as well as among the different karyotypic races and species of the bottae group of pocket gophers.

DNase I sensitivity in facultative and constitutive heterochromatin.

Abstract: In situ nick translation allows the detection of DNase I sensitive and insensitive regions in fixed mammalian mitotic chromosomes. We have determined the difference in DNase I sensitivity between the active and inactive X chromosomes in Microtus agrestis (rodent) cells, along both their euchromatic and constitutive heterochromatic regions. In addition, we analysed the DNase I sensitivity of the constitutive heterochromatic regions in mouse chromosomes. In Microtus agrestis female cells the active X chromosome is sensitive to DNase I along its euchromatic region while the inactive X chromosome is insensitive except for an early replicating region at its distal end. The late replicating constitutive heterochromatic regions, however, in both the active and inactive X chromosome are sensitive to DNase I. In mouse cells on the other hand, the constitutive heterochromatin is insensitive to DNase I both in mitotic chromosomes and interphase nuclei.

In situ nick translation of metaphase chromosomes with biotin-labeled d-UTP

Abstract: Limited nick translation experiments on fixed chromosomes were performed. Sites of preferential DNase-I nicking were made visible by the incorporation of biotin-labeled dUTP and subsequent binding of the streptavidin-peroxidase complex. This procedure leads to a banding pattern on the chromosomes which is strongly DNase-I concentration dependent. Along the chromosome arms, regions of enhanced DNase-I sensitivity alternate with regions of lower DNase-I sensitivity. No complete G- or R-type banding pattern was observed. The easily identifiable human Y chromosome was studied more intensively. Compiled data show the heterochromatin of the Y chromosome stained as heavily as the euchromatin. The boundary between the eu- and heterochromatin on the long arm appears to be a site of preferential DNase-I sensitivity.

Distribution and clustering of two highly repeated sequences in the A and B chromosomes of maize.

Abstract: Clones from a family of highly repeated sequences present in a heterochromatin rich maize line have been characterized by sequencing and chromosome location. The repeats differ from each other in length and degree of sequence homology, and show areas rich in purine and pyrimidine. In “situ” hybridization experiments indicate that the repeats are mainly located in the knob heterochromatin of the A chromosomes and the centromeric heterochromatin of the B chromosome. However, in addition to previously published data, some copies are also distributed in euchromatic regions of the A chromosomes and in the distal heterochromatic block of the B chromosome. The results are discussed in relation to the centromeric activity of maize heterochromatin.

Functional properties of the heterochromatic sequences inducing w m4 position-effect variegation in Drosophila melanogaster

Abstract: In position-effect variegation euchromatic genes are brought into the vicinity of heterochromatic sequences as a result of chromosomal rearrangements. This results in the inactivation of these genes in a proportion of cells causing a variegated phenotype. Tartof et al. (1984) have shown that the flanking heterochromatin in the w m4 variegating rearrangement in Drosophila melanogaster is homologous to the Type I inserts found in some portions of the rDNA repeats. We have studied the functional properties of these sequences using 51 revertant chromosomes, several variant lines of w m4 , strong enhancer mutations of position-effect variegation and X heterochromatin deletions. Our results suggest an array of tandemly repeated sequences showing additive effects and probably subject to magnification and reduction in number. Since only 3 of the 51 revertants isolated do not show variegation if strong enhancer mutations are introduced, only a very short sequence must be essential for the induction of white gene inactivation in w m4 . This suggests that the heterochromatic junction itself is sufficient to initiate the variegation of an adjacent gene. Parental source as well as paternal effects on the activity of these sequences have been detected. Revertant chromosomes of w m4 can be found after P-directed mutagenesis in hybrid dysgenic crosses suggesting mobile genetic elements at the breakpoints of inversion w m4 . These results are discussed with respect to the structural basis of positioneffect variegation as well as the function of certain heterochromatic sequences.

The fine structure of euchromatin and centromeric heterochromatin in Tenebrio molitor chromosomes.

