Showing papers in "Chromosoma in 1971"

Reptitive DNA sequences in drosophila.

Abstract: The satellite DNAs of Drosophila melanogaster and D. virilis have been examined by isopycnic centrifugation, thermal denaturation, and in situ molecular hybridization. The satellites melt over a narrow temperature range, reassociate rapidly after denaturation, and separate into strands of differing buoyant density in alkaline CsCl. In D. virilis and D. melanogaster the satellites constitute respectively 41% and 8% of the DNA isolated from diploid tissue. The satellites make up only a minute fraction of the DNA isolated from polytene tissue. Complementary RNA synthesized in vitro from the largest satellite of D. virilis hybridized to the centromeric heterochromatin of mitotic chromosomes, although binding to the Y chromosome was low. The same cRNA hybridized primarily to the α-heterochromatin in the chromocenter of salivary gland nuclei. The level of hybridization in diploid and polytene nuclei was similar, despite the great difference in total DNA content. The centrifugation and hybridization data imply that the α-heterochromatin either does not replicate or replicates only slightly during polytenization. Similar but less extensive data are presented for D. melanogaster. — In D. melanogaster cRNA synthesized from total DNA hybridized to the entire chromocenter (α- and β-heterochromatin) and less intensely to many bands on the chromosome arms. The X chromosome was more heavily labeled than the autosomes. In D. virilis the X chromosome showed a similar preferential binding of cRNA copied from main peak sequences.—It is concluded that the majority of repetitive sequences in D. virilis and D. melanogaster are located in the α- and β-heterochromatin. Repetitive sequences constitute only a small percentage of the euchromatin, but they are widely distributed in the chromosomes. During polytenization the α-heterochromatin probably does not replicate, but some or all of the repetitive sequences in the β-heterochromatin and the euchromatin do replicate.

DNA of ciliated protozoa

Abstract: DNA was isolated from macronuclei and micronuclei of the ciliated protozoan, Stylonychia mytilus under conditions that minimize the possibility of DNA degradation Macronuclear DNA has an S value of 10 to 11 in sucrose gradients Macronuclear DNA has an average molecular weight of 115×106 daltons and a range of molecular weights of 10×106 to 195×106 daltons The average length of macronuclear DNA, measured by electron microscopy, is 080 microns and the range is 02 to 22 microns Almost all micronuclear DNA pieces are too long to be measured by electron microscopy The shortest piece of micronuclear DNA found was 150 microns in length

Chromatid structure: relationship between DNA content and nucleotide sequence diversity.

Abstract: Models of chromatid structure are based on inferences made from genetic, cytological, and cytochemical observations. An alternative approach can provide limits as to the number of identical subunits present in chromatids. This method is based on the demonstration that nucleotide sequence diversity may be estimated from the kinetics of renaturation of denatured DNA. Measurements of DNA content and renaturation rate constants are given for several eukaryotic DNAs. Control experiments involved measurements of renaturation kinetics of DNAs from bacteria and bacteriophage. These estimates show that most of the nucleotide sequences in mouse, Drosophila, and Ciona DNA are present only once per sperm. Since the reduction of DNA content during meiosis indicates that mouse sperm contain a haploid set of chromatids, it follows that a set of mouse meiotic chromatids contains a single copy of most sequences. Models of chromatid structure which postulate multiple subunits with identical nucleotide sequences are therefore not tenable for mouse meiotic chromatids. This method of analyzing nucleotide sequence diversity may be of general use in designing models of chromatid structure in other organisms.

Arrangement of centromeres in mouse cells.

Abstract: Applying a staining procedure which reveals constitutive heterochromatin to cytological preparations of the mouse (Mus musculus), one detects heterochromatin pieces at the centromeric areas of all chromosomes except the Y. The Y chromosome is somewhat heteropyenotic in general but possesses no intensely stained centromeric heterochromatin. The arrangement of the centromeric heterochromatin in interphase cells is apparently specific for a given cell type. In meiotic prophase, centromeric heterochromatin may form clusters among bivalents. From the location of the centromeric heterochromatin of the X chromosome in the sex bivalent, it is concluded that the association between the X and Y (common end) in meiosis is limited to the distal portions of the sex elements.

