Showing papers on "Heterochromatin published in 1978"

Heterochromatin and highly repeated DNA sequences in rye (Secale cereale)

Abstract: Secale cereale DNA, of mean fragment length 500 bp, was fractionated by hydroxylapatite chromatography to allow recovery of a very rapidly renaturing fraction (C0t 0–0.02). This DNA fraction was shown to contain several families of highly repeated sequence DNA. Two highly repeated families were purified; (1) a fraction which renatured to a density of 1.701 g/ cc and comprised 2–4% of the total genome, and (2) polypyrimidine tract DNA which comprised 0.1% of the total genome. The 1.701 g/cc DNA consisted of short sequence repeat units (5–50 bp long) tandemly repeated in blocks 30 kb long, while a portion of the polypyrimidine tract DNA behaved as part of a much larger block of tandemly repeated sequences. The chromosomal location of these sequences was determined by the in situ hybridisation of radioactive, complementary RNA to root tip mitotic chromosomes and showed the 1.701 g/cc sequences to be largely limited to the telomeric blocks of heterochromatin, accounting for 25–50% of the DNA present in these parts of the chromosomes. The polypyrimidine tracts were distributed at interstitial locations with 20–30% of the sequences at three well defined sites. The combined distributions of the 1.701 g/cc DNA sequences and polypyrimidine tracts effectively “individualised” each rye chromosome thus providing a sensitive means of identifying these chromosomes. The B chromosomes present in Secale cereale cv. Unevita, did not show defined locations for the sequences analysed. — The data are discussed in terms of the structure of the rye genome and the generality of the observed genomic arrangement of highly repeated sequence DNA.

Genetic studies on heterochromatin in Drosophila melanogaster and their implications for the functions of satellite DNA

Abstract: In Drosophila melanogaster the centromeric heterochromatin of all chromosomes consists almost entirely of several different satellite DNA sequences. In view of this we have examined by genetic means the meiotic consequences of X chromosomes with partial deletions of their heterochromatin, and have found that the amount and position of recombination on each heterochromatically deleted X is substantially different from that of a normal X. It appears that the amount of heterochromatin is important in modifying the "centromere effect" on recombination.--In all the deleted Xs tested, chromosome segregation is not appreciably altered from that of a nondeleted control chromosome. Thus satellite DNA does not appear to be an important factor in determining the regular segregation of sex chromosomes in Drosophila. Additionally, since X chromosomes with massive satellite DNA deficiencies are able to participate in a chromocenter within salivary gland nuclei, a major role of satellite DNA in chromocenter formation in this tissue is also quite unlikely.--In order to examine the mechanisms by which the amount of satellite DNA is increased or decreased in vivo, we have measured cytologically the frequency of spontaneous sister chromatid exchanges in a ring Y chromosome which is entirely heterochromatic and consists almost exclusively of satellite DNA. In larval neuroblast cells the frequency of spontaneous SCE in this Y is approximately 0.3% per cell division. Since there is no meiotic recombination in D. melanogaster males and since meiotic recombination in the female does not occur in heterochromatin, our results provide a minimum estimate of the in vivo frequency of SCE in C-banded heterochromatin (which is predominantly simple sequence DNA), without the usual complications of substituted base analogs, incorporated radioactive label or substantial genetic content.--We emphasise that: (a) satellite DNA is not implicated in any major way in recognition processes such as meiotic homologue recognition or chromocenter formation in salivaries, (b) there is likely to be continuous variation in the amount of satellite DNA between individuals of a species; and (c) the amount of satellite DNA can have a crucial functional role in the meiotic recombination system.

Distribution of heterochromatin in a reconstructed karyotype of Vicia faba as identified by banding- and DNA-late replication patterns