Abstract: The fine structure of constitutive heterochromatin and euchromatin was compared in electron microscope whole-mount preparations of Tenebrio molitor (Insecta, Coleoptera) spermatocyte nuclei. Tenebrio molitor pachytene chromosomes display extended segments of centromeric heterochromatin and thus are especially suitable for this purpose. When nuclei were incubated in solutions containing different concentrations of NaCl or of MgCl2, two levels of chromatin fine structures were observed in the euchromatic segments: nucleosome fibers (0.1 mM–20 mM NaCl) and supranucleosomal fibers with 28 nm in diameter (40 mM–100 mM NaCl, 0.2 mM–1.0 mM MgCl2). The fine structure in the heterochromatic segments was the same as that in the euchromatic segments in all NaCl concentrations and in MgCl2 concentrations up to 0.4 mM. In higher MgCl2 concentrations the heterochromatin remained more compact than the euchromatin and consisted of 37-nm-thick fibers in 0.6 mM MgCl2 and of 65-nm-thick fibers in 1.0 mM MgCl2. After the 37-nm and the 65-nm fibers had been dispersed in Mg2+-free solutions they could be recondensed by incubation in 0.6 mM and 1.0 mM MgCl2, respectively. It is concluded that a Mg2+-sensitive component of the heterochromatin is responsible for the folding of the nucleosome chain to heterochromatin-specific supranucleosomal structures.

Further Characterization of Genetic Elements Associated with the Segregation Distorter Phenomenon in DROSOPHILA MELANOGASTER.

Abstract: Segregation Distorter, SD, associated with the second chromosome of Drosophila melanogaster, is known to cause sperm bearing the non-SD homologue to dysfunction in heterozygous males. In earlier studies, using different, independently derived, SD chromosomes, three major loci were identified as contributing to the distortion of segregation ratios in males. In this study the genetic components of the SD-5 chromosome have been the subjects of further investigation, and our findings offer the following information. Crossover analysis confirms the mapping of the Sd locus to a position distal to but closely linked with the genetic marker pr. Spontaneous and radiation-induced recombinational analyses and deficiency studies provide firm support to the notion that the Rsp (Responder) locus lies within the proximal heterochromatin of chromosome 2, between the genetic markers lt and rl and most likely in the heterochromatin of the right arm. The major focus of this study, however, has been on providing a better definition of the genetic properties of the Enhancer of SD [E(SD)]. Our findings place this locus within the region of the two most proximal essential genes in the heterochromatin of the left arm of chromosome 2. Moreover, our analysis reveals a probable association of the E(SD) locus with a meiotic drive independent of that caused by Sd.

Genomic differentiation and its effect on gene flow

Abstract: Genomic differentiation occurs by structural rearrangements of chromosomes, by addition or deletion of DNA segments, and by mutations that affect the behavior of chromosomes or entire genomes. Depending on their size and other factors, structural changes can cause varying degrees of sterility in hybrids without regard to taxonomic level. Mitotic instability due to different kinds of mutations, structural rearrangements, and asynchronous behavior of heterochromatin can lead to hybrid weakness and segregational sterility in meiosis. Many structural changes of chro- mosomes are undoubtedly due to transposable elements. Hologenomic mutations, affecting entire genomes, may exert their effect in somatic tissue by causing certain kinds of mitotic instability such as centromere inactivation. However, some of these mutations may be expressed only in meiosis where they prevent normal chromosome pairing and/or failure of chiasma formation, so sterility in hybrids is caused by unbalanced gametes resulting from random disjunction of univalents. In analyzing modes and mechanisms of plant evolution and speciation, we are concerned primarily with the products of mutation, re- combination, selection, drift, and migration. More precisely, we are trying to deduce the mechanisms leading to plant speciation long after the process occurred. This has the inher- ent danger of including post-speciational

Cytogenetics of Rutaceae. III. Heterochromatin Patterns

Abstract: The patterns of heterochromatin distribution was analysed in 22 species pertaining to 15 genera of Rutaceae. The chromosomes were stained by diferent techniques in an attempt to distinguish the het...

The heterochromatin of grasshoppers from the Caledia captiva species complex: II. Cytological organisation of tandemly repeated DNA sequences

