End-to-end chromosome attachments in mitotic interphase and their possible significance to meiotic chromosome pairing

The telomere bouquet is a hub where meiotic double-strand breaks, synapsis, and stable homolog juxtaposition are coordinated in the zebrafish, Danio rerio

Meiosis is a cellular program that generates haploid gametes for sexual reproduction. While chromosome events that contribute to reducing ploidy (homologous chromosome pairing, synapsis, and recombination) are well conserved, their execution varies across species and even between sexes of the same species. The telomere bouquet is a conserved feature of meiosis that was first described nearly a century ago, yet its role is still debated. Here we took advantage of the prominent telomere bouquet in zebrafish, Danio rerio, and super-resolution microscopy to show that axis morphogenesis, synapsis, and the formation of double-strand breaks (DSBs) all take place within the immediate vicinity of telomeres. We established a coherent timeline of events and tested the dependence of each event on the formation of Spo11-induced DSBs. First, we found that the axis protein Sycp3 loads adjacent to telomeres and extends inward, suggesting a specific feature common to all telomeres seeds the development of the axis. Second, we found that newly formed axes near telomeres engage in presynaptic co-alignment by a mechanism that depends on DSBs, even when stable juxtaposition of homologous chromosomes at interstitial regions is not yet evident. Third, we were surprised to discover that ~30% of telomeres in early prophase I engage in associations between two or more chromosome ends and these interactions decrease in later stages. Finally, while pairing and synapsis were disrupted in both spo11 males and females, their reproductive phenotypes were starkly different; spo11 mutant males failed to produce sperm while females produced offspring with severe developmental defects. Our results support zebrafish as an important vertebrate model for meiosis with implications for differences in fertility and genetically derived birth defects in males and females.

Inherent to reproduction is the transmission of genetic information from one generation to the next. In sexually reproducing organisms, each parent contributes an equal amount of genetic information, packaged in chromosomes, to the offspring. Diploid organisms, like humans, have two copies of every chromosome, while their haploid gametes (e.g. eggs and sperm) have only one. This reduction in ploidy depends on the segregation of chromosomes during meiosis, resulting in gametes with one copy of each chromosome. Missegregation of the chromosomes in the parents leads to abnormal chromosome numbers in the offspring, which is usually lethal or has detrimental developmental effects. While it has been known for over a century that homologous chromosomes pair and recombine to facilitate proper segregation, how homologs find their partners has remained elusive. A structure that has been central to the discussion of homolog pairing is the bouquet, or the dynamic clustering of telomeres during early stages of meiosis. Here we use zebrafish to show that the telomere bouquet is the site where key events leading to homologous chromosome pairing are coordinated. Furthermore, we show that deletion of spo11, a gene required for proper recombination in most studied organisms, resulted in very different effects in males and females where males were sterile while females produced deformed progeny.

3.—The Cytology of the Parthenogenetic Australian Weevil Listroderes costirostris Schönh

Karyograms prepared from ovarian and blastoderm cells of the parthenogenetic Australian Brown Vegetable Weevil demonstrate a consistent triploid condition with 30 chromosomes which can be grouped into 10 sets of homologues. Meiosis is replaced by a single mitotic-like division in which 30 univalent chromosomes, each composed of two chromatids, divide equationally between an ootid nucleus and a single polar nucleus. Prior to the differentiation of the oocytes a peculiar bouquet stage occurs in the cells of the end chamber of each ovariole, but the significance of this phase is not known. Arrested development in eggs from individuals of low fertility is investigated and the relationship of body size and chromosome number is discussed.

Somatic Association in Avena sativa L

Root tip cells of hexaploid oats Avena sativa L. were examined at mitotic metaphase, and distances between homologous as well as between non-homologous chromosomes were measured and their frequency distributions compared. Nonhomologous chromosomes were scattered at random in the cells studied. In contrast, the mean distance between homologous chromosomes was significantly shorter. There is a tendency this species. for somatic association of homologs in this species.

Mammalian meiotic telomeres: protein composition and redistribution in relation to nuclear pores

Mammalian telomeres consist of TTAGGG repeats, telomeric repeat binding factor (TRF), and other proteins, resulting in a protective structure at chromosome ends. Although structure and function of the somatic telomeric complex has been elucidated in some detail, the protein composition of mammalian meiotic telomeres is undetermined. Here we show, by indirect immunofluorescence (IF), that the meiotic telomere complex is similar to its somatic counterpart and contains significant amounts of TRF1, TRF2, and hRap1, while tankyrase, a poly-(ADP-ribose)polymerase at somatic telomeres and nuclear pores, forms small signals at ends of human meiotic chromosome cores. Analysis of rodent spermatocytes reveals Trf1 at mouse, TRF2 at rat, and mammalian Rap1 at meiotic telomeres of both rodents. Moreover, we demonstrate that telomere repositioning during meiotic prophase occurs in sectors of the nuclear envelope that are distinct from nuclear pore-dense areas. The latter form during preleptotene/leptotene and are present during entire prophase I.