The DNA content of sperm of Drosophila melanogaster.

Abstract: Amounts of Feulgen staining in individual sperm nuclei of Drosophila melanogaster were determined with a scanning microdensitometer and compared with the DNA-Feulgen levels found for hen erythrocyte nuclei, taken as a presumed cytophotometric standard of 2.5×10−12 g DNA per cell. Under these conditions, the amount of DNA estimated for the haploid male genome of Drosophila was about 0.18×10−12 g DNA, corresponding to a molecular weight of roughly 11× 1010 daltons. From a comparable series of measurements on Feulgen-stained nuclei of hemocytes and epithelial cells of the imaginal soma, values of approximately 0.35–0.36×10−12 g DNA were estimated for the diploid or 2C male genome of this species.

Distribution of constitutive heterochromatin in mammalian chromosomes

Abstract: Using a special staining technique, a survey of the chromosomes of many mammalian species showed that constitutive heterochromatin is present in all cases and that the heterochromatin pattern appears to be specific and consistent or each chromosome and each taxon. Usually heavy heterochromatin is found in the centromeric areas, but terminal heterochromatin is not uncommon. Occasionally interstitial heterochromatin bands occur. In some species, such as the Syrian hamster and Peromyscus, many chromosome arms are completely heterochromatic.

Karyotypic changes in callus cultures from haploid and diploid plants of Crepis capillaris (L.) Wallr

Abstract: Two auxin-heterotrophic callus cultures of Crepis capillaris, one coming from an haploid plant and the other from a diploid one, were studied in regard to karyotypic changes for over a year. The degree of polyploidisation of the originally haploid culture was considerably higher than that of the diploid culture. The frequency of chromosome rearrangements was significantly higher in polyploidised karyotypes than in not polyploidised karyotypes and correspondingly greater in the “haploid” culture. However, the cytogenetical stability of the cultures cannot be measured only through their degree of polyploidisation: it has been found that new karyotypes also originate through chromosome rearrangements at the same ploidy level as the original explant.

Morphology and development of the macronuclei of the ciliates Stylonychia mytilus and Euplotes aediculatus.

Abstract: Some stages of macronuclear anlagen development, known from earlier investigations (see Fig. 1), were studied in detail. The results are: a) The giant chromosomes of Stylonychia mytilus are not somatically paired, but are connected end-to-end to form one or a few composite chromosomes. When they later disintegrate, the bands become isolated granules. b) Spectrophotometric measurements show that during the DNA-poor stage which follows the disintegration of the chromosomes, the macronuclear anlagen of Euplotes have a DNA content of 21 c, while the syncaryotic (deriving from syncarya) and hemicaryotic (deriving from haploid hemicarya) anlagen of Stylonychia have the DNA content of diploid micronuclei (2c). Nevertheless the syncaryotic anlagen of Stylonychia and Euplotes initially develop two nucleoli at the end of this stage, the hemicaryotic anlagen of Stylonychia only one. From this it is concluded that the genes of one giant chromosome band stay together in one granule, c) Labeled DNA from the giant chromosomes which remains in the anlagen during the DNA-poor stage is distributed approximately equally to the daughter nuclei during the first few fissions of the exconjugants.-Autoradiographic experiments showed that the DNA of the macronuclei of Stylonychia that is duplicated at one time in a replication band is not duplicated simultaneously during the next DNA-duplication. The DNA duplications during the second polyploidization stage of the macronuclear anlagen development are exceptions, because the mixing of the macronuclear DNA which occurs before every fission does not occur during the second polyploidization stage.—The pseudomicronuclei which sometimes are formed from the macronuclei in emicronucleated strains of Stylonychia contain numerous elements which are much smaller than the chromosomes.—The macronucleus of Stylonychia is very insensitive to irradiation with X-rays.—The results lead to the following hypothesis: The macronuclei of the two hypotrich ciliates contain unconnected chromomeres or small aggregates which are distributed at random to the two daughter nuclei during the divisions.

Localization of repetitive DNA in the chromosomes of Microtus agrestis by means of in situ hybridization.