Abstract: C-band variation between the Caledia taxa is extensive with numerous large interstitial and telomeric blocks of heterochromatin being present in the South-east Australian and Moreton taxa while the Torresian types possess small centromeric or telomeric C-bands. In situ hybridization using 3H-cRNA from a 168 bp (base pairs) highly repeated sequence, originally isolated from the South-east Australian taxon, defined further variation between the C. captiva taxa. This sequence family is present in each of the interstitial and telomeric constitutive heterochromatic blocks in the South-east Australian and Moreton taxa. However, it is represented in only a fraction of the heterochromatic regions, defined by C-banding, within the three Torresian types. A second, unrelated 144 bp sequence family, originally isolated from the Daintree taxon, is restricted to the procentric blocks of heterochromatin of chromosomes 2–7, 9 and 10 in the Daintree taxon. This sequence is A-T rich and possesses a region of dyad symmetry. Quantitative measurements for the two sequence families revealed a wide range of copy numbers between the C. captiva taxa. The 168 bp family has approximately 150,000, 35,000 and 4,000 copies, respectively, in the South-east Australian/ Moreton, Torresian and Daintree genomes. There are 2,000,000 and 100,000 copies of the 144 bp sequence in the Daintree and Papuan Torresian taxa, respectively. The distributional, quantitative and sequence characteristics of these repeat families imply that past amplification or introgression has played a major role in the evolution of these sequences. There is an overall negative correlation between the quantity of the 168 bp sequence and the levels of reproductive isolation and genie divergence between the various taxa. It is possible that some of the reduction in the viability of the hybrid individuals is due to the quantitative changes in these sequences. Moreover, the quantitative and qualitative characteristics of highly repeated DNA families may play a role in the modulation of such essential cellular functions as cell cycle duration, nuclear organization and gene expression.

Extra euchromatic band in the qh region of chromosome 9.

Abstract: two centromeres indicated by two distinct C bands but only one primary constriction at the proximal C band. The two C bands were separated by chromosomal material staining pale in G banding and intensely dark in R banding (fig 1). Both NORs could be observed in satellite associations (fig 2). The chromosome was therefore defined as pseudodicentric chromosome 21 (pseudic 21). The same chromosome was found in the proband's father and paternal grandmother. Acrocentric chromosomes with a short arm morphology similar to that presented here have been reported by Balicek and Zizka.' These authors paid no attention to the activity of the centromeres. The suppression of additional centromeres is indicated by the presence of only one primary constriction as shown by Ing and Smith2 in a dicentric (Y;13) transtocation. Variants of acrocentric chromosomes are often observed in patients with congenital anomalies.' 3 The occurrence of the pseudodicentric chromosome 21 in the proband and her phenotypically normal father and grandmother indicates that there is no association between the chromosomal variant and the proband's congenital anomalies. I HANCKE AND K MILLER Department of Human Genetics, Medizinische Hochschule Hannover, Hannover, Federal Republic of Germany.

Chromosome banding in Podisma pedestris

Abstract: C-banding of P. pedestris chromosomes demonstrated the presence of small procentromeric heterochromatic blocks on all members of the complement. The telocentric X-chromosomes and all of the medium sized autosomes also had small distally located C-bands and the megarmeric M7 chromosome consistently had a number of interstitial bands. The metacentric neo X chromosome found in some populations has a distinctive C-banding pattern in that although the distal C-block of the free X is retained, the centromeric heterochromatin is quite asymetrical with most of the procentric C-band of the X apparently having been lost during or subsequent to the X-autosome fusion event. Two banding polymorphisms were noted, an extra C-band on the S10 chromosome and variability in size of the distal C-band on the M6. B-chromosomes did not appear to have any C-band material other than a small procentric block. G-banding gave no additional information as the chromosomes appeared light staining only in those regions which were C-positive. Use of the fluorochromes Hoechst 33258 and Chromomycin A3 showed that the heterochomatin of P. pedestris is relatively rich in G + C bases.

Heterochromatin diversity in two species of Pellia (Hepaticae) as revealed by C-, Q-, N- and Hoechst 33258-banding

Abstract: Giemsa C-banding of two liverwort species, Pellia epiphylla (L.) Corda (n=9) and P. neesiana (Gott.) Limpr. (n = X/Y + 8), identified each chromosome as unique. Almost the full complement of C-bands was defined by N-banding. Apart from minor variation in the prominence of certain bands, which were designated according to principles adopted in human cytology, the chief exceptions were the absence of bands 1p2, 1p4, 1q2, 1q4 and 7p6 from the N-banded karyotype of P. epiphylla and of band 7q25 from P. neesiana. Both quinacrine dihydrochloride and Hoechst 33258 showed reduced fluorescence in the constitutive heterochromatin of P. epiphylla. The effects of these two fluorochromes on P. neesiana, however, differed from each other and from their effect on P. epiphylla. Only one Q-band, the brightly fluorescing 1(X/Y)q24, occurred, whereas Hoechst 33258 resulted in considerable differentiation. The majority of C-bands were correlated with bright fluorescence with Hoechst 33258 but a few were dim. On the basis of these results, four major types of heterochromatin were identified in P. neesiana and two further types in P. epiphylla. They are discussed in the context of previously reported early replication of P. neesiana heterochromatin and point to considerable cytological evolution within these two species.