Abstract: 1) The distribution pattern of heterochromatin characterized by Giemsa-banding, Quinacrine-banding and DNA-late replication has been studied in a reconstructed karyotype of Vicia faba with all chromosome pairs interdistinguishable. 2) By means of two Giemsa-banding methods both an interstitial and a centromeric Giemsa-banding pattern are described. The former one comprehends 14 “marker” and 18 “additional” bands of lower but characteristic visualization frequencies. The centromeric Giemsa-banding pattern consists of 7 bands, located in the centromeric and in the secondary constrictions of the metaphase chromosomes. Chromosomes with banding patterns intermediate between the interstitial and the centromeric Giemsa-banding have also been observed. 3) Quinacrine-banding revealed 10–12 brightly fluorescent bands and 1–2 regions of dim fluorescence. Most Q-bands occupy chromosomal positions also characterized by interstitial Giemsa bands. 4) The DNA-late replication pattern, analyzed both by autoradiography and by FPG-technique, revealed 9 late replicating chromosome regions; all of these correspond positionally to the sites of interstitial Giemsa bands. 5) The results are discussed with respect to (a) the relationships between the banding- and the DNA-late replication pattern; (b) banding and heterochromatin characteristics; (c) the correlations between the distribution of chromatid aberrations and special types of heterochromatin. — The patterns of heterochromatin distribution found are in basic conformity with the corresponding patterns reported for the standard karyotype of Vicia faba. The heterochromatin type characterized by both Giemsabanding and late replication is characteristic of all those chromosome regions which after mutagen treatments show up as aberration hot spots. Positional correlations between interstitial Giemsa marker bands and chemically induced isochromatid breaks are indicative of preferential aberration clustering in heterochromatin/euchromatin junctions.

Studies on the Ribosomal RNA Cistrons in Interspecific Drosophila Hybrids. II. Heterochromatic Regions Mediating Nucleolar Dominance.

Abstract: Interspecific hybrids of D. melanogaster and D. simulans normally exhibit a secondary constriction only at the D. melanogaster nucleolus organizer (NO). This phenomenon, termed nucleolar dominance, occurs only when the NO-bearing sex chromosomes of both species are present in conjunction. Experiments were initiated to localize regions on the sex chromosomes of D. melanogaster involved in mediating this suppression. Sex chromosome heterochromatic rearrangements and deficiencies were introduced into F(1) hybrids and their corresponding effect on simulans NO constriction formation was examined in hybrid mitotic neuroblast tissue. Sex chromosomes deficient for both the D. melanogaster NO and adjacent heterochromatin were unable to restrict the formation of a constriction at the D. simulans NO. The presence of a D. melanogaster NO, however, was not sufficient for the establishment of nucleolar dominance. Results from an array of NO-bearing X and Y chromosome rearrangements and deficiencies indicate that at least one heterochromatic region, proximal to the NO on the D. melanogaster X and distal to the NO on the D. melanogaster Y, affects the induction of this interchromosomal phenomenon.

The identification of a repeated DNA sequence involved in the karyotype polymorphism of the human Y chromosome.

Abstract: We show that individual men are polymorphic for the amount of two different repeated DNA sequences. The amount of one of these sequences is proportional to the length of the brightly fluorescent heterochromatin on the Y chromosome. There are no detectable alterations in sequence between polymorphic individuals. Female DNA contains sequences complementary to those found on the Y, but at a much reduced level.

The effect of C-heterochromatin in chiasma terminalisation in Chorthippus biguttulus L. ( Acrididae, Orthoptera )

Abstract: Using the Giemsa C-banding procedure, a polymorphism in chromosome banding pattern has been found in a spanish population of Chorthippus biguttulus. The variation in C-banding pattern shown by bivalent M 6 allowed to study the effect of C-heterochromatin on chiasma terminalisation. The results indicate that interstitial heterochromatin acts as a barrier preventing chiasmata to pass. Anaphase separation seems to be normal but could be slightly delayed. A similar role for telomeric C-heterochromatin is suggested.

Conservation and chromosomal localization of DNA satellites in balenopterid whales.