Abstract: Heterochromatin in the European field vole, Microtus agrestis, was studied using a special staining technique and DNA/RNA in situ hybridization. The heterochromatin composed the proximal 1/4 of the short arm and the entire long arm of the X chromosome, practically the entire Y chromosome and the centromeric areas of the autosomes. By using the DNA/RNA in situ hybridization technique, repeated nucleotide sequences are shown to be in the heterochromatin of the sex chromosomes.

Cytological mapping of human chromosomes: results obtained with Quinacrine fluorescence and the Acetic-Saline-Giemsa techniques

Abstract: The similarities and differences between the banding patterns obtained in human chromosomes with the Quinacrine fluorescence and the Acetic-Saline-Giemsa (ASG) techniques are described. The use of these techniques to identify each chromosome pair in the human karyotype is discussed, as also is the use of the methods to identify aberrant chromosomes and to map points of exchange in translocations and inversions. A number of examples are used to illustrate the resolution permitted by these new methods. Seven polymorphic regions on normal chromosomes are described, which include four identified by fluorescence on chromosomes 3,4, 13, and 22. The secondary constrictions on chromosomes 1, 9, and 16, which had previously been observed in conventionally stained preparations from favourable material, are particularly clear in all cells treated with the Giemsa techniques. The new methods make it possible to detect small differences in size between the heterochromatic blocks at these regions in homologous chromosomes. The benefit to human genetics of studying the familial segregation of both structurally rearranged and normal, but polymorphic chromosomes, where the chromosomes or parts of chromosomes can be unambiguously identified is stressed.

The relationship between patterns of DNA replication and of quinacrine fluorescence in the human chromosome complement.

Abstract: Cultured human peripheral blood lymphocytes were labelled with 3H-thymidine in the early or late S phase prior to mitosis. Quinacrine fluorescence patterns in metaphase chromosomes were then recorded photographically and the slides reprocessed for autoradiography so that the same metaphase cells were examined with the two techniques. The intensity and distribution of 3H-thymidine labelling was compared with the intensity and distribution of Q fluorescence with particular reference to chromosomes 1, 13, 14, 15, 17, 18, 19, 20, 21 and 22. It was found that chromosome regions showing bright fluorescence were also late replicating and that, in general, patterns of late replications reflected the patterns of fluorescence. Exceptions to this generalisation included the late labelling X chromosome in cells of female origin and areas near the centromeres on chromosomes 1, 9, 16 and 22. These centromeric regions show a dull fluorescence but, with exception of chromosome 9, are strongly Giemsa-positive in the ASG staining technique. On the basis of staining reaction, late replicating heterochromatic regions fall into five categories, the relationships and functional significance of these categories is discussed.

Architecture of the Chinese hamster metaphase chromosome.

Abstract: The development of procedures for the isolation of unfixed metaphase chromosomes has made feasible a direct analysis of their morphology. Wholemount stereo electron microscopy was used to examine intact and partially disrupted chromosomes produced by physical shearing and extraction with salt and urea solutions. A model of chromosome architecture was developed to accommodate evidence from studies using both light and electron microscopy. In the proposed model the chromatid (anaphase chromosome) consists of two half-chromatids; each half-chromatid contains two deoxyribonucleoprotein ribbons wound into a single fiber (termed the core), with many loops of chromatin (termed epichromatin) attached along its length. The core ribbons are each about 50 A thick by 4000 A wide and are composed of many parallel deoxyribonucleoprotein strands. The epichromatin loops appear to be 250 A supercoiled fibers containing about 75 per cent of the chromosomal DNA. The epichromatin can be selectively removed from the core fibers by extraction with 2.0 M NaCl or 6.0 M urea solutions.

Analysis of the human karyotype using a reassociation technique.

Abstract: A characteristic banding pattern can be made visible in human chromosomes by a denaturating and renaturating procedure performed on cytological preparations. The banding pattern is characteristic of each chromosome pair, allowing easy identification of all human chromosomes. The method is likely to provide a useful tool in the identification of mammalian chromosomes and in the study of aberrations and variations in the chromosome set.

Karyotype analysis utilizing differentially stained constitutive heterochromatin of human and murine chromosomes.