Sequence of centromere separation: influence of pericentromeric heterochromatin (repetitive DNA) inMus

Abstract: A relationship between the sequence of centromere separation and quantity of pericentromeric constitutive heterochromatin was studied using bone marrow cells ofMus musculus molossinus and three cell lines, viz, SEWA-Rec 4, brain tumor and L-cells, ofM m domesticus origin The timing of separation of a centromere into two daughter centromeres is related to the quantity of pericentromeric heterochromatin In these genomes, having qualitatively uniform DNA in their heterochromatin fraction, the chromosomes with none or small quantities of heterochromatin separate first These are followed by those chromosomes which have increasingly larger quantities of heterochromatin It appears that one function of repetitive DNA (pericentromeric heterochromatin) is to regulate the timing of separation of centromeres

C-Banding in Maize. I. Band Patterns

Abstract: SUMMARYThe C-banded karyotypes of several maize stocks, including one with knobs in almost all positions known in maize chromosomes, are presented together with the evidence that large C-bands or knobs alter the relative arm length of the chromosomes involved. All the aspects of the differential Giemsa staining are discussed, including the centromeric heterochromatin, Nucleolar Organizing Region (NOR) heterochromatin, the satellite of chromosome 6, as well as the heterochromatic segments of the B-chromosomes observed both in mitotic metaphase and at the pachytene stage of meiosis.

Content of histone H1 and histone phosphorylation in relation to the higher order structures of chromatin in Drosophila.

Abstract: The amount of histone H1 relative to core histones has been determined in three Drosophila species (D. melanogaster, D. texana and D. virilis) in chromatin from several tissues differing in chromatin structure and genetic activity. Low levels of H1 were found in relatively undifferentiated, early embryos as well as in a line of cultured cells. In late embryos the content of H1 was highest in D. virilis which possesses larger amounts of and a partially more compacted constitutive heterochromatin than the two other species. Polytene chromatin from larval salivary glands showed increased levels of H1 compared with diploid chromatin and the degree of phosphorylation of this histone was relatively low. The degree of phosphorylation of H2A was found to be drastically reduced in polytene as compared with diploid embryonic chromatin, which parallels the extensive underreplication of constitutive heterochromatin. Also, in diploid chromatin a qualitative correlation was observed between the relative amounts of heterochromatin and the levels of H2A phosphorylation. These findings suggest a connection between H2A phosphorylation and heavy compaction of interphase chromatin.

The distribution of quinacrine on chromosomes as determined by X-ray microanalysis. II. Comparison of heterochromatic and euchromatic regions of mouse chromosomes.

Abstract: The distribution of quinacrine and protein sulphur has been compared with that of DNA in euchromatic and heterochromatic regions of mouse chromosomes stained with the fluorescent dye quinacrine, using X-ray microanalysis. Heterochromatin tends to bind relatively more quinacrine than euchromatin, and contains a greater concentration of sulphur. Measurements of quinacrine fluorescence, when compared with quinacrine binding, show that the excitation of fluorescence is more efficient when the dye is bound to euchromatin than when it is bound to heterochromatin. Although this observation is consistent with the hypothesis that the dull quinacrine fluorescence of mouse centromeres is due to quenching by guanine residues, two other factors should also be considered: the lower absolute amount of dye bound to the centromeres, and a concentration-dependent quenching of fluorescence.

Metaphase karyotype identity in four homosequential Drosophila species from Hawaii

Abstract: Four recently evolved species of Hawaiian Drosophila (silvestris, heteroneura, dijferens, and planitibia) have previously been shown to be homosequential in all five polytene chromosome arms. This suggests that the changes involved in speciation are at the genic level and hence are not evident in the polytene banding sequences. Because this does not rule out the occurrence of heterochromatic differences between these homosequential species, the present study was carried out to examine this possibility. These species are now shown to have identical heterochromatin distributions in mitotic metaphase chromosomes. This proves that neither gross chromosomal rearrangements nor novel heterochromatic blocks have been involved in the divergence of these four species. A fifth, and evolutionarily more distant, species (hemipeza) belonging to the same subgroup has a significantly different heterochromatin distribution from the other four species.Key words: heterochromatin, metaphase karyotypes, Hawaiian Drosophila.