Abstract: DNA satellites were isolated from three balenopterid species, viz. the minke, sei, and fin whales. In each of them at least two DNA satellites were recognizable with buoyant densities in neutral CsCl of ρ=1.702/1.703 and ρ=1.710/1.711, respectively. cRNAs from each satellite group were used for filter and in situ hybridisations. Homo- and heterologous DNA-cRNA hybrids within each satellite group yielded virtually identical melting curve profiles showing conservation of at least a considerable part of the DNA satellite sequences. There was no evident sequence homology between the ρ=1.702/1.703 and the ρ=1.710/1.711 satellites by filter hybridisation. — The in situ hybridisation showed that in each species the ρ=1.702/1.703 satellite was located in centromeric-paracentromeric C-bands in a few pairs, whereas the ρ=1.710/1.711 satellite was located in terminal C-bands throughout the karyotypes. — The data on the whale DNA satellites indicate that the quantitative evolution of the satellite DNA sequences preceded species divergence of the balenopterids and that the satellite sequences have remained relatively unaltered since the divergence took place. The function of satellite DNA is considered to imply the introduction of both chromosomal and genic polymorphisms and thus being of great importance in speciation. Based upon these concepts a model is postulated for the function of satellite DNA. According to this model at meiotic pairing euchromatin-heterochromatin overlapping between homologous chromosomes is considered to be of a general occurrence. This overlapping is presumed to be accentuated by the size heteromorphism frequently observed between homologous heterochromatic segments (C-bands). In the region of such euchromatin-heterochromatin overlapping, crossing-over would be excluded. The overlapping is suggested to be rectified progressively in the chromosome arms, leaving unaffected crossing-over distant to the euchromatin-heterochromatin junctions. The consequence of this will be that genes in the proximity of the junctions are collectively inherited and selected, whereas genes distant to the heterochromatin will be independently assorted and selected.

A Comparison of Heat and Interchromosomal Effects on Recombination and Interference in DROSOPHILA MELANOGASTER.

Abstract: Heat and interchromosomal effects on recombination have been compared for 23 regions comprising the predominantly euchromatic portions of the five arms of the Drosophila genome Patterns of response are strikingly similar, with both modifiers causing proximal and distal increases and minimal effects in the middle of the arms Changes in interference for the same regions in the presence of the two modifiers reveal little similarity, except for the X chromosome The question of independent control of interference and recombination, as well as alternatives for their temporal sequence, is discussed Recombination response to the two modifiers in the centric heterochromatin of chromosoaime 2 is markedly different from that found in euchromatin The interchromosomal effect is absent here, whereas heat induces an increase roughly an order of magnitude greater than that found in euchromatin and totally unlike the lack of response in the proximal heterochromatin of the X chromosome It is proposed that the sequestering of DNA satellite I (thermal dissociation 9–20° lower than that of the other major satellites) in the centromeric heterochromatin of chromosome 2 (but not in X or 3) may account for the increase

Two types of constitutive heterochromatin in the chromosomes of some Fritillaria species

Abstract: A Giemsa banding technique has been used to study C-banding in mitotic chromosomes in root tips of Fritillaria graeca, F. crassifolia and F. rhodocanakis, all diploids (2n=24) belonging to the graeca group. In the first two the C-bands were of two types, diverging in respect of staining regularly and specifically within chromosomes. In one type it was weak, being intermediate between that of intensely stained ones, representing the other class, and the euchromatin. In F. graeca the pale bands were proximally localized and confined to 5 pairs, whereas in F. crassifolia they occurred only in the 4 M chromosomes, in each within the centromeric constriction as a large inclusion. The interphase nuclei of both species contained pale and heavily stained chromocentres. No pale ones occurred in such nuclei of F. rhodocanakis. The probability is discussed that the two classes of C-band represent distinct types of heterochromatin, differing both in respect of condensation throughout the whole mitotic cycle and in the repetitive DNA sequences they most likely contain. In all 3 species pairs of Giemsa-positive centromeric dots, representing the centromeres, were masked both by proximally or centromerically localized bands, irrespective of the class of heterochromatin they represented.

Selective digestion of mouse metaphase chromosomes

Abstract: Metaphase chromosomes prepared from colcemid-treated mouse L929 cells by non-ionic detergent lysis exhibit distinct heterochromatic centromere regions and associated kinetochores when viewed by whole mount electron microscopy. Deoxyribonuclease I treatment of these chromosomes results in the preferential digestion of the chromosomal arms leaving the centromeric heterochromatin and kinetochores apparently intact. Enrichment in centromere material after DNase I digestion was quantitated by examining the increase in 10,000xg pellets of the 1.691 g/cc satellite DNA relative to main band DNA. This satellite species has been localized at the centromeres of mouse chromosomes by in situ hybridization. From our analysis it was determined that DNase I digestion results in a five to six-fold increase in centromeric material. In contrast to the effect of DNase I, micrococcal nuclease was found to be less selective in its action. Digestion with this enzyme solubilized both chromosome arms and centromeres leaving only a small amount of chromatin and intact kinetochores.