Abstract: Constitutive heterochromatin of chromosomes can be visualized utilizing a new differential staining technique which was originally developed by Gall and Pardue (1971). The method facilitates the more certain identification of specific chromosomes within and between cell populations of different origins. Marker chromosomes can be identified in established cell lines over many months of serial passage. Chromosomes of similar morphology within karyotypes of man and mouse can be distinguished in a number of instances. For example, the Y chromosomes of both mouse and man can now be easily detected. The hetero-chromatic staining method also permits discrimination between mouse and human chromosomes in somatic cell hybrids, thus facilitating the assignment of gene markers to chromosomes in somatic cell genetics systems. Instances of translocation of centric heterochromatin to other parts of chromosomes in established tissue culture cell lines are described. An instance of the inheritance of a polymorphic variation in autosomal heterochromatin in man is reported. It is postulated that polymorphisms in the centric heterochromatin may account largely for small heritable chromosome length variations previously described in human populations and termed minor chromosome variants.

The nucleolus organizer region of maize (Zea mays L.): Chromosomal site of DNA complementary to ribosomal RNA

Abstract: A maize genetic marker strain (designated as the 2NOR strain) was found to carry a cytologically-visible “duplication” of the nucleolar organizer region (NOR). The 2 NOR condition is a simply inherited cytological marker which is transmitted normally through mega- and microsporogenesis and is associated when homozygous with a nucleolus approximately 64% greater in volume than that of two representative normal inbred lines of maize.—Utilizing DNA · rRNA hybridization techniques and the 2 NOR strain plus two unrelated inbred lines with normal NOR's, the chromosomal site of DNA complementary to rRNA was shown to be localized in the NOR. To our knowledge, this is the first report on the localization of rDNA cistrons in the NOR of plants. An estimate was made of 17,000 rDNA cistrons per diploid nucleus for normal maize.—The 2 NOR strain was shown to possess approximately twice as many rDNA cistrons (34,000) as normal maize. The usefulness of this 2 NOR condition for plant protein improvement programs is being investigated.

Genome relationships between Hordeum vulgare L. and H. bulbosum L.

Abstract: Interrelationships between H. vulgare (2x=14) and H. bulbosum (2x=14; 4x=28) were estimated on the basis of the karyotypes and the pairing behaviour of the chromosomes in diploid, triploid and tetraploid hybrids obtained with the aid of embryo culture. — A comparison of the karyotypes of the two species revealed similarities as well as differences. It was concluded that at least 4 or more of the chromosomes were similar in morphology and probably closely related. — Diploid and tetraploid hybrids are rarely obtained and their chromosome numbers tend to be unstable whereas triploid hybrids (1 vulgare + 2 bulbosum genomes) were stable and relatively easy to produce. In the diploid hybrid only 40% of the meiotic cells contained 14 chromosomes while the numbers ranged from 7 to 16 in other cells. All hybrids exhibited pairing between the chromosomes of the two species. Diploid hybrids had a mean of 5.0 and a maximum of 7 bivalents per cell in those cells having 14 chromosomes. Triploid hybrids from crosses between 2x H. vulgare and 4x H. bulbosum exhibited a mean of 1.5 and a maximum of 5 trivalents per cell. In a hexaploid sector found following colchicine treatment of a triploid the mean frequencies of chromosome associations per cell were: 5.5I+8.0II+0.7III+3.7IV+0.3V+0.4VI. One unstable 27 chromosome hybrid obtained from crosses between the autotetraploid forms had a mean of 1.1 and a maximum of 4 quadrivalents per cell. The chromosome associations observed in these hybrids are consistent and are taken as evidence of homoeologous pairing between the chromosomes of the two species. Interspecific hybridization between these two species also reveals that chromosome stable hybrids are only obtained when the genomes are present in a ratio of 1 vulgare∶2 bulbosum. Based upon the results obtained, the possibility of transferring genetic characters from H. bulbosum into cultivated barley is discussed.

The chromosomal localization of a heavy satellite DNA in the testis of Plethodon c. cinereus.