C-banding analysis of gamma-radiation-induced chromosomal interchanges in rye: the role of C-heterochromatin in chromosome repatterning

Abstract: Gamma-radiation-induced chromosomal interchanges in two inbred lines of rye were analyzed by using the C-banding technique. Neither the doses of radiation nor the time elapsed between radiation and cytological observation influenced the contribution of individual chromosomes to the observed interchanges. Dicentric are more frequent than monocentric interchanges. Neither the individual chromosome contribution to interchanges nor the chromosome combinations fit a random distribution. Chromosomes 6(5R), 4(4R) and 7(1R) of the inbred line Pool and 6(5R), 5(6R) and 4(4R) of line Riodeva, which are carriers of large amounts of heterochromatin, are much more frequently involved in interchanges than expected. Likewise, they are involved in the most frequent combinations observed (6-4, 6-7, 4-5, 4-7 and 6-3 (2R) in line Pool and 6-5, 6-4, 4-5 and 6-7 in line Riodeva). The role of C-heterochromatin in radiation-induced chromosomal interchanges is discussed.

Karyotypic architecture of the false vampire bat Megaderma lyra

Abstract: Chromosomal architecture of Megaderma lyra, a microchiropteran bat collected from South India, was analysed for the first time using G- and C-banding techniques. The diploid number, FN value and categorisation of autosomes were in confirmity with the previous report which utilised a North Indian population for analysis. However, variations with regard to the nature of the X and Y chromosomes were encountered in these two populations. The important feature noticed in the G-banded karyptype is the G-positive nature of thesatellite of the 8th pair of hetero-morphic autosomes and centromeric region of 5th, 6th, 7th and 18th autosomal pairs. The C-banded chromosomes revealed extensive occurrence of C-positive blocks of heterochromatin. The amount of heterochromatin (44.76%) measured for M. lyra is the highest for any bat studied till today. Present observations implicate pericentric inversion and addition or deletion of heterochromatin as causative agents for inter-population sex chromosome variations. It is also proposed that the high amount of consitiutive heterochromatin possibly had a role in the phylogenetic differentiation of this species.

Segregation of centric Y-autosome translocations in Drosophila melanogaster. II. Segregation determinants in females.

Abstract: This is a study of the chromosomal segregation patterns in females of 15 Experimental stocks of Drosophila melanogaster, each carrying one element of a T (Y; 2) with a centric break-point. In each Experimental stock the relative frequency of all eight possible meiotic configurations of four relevant chromosomal elements was followed: an attached-X chromosome, a multiply-inverted chromosome 2, a free arm of chromosome 2, and a half-translocation element. Although the 15 translocation elements were broken at different sites, there were no basic differences among the Experimental stocks in their segregation patterns. The three two-by-two configurations were the most common. Comparison of this pattern with that of the segregation pattern of stocks similar but for an inversion-free chromosome 2, showed that in the Experimental stocks exchange pairing did not play a significant role in the determination of the segregation pattern.The results of these experiments, together with the analysis of results from other published studies provide evidence against the model that had been forwarded by Grell. According to this model, chromosomes that did not participate in exchange pairing undergo another pairing cycle, in which total chromosome length is a factor in the determination of segregation.We support a modified version of Novitski's model of premetaphase chromocenter-like chromosome aggregation. Disjunction of non-exchange chromosomes is regulated by determinants located in the proximal heterochromatin of the sex chromosomes and the autosomes. However, the specificity, especially that of the autosomal determinants, is not high. Thus, if an autosome and a sex chromosome are available, their determinants may interact-to-disjoin by default. More frequently, the determinants of the left-arm autosomal element may interact-to-disjoin with those of the right-arm chromosomal element.

Evolution of the Karyotype in Cells of Mouse Ascites Tumour (ATP C +) 25 Years After Its Origin

Abstract: SUMMARYThe authors examined the karyotype of mouse ascites tumour (ATP C +) to study the changes which had occurred over 25 years. From the chromosome analyses carried out on 600 metaphase cells it resulted that this cell population consisted for the most part of cells with 46 chromosomes. Besides these it is possible to find hypertetraploid cells whose karyotype had chromosomes varying in number from 86 to 92. By G-banding it was possible to identify only 28 of the 40 mouse normal karyotype chromosomes and a partial trisomy 13 probably present since when they originated from a spontaneous mammary tumour. The other chromosomes revealed remarkable structural rearrangements, while C-banding shows heterochromatin outwith the centromeric region only in two chromosomes.