A comparison of heat and interchromosomal effects on recombination and interference

Abstract: Heat and interchromosomal effects on recombination have been compared for 23 regions comprising the predominantly euchromatic portions of the five arms of the Drosophila genome. Patterns of response are strikingly similar, with both modifiers causing proximal and distal increases and minimal effects in the middle of the arms. Changes in interference for the same regions in the presence of the two modifiers reveal little similarity, except for the X chromosome. The question of independent control of interference and recombination, as well as alternatives for their temporal sequence, is discussed. Recombination response to the two modifiers in the centric heterochromatin of chromosoaime 2 is markedly different from that found in euchromatin. The interchromosomal effect is absent here, whereas heat induces an increase roughly an order of magnitude greater than that found in euchromatin and totally unlike the lack of response in the proximal heterochromatin of the X chromosome. It is proposed that the sequestering of DNA satellite I (thermal dissociation 9-20" lower than that of the other major satellites) in the centromeric heterochromatin of chromosome 2 (but not in X or 3) may account for the increase.

Variability and familial transmission of constitutive heterochromatin of human chromosomes evaluated by the method of linear measurement

Abstract: Using the method of linear measurement, the lengths of constitutive heterochromatin of chromosomes 1, 9, 16, and Y were determined in 125 unrelated individuals, and in 30 members of ten families. The method used eliminates the variations in the C-band length due to different degrees of contraction of chromosomes in different mitoses, and enables the size of heterochromatin blocks to be expressed. It was found that the distribution of C-band lengths in the group of 125 individuals was normal, i.e., Gaussian, for all four classes of chromosomes measured. On the basis of length distribution and by computing the P1, P10, P90 and P99 percentiles, the actual numerical limits could be proposed for the five-step evaluation of heterochromatin length according to the Paris Conference (1971), Supplement (1975), for chromosomes 1, 9, 16, and in a preliminary way also for Y. When applying the proposed limits to data obtained in the present study, 165 C-band variants could be identified among the 125 individuals. In ten families, C-block lengths of the chromosomes transmitted from parents to progeny could be determined in 63 cases. The mean difference in C-band length of transmitted chromosomes, as measured in parents and in children, was 0.46×10-7 m. An analysis was carried out to detect the factors upon which the magnitude of this difference depends, and to define what differences are attributable to methodological errors. The results revealed that the difference rises slightly with the increasing length of the measured C block. Three degrees, defined by concrete ranges of difference in C-block length, were proposed for expressing the probability that the compared chromosomes had been transmitted. The study further attests to the effectiveness of the method of constitutive heterochromatin measurement for paternity testing. In our set of ten families, the comparison of C-band lengths of chromosomes 1, 9, 16, and Y led to rejection of paternity in 64% of unrelated individuals; excluding the Y chromosome, the percentage decreased to 61. As many as 47% of the individuals were rejected by a difference higher than two units (i.e., transmission of the compared chromosome highly improbable).

The effects of derived B-chromosomes on meiosis in pearl millet Pennisetum typhoides

Abstract: The cytological behaviour of Standard, Deficient and Iso-Bs and their effects on the endophenotype were studied in Pennisetum typhoides. The Standard and Deficient Bs were acrocentric with the short arms consisting entirely of heterochromatin. The Deficient B was derived from the Standard B by spontaneous loss of short segment extending over two chromomeres in the euchromatic long arm. The Iso-B was derived from the Deficient by centromere misdivision, the long arm forming the Iso-B. The Iso-Bs were, therefore, less heterochromatic than either the Standard or the Deficient Bs. These three types of Bs had different effects on the mean cell A-chiasma frequency. The Deficients had a depressing effect, the Iso-Bs had an enhancing effect, while the Standard Bs had no effect on the mean A-chiasma frequency but did affect its variance. When the Standard and Deficient Bs were present in combination with Iso-Bs in different numbers, the values of mean A-chiasma frequencies were intermediate between the values of Standard, Deficient and Iso-Bs present separately in corresponding numbers. It is suggested that a balance between the amount of heterochromatin and euchromatin present in the B-chromosomes is responsible for the differential effects of these three types of Bs on mean A-chiasma frequency. In this species, the extra euchromatin present in the form of Iso-Bs seems to have an enhancing effect on the mean A-chromosome chiasma frequency whereas the extra heterochromatin has a repressing effect.