Abstract: When DNA from blood or liver of Plethodon c. cinereus is centrifuged to equilibrium in cesium chloride it separates out into 2 components. The smaller or satellite component is relatively rich in G + C and is therefore heavy, and it amounts to about 2% of the total DNA. The heavy satellite does not include the ribosomal cistrons, and it is unrelated to the nucleolar organizer. When squash preparations of cells from the testis of P. c. cinereus are incubated in synthetic E3RNA complementary to the satellite DNA, the RNA anneals specifically to the centromeric heterochromatin of spermatogonia, spermatocytes, and spermatids, and to the centromeric regions of all discernible chromosomes. RNA/DNA hybrids were located by autoradiography. H3RNA complementary to the major component of the DNA anneals to all nuclei and to all parts of the chromosomes. H3RNA complementary to nucleolar DNA from Xenopus laevis anneals specifically to the chromatin associated with nucleoli in nuclei at various stages of the meiotic divisions. The nature of the centromeric heterochromatin and its role in the meiotic divisions are discussed.

Satellite DNA associated with heterochromatin in Rhynchosciara.

Abstract: The DNA of Rhynchosciara hollaenderi was examined using isopycnic centrifugation in neutral CsCl. Two low density minor bands (collectively termed satellite DNA) were detected in addition to the main band DNA. Main band DNA has a buoyant density of 1.695 g/cm3. The larger of the two minor bands has a buoyant density of 1.680 g/cm3 while the smaller of the two minor bands has a buoyant density of about 1.675 g/cm3. Thermal denaturation studies have confirmed the presence of the two minor classes of DNA.—The satellite and main band DNAs were isolated in relatively pure form and were transcribed in vitro using DNA-dependent RNA polymerase from Escherichia coli. Annealing of the two complementary RNAs (cRNAs) with main band and satellite DNA was examined using filter hybridization techniques.—The chromosomal distribution of the satellite DNA was determined by in situ molecular hybridization of satellite-cRNA with Rhynchosciara salivary gland chromosomes. Satellite-cRNA hybridized with the centromeric heterochromatin of each of the four chromosomes (A, B, C, and X) and with certain densely staining bands in the telomere regions of the A and C chromosomes. Main band-cRNA annealed with many loci scattered throughout the chromosomes including areas containing satellite DNA.

Effect of -amantine on puffing and intranuclear RNA synthesis in Chironomus salivary glands.

Abstract: Incubation of Chironomus salivary glands with α-amanitine in concentrations from 1 to 10 γ/ml results in the suppression of puffing and chromosomal 3H-uridine incorporation after 30 to 60 min in 80% of the cells. Nucleolar 3H-uridine incorporation remains completely unaffected. Even 4 h after the injection of high doses of α-amanitine into living larvae, nucleolar incorporation is still pronounced. The distribution of “resistant” cells within the salivary glands suggests that the uptake of α-amanitine is subject to physiological restrictions.—A puff typically induced during in vitro incubation of salivary glands was found to be less sensitive to α-amanitine than the Balbiani rings.

Evolution of sex-chromosomes and formation of W-chromatin in snakes

Abstract: The analysis of sex-chromosome complexes and formation of W-chromatin in 16 species of snakes of the families Boidae, Colubridae, Elapidae, and Hydrophiidae, reveal three very pertinent facts. First, the snakes exhibit various states of the differentiation of the Z and W chromosomes, apparently according to the evolutionary status of the families, being homomorphic in primitive families and well differentiated in highly evolved ones. Second, the demonstration of a heteropycnotic body in the interphase nuclei of the families of a large number of species of snakes has definitely shown that the nuclear sexing is possible not only in those species of snakes where the W chromosome is morphologically distinguishable from the Z, but also in those species where it is not so, but shows an asynchrony in the replicating pattern of W. It is suggested that development of allocycly rather than establishment of structural changes is the first step in the differentiation of the W from the Z in snakes. Third, the absence of coexistence of nucleolus and W-chromatin in a condensed state in the interphase nuclei of different tissues in a few species of snakes reported in this paper suggests that the W-chromatin is responsible for the synthesis of the nucleolus in these snakes.

Meiosis in haploid barley — An interpretation of non-homologous chromosome associations

Abstract: Detailed meiotic studies were conducted on ten haploid plants representing six different genotypes of barley (Hordeum vulgare, 2n=14) At pachytene stages the non-homologous chromosomes were observed to pair as intimately as homologous chromosomes in many cells Foldback pairing, involving single chromosomes, and multivalent associations were common At diplotene, up to 4 chiasmatalike structures were observed in paired chromosomes but it is not likely that they resulted from crossing over At diakinesis the bivalent frequency mean was from 1 to 13 per cell whereas by metaphase I the paired associations were rare with a single rod bivalent being observed in 3 to 5% of the cells The frequencies of various types of secondary associations at metaphase were also recorded — The origin and significance of bivalents and secondary associations in haploids is reviewed and discussed Caution is urged in the interpretation that low levels of chromosome pairing in haploids is evidence of homology It is concluded that very little chromosome duplication is likely to be found within the haploid set of barley chromosomes and that the basic chromosome number is seven

Intraspecific variation of DNA per cell between Picea sitchensis (Bong.) Carr. provenances.

Abstract: Quantitative intraspecific variation of DNA per cell was established by means of biochemical, double wavelength microspectrophotometry and micro-densitometric analyses of root tips from 7 seed sources of Picea sitchensis (Bong) Carr Northern seed sources possessed more DNA per cell than southern provenances This, perhaps, is an indication of DNA adaptability through plasticity resulting from changing environmental stress over the natural range

Differential response of constitutive and facultative heterochromatin in the manifestation of mitomycin induced chromosome aberrations in Chinese hamster cells in vitro

Abstract: The distribution of chromatid aberrations induced by mitomycin C among the individual chromosomes of female and male Chinese hamster cells in vitro was studied. The aberrations were found to be non-randomly distributed. Among the autosomes, the chromosomes possessing constitutive heterochromatin were more often involved in aberrations as well as in homologous exchanges. The inactivated X chromosomes in the female cells offer a situation where the short arm is facultatively heterochromatic and the long arm constitutively heterochromatic, thus enabling an analysis of their response for aberration formation. The short arm was seldom found to be involved in the aberration. The long arm of the inactivated X was more often affected (5 to 10 times) than the long arm of the functional X though both are constitutively heterochromatic. The possible role of (a) structure of heterochromatin, (b) the chromocenter formation and their association, (c) allocycly, and (d) the qualitative differences in the DNA of different types of heterochromatin are discussed in relation to the formation of chromatid aberrations.

New Karyotypes of Vicia faba L.

Abstract: Five new karyotypes of V. faba are presented and the types and positions of the structural changes combined in the construction of these new karyotypes are described. All their chromosomes are easily distinguishable by morphological criteria. These new chromosome complements are presently used to study the inter- and intrachromosomal distribution of induced chromatid aberrations and related problems.

RNA metabolism during puff induction in Drosophila melanogaster.

Abstract: RNA metabolism of the salivary glands of Drosophila melanogaster was studied for possible changes coinciding with the induction of new puffs by heat treatment.—The rate of 3H-uridine incorporation into RNA is identical at 37° C and at 24° C. It declines with time of incubation, possibly indicating the existence of a class of rapidly turning over RNA.—RNA extracted from glands pulselabelled at either 24° or at 37° C displays similar profiles if subjected to gel electrophoresis. Processing of the 38s ribosomal RNA precursor comes to a halt at 37° between 30 and 60 minutes of incubation, i.e., some time after puff induction is completed. At both temperatures newly synthesized pre-ribosomal RNA accumulates with time of incubation more rapidly than heterodisperse RNA, again suggesting that some heterodisperse RNA is of relatively short life span. After short pulses the portion of heterodisperse RNA is larger in glands kept at 37° C than in glands kept at 24° C. With increasing time this difference disappears.—Some of the pulse-labelled, high molecular weight heterodisperse RNA is rapidly degraded, if RNA synthesis is blocked by actinomycin D. If the chase is performed at 24° C, about 30% of the newly synthesized RNA is degraded within about 15 minutes. At 37° C the beginning of degradation appears delayed for about 30 minutes; subsequently the same percentage of RNA is degraded as at 24° C.—The possibility is considered that the local RNA accumulation visualized by the heat-induced puffs may have resulted from a change in RNA degradation rather than from a local stimulation of RNA synthesis.