Fine-Structure Analysis and Genetic Organization at the Base of the X Chromosome in DROSOPHILA MELANOGASTER.

Abstract: Genetic organization at the base of the X chromosome was studied through the analysis of X-ray-induced deficiencies. Deficiencies were recovered so as to have a preselected right end "anchored" in the centric heterochromatin to the right of the su(f) locus. "Free" ends of deficiencies occurred at any of 22 intervals in Section 20 and in the proximal portion of Section 19 of Bridges9 (1938) polytene chromosome map. The distribution of 130 such free ends of deficiencies induced in normal, In(1)sc 8 , and In(1)w m4 chromosomes suggests that on the single section level, genes are flanked by "hot" or "cold" sites for X-ray-induced breaks, and that occurrence of the hot spots is dependent on their interaction with the fixed-end sites in the centric heterochromatin. In the light of these results, it is argued that long heterochromatic sequences separate the relatively few genes in Section 20, and thus endow it with several characteristics typical of heterochromatic regions. Section 20 is considered to be a transition region between the mostly heterochromatic and mostly euchromatic regions of the X chromosome; the differences between them are suggested as being merely quantitative.

A Mechanism of Chromosomal Rearrangements: The Role of Heterochromatin and Ectopic Joining

Abstract: Clusters of breaks at certain intercalary heterochromatin sites producing chromosomal rearrangements are reported in four endemic species (24 strains) of Hawaiian Drosophila. In laboratory strains of these species we observed some types of changes in chromosome structure that were predicted in our earlier studies (Yoon and Richardson 1976a).-We outline the pseudochromocenter model for the production of chromosomal rearrangements. First, nonhomologous sites that are heterochromatic and contain similar base sequences of highly repetitious DNA join in a chromocenter-like configuration. Second, chromatid exchanges by breakage and reunion occur at the ectopically joined sites. Based on this model, one expects many new chromosomal rerrangements, some of which have been observed and used to differentiate species.-Inversions with identical breakpoints may occur with much greater frequency than previously assumed. Chromosome phylogenies, based on the assumption that inversions are unique events, still would be accurate if the incorporation of an inversion into the karyotype was rare. This would be the case if a rare combination of genes was necessarily contained in the inversion before it was likely to be incorporated into the gamete pool and thereby become a characteristic feature of the species.

Characterization of two moderately repetitive DNA components localized within the β-heterochromatin of Drosophila hydei

Abstract: Two nuclear DNA fractions from Drosophila hydei were isolated by silver ion and actinomycin D binding and centrifugation in isopycnic salt gradients. Neither fraction is composed of nor does it contain any highly repetitive simple sequence DNA, as shown by melting and reassociation studies. — One fraction has a CsCl density of 1.702 g/cm3 and hybridizes in situ with the β-heterochromatin of the chromocenter in polytene cells. This DNA fraction was found to be replicated during polytenization. In diploid cells this 1.702 g/cm3 component hybridizes to the heterochromatin of all four large autosome pairs, the middle part of the long arm of the Y-chromosome, but not to the X-heterochromatin. — A second fraction has a CsCl density of 1.697 g/cm3 and hybridizes in situ with polytene cells to the chromocenter and the nucleolus, but on metaphase chromosomes only to the nucleolus organizer regions.

High frequency recombination in centromeric and histone regions of Drosophila genomes.

Abstract: DNA SYNTHESIS in eukaryotes is asynchronous, with heterochromatin replicating later than euchromatin1. In Drosophila, autoradiographic studies of metaphase chromosomes from embryonic2 and cultured3 cells have shown that the centromeric heterochromatin of chromosome 2 is late replicating. The histone genes have been localised to subsections 39D and 39E in the left arm chromosome 2, close to the centromere4, and may be part of this late-replicating region for the following reasons. First, the five tandemly arranged histone coding sequences are reiterated approximately 50 times, and late-replicating DNA has been shown to contain significantly more repeated (but non-satellite) sequences than the genome as a whole5. Second, the histone genes are transcriptionally active in all but the final hours of the S phase, suggesting that their replication may be delayed6. Third, they are in close proximity to the centromere. Asynchrony has been detected in the recombination process as well. Heat treatment of Drosophila oocytes at sequential intervals during premeiotic-S phase elicits increases in recombination that follow a well-ordered pattern7. To investigate the possibility that the pattern of heat response parallels that of replication, the centromeric heterochromatin of chromosome 2 and the contiguous histone region have been examined for the time of their recombinational response to heat during the S phase. We present here evidence for late response in both regions. In addition, a level of response roughly two orders of magnitude greater than the average encountered in the genome has been found in these regions.

Heterochromatin in mitotic chromosomes of Drosophila nasuta.

Abstract: The heterochromatin in mitotic cells of larval neural ganglia of Drosophila nasuta has been analysed by C-banding and by fluorescence studies. All chromosomes, except the 'dot' like 4th chromosome pair, carry large blocks of heterochromatin which are darkly stained by C-banding, and which fluoresce uniformly brightly with Hoechst 33258, quinacrine mustard, and acridine orange. These heterochromatic segments make up about 40% of the total metaphase chromosome length, The heterochromatic segments also fluoresce brightly with all the dyes at the anaphase stage. In interphase nuclei, all the heterochromatic segments form a common, compact and homogeneous mass which fluoresces brightly with the three fluorochromes used. The size of the bright chromocentre is similar with all the dyes. It is suggested that the heterochromatic segments of different chromosomes are relatively homogeneous in their DNA base sequences, which are likely to be A-T rich in view of their bright fluorescence with Hoechst 33258 as well as quinacrine mustard.

Two highly repetitive DNA satellites of Drosophila hydei localized within the α-heterochromatin of specific chromosomes

Abstract: Two major highly repetitive satellites have been isolated from nuclear DNA of Drosophila hydei by sequential centrifugations in Ag+/Cs2SO4, actinomycin D/CsCl and CsCl. Their CsCl-densities are 1.696 and 1.714 g/cm3. In diploid larval brains they comprise about 13% and 4% respectively of the DNA. Both satellites are localized and chromosome specific. The 1.696 component was shown to be derived from the X-heterochromatin by comparison of different stocks containing different amounts of X-heterochromatin and by in situ hybridization of the 125I-labelled light single strand of this satellite. Since the amount of X-heterochromatin equals the amount of this satellite it was concluded that the 1.696 satellite is the only major DNA component of the X-heterochromatin besides minor DNA fractions (e.g. rDNA). The other highly repetitive satellite (1.714 g/cm3) hybridized in situ to all four acrocentric autosome pairs of D. hydei, but neither to the X nor to the small dot-like sixth chromosome, and not to the Y.

The constitutive heterochromatin in chromosomes of Fritillaria Sp., as revealed by Giemsa banding.

Abstract: The incidence of C-bands (constitutive heterochromatin), as determined by differential Giemsa staining, was studied in the chromosomes of 56 species, varietal forms and subgenera of Fritillaria and 30 of them are illustrated. With the exception of the subgenera Korolkowi, a supposed link between lilies and fritillaries, the chromosome complements of all plants contained bands. There were wide differences in the size and number of these bands among species both within and between groups. In those with the largest and most abundant bands, there was a pronounced tendency for centromeric localization, both in Old and New World species. The Giemsapositive centromeres were masked when this occurred. Heteromorphy in respect of banding occurred in most species. The relation of repetitive DNA sequences with heterochromatin is discussed, as is also the problem of evolution in Fritillaria.

Evolution in hawaiian drosophilidae. iii. the microchromosome and heterochromatin of drosophila.