The ultrastructure of the non-localized kinetochores of Luzula and Cyperus.

Abstract: Profiles of kinetochores of Luzula appear fibrillar, less electron-dense than the rest of the chromatin, and are located in recesses on the poleward surfaces of the chromosomes. Cyperus kinetochores also appear less electron-dense than the majority of the chromatin; but Cyperus kinetochores are not as distinct as those of Luzula and are not located in recesses. Serial sections reveal that the non-localized kinetochores of both Luzula and Cyperus consist of several packages of kinetochore material along the poleward surfaces of the chromosomes instead of being truly diffuse as suggested by earlier light microscopists. Interchromosomal connections are observed, but no structural relationships to the non-localized kinetochore condition are evident.

A fresh look at meiosis and centromeric heterochromatin in the red-backed salamander, Plethodon cinereus cinereus (Green)

Abstract: Male meiosis, with special regard to the centromeric heterochromatin and to centromeric structure, has been studied in the salamander, Plethodon cinereus cinereus. In this salamander, n = 14. Early meiotic prophase proceeds as described by other authors. Pachytene is followed by a “diffuse” stage in which much of the chromosomal DNA becomes reorganized into fine lateral loops which spring from the bivalent axes. These loops can be seen along the bivalent axes as early as zygotene. Loops are maximally extended in the diffuse stage. The formation of diplotene bivalents involves a return of this extended DNA into the axes of the bivalents. — At leptotone, centromeric heterochromatin is in one or a few large masses. These masses break up during zygotene. At pachytene there is one mass of heterochromatin at the centromeric region of each bivalent. The heterochromatin remains condensed in the diffuse stage. During diplotene, centromeric heterochromatin becomes less conspicuous, and it is possible to see 4 centromere granules in each diplotene bivalent. These observations support the view that centromeres replicate at pre-meiotic S-phase when the associated hetero-chromatin is replicated. In the interphase before the 2nd division, the hetero-chromatin often forms a broken ring corresponding to the positions of the centromeres at the end of anaphase 1. There are 14 masses of heterochromatin in nuclei at prophase of the 2nd division. In spermatids, the heterochromatin appears as a single solid mass or a broken ring.

Cytological studies of ameiotic and normal maize with reference to premeiotic pairing

Abstract: This study reports cytological observations in maize plants homozygous for the recessive am allele which suppresses meiosis in both male and female meiocytes. The ultimate premeiotic mitosis in anthers from ameiotic plants is normal, but the resulting nuclei do not undergo meiosis. Instead, a synchronized mitosis occurs after which the cells degenerate. No evidence was found for a non-random association of homologous chromosomes in the premeiotic or ameiotic mitoses of homozygous ameiotic plants or in the premeiotic mitosis of normal sibs. These observations are in agreement with the classical view that synapsis of homologous chromosomes does not occur until zygotene.

Chromosomal basis of dosage compensation in Drosophila - V. Puffwise analysis of gene activity in the X-chromosome of male and female Drosophila hydei

Abstract: A critical analysis of the puffing activity and transcribing activity patterns of different sites of the X-chromosome of the male and female larval salivary glands of Drosophila hydei has been presented. The results show that within the limitations of the resolving power of the technique and variability inherent in the general chromosomal conditions the puffing activities of the different sites of the X-chromosome are very much alike in the two sexes. Of the 15 puffing sites in the X-chromosome, most of the sites either show good concordance in the two sexes or resemble in their highest class value. Only 4 sites (4CD, 8A, 16C and 20B) show considerable discordance in the activity pattern between male and female. Incorporation of 3H-uridine in the X-chromosome also reveals that there is indeed a reasonable degree of superimposition of the number of silver grains in the X-chromosomal puffs of the two sexes. Whatever disparity that exists between the grain numbers in the two sexes can be explained on the basis of sister-class compensation. These results have been interpreted as evidence in support of the piece-meal mechanism of dosage compensation in Drosophila, operating through hyperactivation in the male.