Abstract: Among the 152 Hawaiian species of Drosophila which have been studied the majority has a metaphase karotype of five pairs of telocentric chromosomes and one pair of microchromosomes (Clayton and Wheeler, 1975; Yoon et al., 1972a, 1972b; Yoon et al., 1975; Yoon and Richardson, 1976b). Only 18 species have karyotypes altered by \"centric fusions\" that result in karyotypes containing one or two metacentric chromosomes derived from two or four telocentrics, respectively. Another 14 species have karyotypic modifications involving variation in the amount and distribution of blocks of heterochromatin. In these species, the heterochromatin is located primarily on the microchromosome, Xand V-chromosomes, and heterochromatin added to the microchromosome produces an acrocentric chromosome which is equal to or longer than the telocentric chromosomes. We are particularly interested in heterochromatic changes because they may be of special evolutionary interest. The X and microchromosome may be of common phylogenetic origin (Stone and Griffen, 1940; Sandler and Novitski, 1956), and over several species these three chromosomes exhibit the following relict features: they are frequently involved in ectopic pairing (Kaufmann and Gay, 1969) and reciprocal translocations (Sandler and Novitski, 1956); they show a variety of genetic regulatory interactions, including loci which may be combined to exhibit cis-dominant variegation effects (Judd, 1955) and position effects on puffing (Kaufmann and Gay, 1969); they exhibit interchromosomal suppressor effects of lethals (Parker, 1954); and they exhibit both dominant and epistatic interactions affecting nucleolar organizer region activity (Bicudo and Richardson, 1977). In D. busckii the microchromosome and the X chromosome are joined, as demonstrated by X-linked mutants in D. busckii which are apparently homologous to microchromosome linked mutants of D. melanogaster (Krivshenko, 1959; Hochman, 1976) . In addition, X-Y translocations may affect mate choice and can lead to reproductive isolation in certain combinations (Tracey and Espinet, 1976). Although the large chromosomes of Hawaiian Drosophila have been studied in detail, and polytene chromosomes have been homologized across species (Yoon and Carson, 1973), the microchromosomes have not been studied systematically. The chromosome is small in either the polytene or the metaphase cells, and rarely appears in a satisfactory condition for detailed analysis (Stalker, 1965; Carson and Stalker, 1968; Carson et al., 1970; Clayton et al., 1972). We report a unique inversion that involves the euchromatic arm of the microchromosome in the Hawaiian endemic subgenera, Engiscaptomyza and Antopocerus. We found in Hawaiian Drosophila large accumulations of heterochromatin on the microchromosome. These accumulations occurred only in the phylogenetically recently derived species which have no differences in gene sequences from their nearest relatives. In addition, we discuss possible changes in gene regulation associated with karyotypic changes of heterochromatin (or loci contained therein).

3h-actinomycin-D binding to mitotic chromosomes of Drosophila melanogaster.

Abstract: The binding of 3H-AMD to the metaphase chromosomes of Drosophila melanogaster has been analyzed after two different periods of exposure to photographic emulsion. The entirely heterochromatic Y chromosome was markedly less labelled than euchromatin and other heterochromatic regions. Moreover, the few grains present on the Y chromosome were clustered in two regions, one localized in the middle of YS and the other in the proximal third of YL. This labelling pattern is not affected by removing histones with a 2-hour treatment with 2N HCl. It is suggested that the specific underlabelling of the Y chromosome reflects a peculiar AT richness.

Number of the Repetitive Euchromatic 5S RNA Genes in Polyploid Tissues of Drosophila hydei

Abstract: Repetitive genes localized within heterochromatin, such as the rDNA in Drosophila, replicate several steps less than the bulk DNA during polytenization. The 5S RNA genes of Drosophila hydei were chosen as a model system to inquire whether underreplication also occurs if the repetitive gene cluster is localized in the euchromatin. Filter saturation hybridization showed that there are 320 5S RNA gene copies in the haploid genome. Setting the diploid number at 100%, it was found that the DNA of polytene salivary glands reached only 79% of this value, and the DNA of polyploid ovarian tissue reached only 72% of this value. Although the latter two saturation values are less than the diploid standard, they are not as low as the 50% saturation value predicted for a one-step reduction. This may reflect a slower replication of these genes compared to the bulk DNA. These results imply that underreplication is not a general characteristic of repetitive genes but depends on their localization in the euchromatic or heterochromatic part of the genome.

[Presence of abundant heterochromatin in the karyotype of Cebus: C. cappucinus and C. nigrivittatus].

Abstract: The karyotypes of Cebus capucinus and C. nigrivittatus (Primates, Platyrrhini) are compared after applying several banding techniques. The chromosomes have abundant intercallary heterochromatin which can be stained by R-, T- and C-band techniques and which are late replicating. The X chromosome resembles that of man and of numerous primates. However, the late replicating pattern of the X in female lymphocytes resembles that of the late replicating X of human fibroblasts rather than of human lymphocytes. Banding patterns of certain chromosomes appear analogous in Cebus and Cattarhini, including Man.