UbcD1, a Drosophila ubiquitin-conjugating enzyme required for proper telomere behavior

The end-to-end association of chromosomes through their telomeres has been observed in normal cells of certain organisms, as well as in senescent and tumor cells. The molecular mechanisms underlying this phenomenon are currently unknown. We show here that five independent mutant alleles in the Drosophila UbcD1 gene cause frequent telomere-telomere attachments during both mitosis and male meiosis that are not seen in wild type. These telomeric associations involve all the telomeres of the D. melanogaster chromosome complement, albeit with different frequencies. The pattern of telomeric associations observed in UbcD1 mutants suggests strongly that the interphase chromosomes of wild-type larval brain cells maintain a Rab1 orientation within the nucleus, with the telomeres and centromeres segregated to opposite sides of the nucleus. The UbcD1 gene encodes a class I ubiquitin-conjugating (E2) enzyme. This indicates that ubiquitin-mediated proteolysis is normally needed to ensure proper telomere behavior during Drosophila cell division. We therefore suggest that at least one of the targets of UbcD1 ubiquitination is a telomere-associated polypeptide that may help maintain proper chromosomal orientation during interphase.

Prediction and analysis of spatial order in haploid chromosome complements

Bennett has proposed a model that predicts a mean ordered arrangement of all the chromosomes in a simple haploid genome, based on associations of pairs of most similar long, and pairs of most similar short, chromosome arms. The model orders a complete simple haploid genome so that each chromosome is associated with two constant neighbours. This paper describes a test of the model with two types of data obtained from the same reconstructed serially sectioned somatic metaphases examined in the electron microscope. First, chromosome arm volumes were estimated and used to identify the chromosomes and to predict their mean spatial order. Secondly, centromere positions in three dimensions were established. In the species and hybrids used, all with 14 chromosomes, there are so many ways of positioning the chromosomes within haploid sets that a computer-aided analysis was developed. With use of only centromere identities and positions, the programs generated all possible orders of centromeres in haploid sets (where each centromere has two neighbours) and computed the sum of distances between centromeres for each order within a cell. Orders were ranked in ascending sequence of sums of distances. Orders that ranked highest were taken as ‘best’. After results for replicate cells had been pooled, orders were ranked from best to worst. A test of the predicted order was then made by finding its position on this summary. In all the four grasses examined, the predicted order was among the 5% of orders judged best by the analysis. To demonstrate and confirm the predicted order in these grasses, only seven to ten reconstructed nuclei were required. Presumably this test is suitable for general application to other materials whose simple haploid genomes contain between six and about ten biarmed chromosomes.

Chromosome rearrangements at telomeric level in hematologic disorders

Following retrospective screening of our karyotype data from 414 consecutive non-childhood, neoplastic, and preneoplastic hematologic diseases, we have isolated 11 cases with alterations involving one or two chromosome termini, including: a) nonclonal telomeric telomeric associations (tas), b) subclonal terminal rearrangements consisting of additional (add) material of unknown origin fused at the end of the chromosome, c) clonal telomere-centromere fusion (t telcen) with pseudodicentric structure. Most of these abnormalities were present in karyotypes with multiple alterations and associated to an evolutive stage of the disease (9 of 94 cases studied in progression, including three of 22 CML studied in blast crisis). The immunophenotype of the cell populations was lymphoid in eight cases, six of which were NHL, and myeloid, erythroid, and undifferentiated in the other three. More data on telomeric abnormalities may clarify whether there is ubiquitous genomic instability of neoplastic cells or an inborn cell lineage predisposition favoring rearrangements involving telomeres.

Telomeres and the functional architecture of the nucleus

The single molecule of DNA that constitutes a eukaryotic chromosome begins and ends with a stretch of repetitive DNA known as a telomere. These sequences appear to be necessary to preserve the integrity of the genetic material through the cell cycle. Telomeric DNA is organized into regions of non-nucleosomal chromatin called the telosome, which can interact with other telosomes and with the nuclear envelope. This review focuses on cytological evidence for these interactions and on recent insights into the molecular organization of the telomeric complex.

On the ordered arrangement of the haploid complement in radial metaphases of secondary meiocytes of male grasshoppers, Euchorthippus pulvinatus gallicus

Five hundred and ninety-three radial metaphase II cells from the male grasshopper, Euchorthippus pulvinatus gallicus, were analyzed to ascertain whether chromosomes in the haploid complement were in a fixed order. As an a posteriori hypothesis, the most probable original order of chromosomes in the metaphases was determined. The genetical significance of a suprachromosomal organization is discussed.

The ordered arrangement of chromosomes in the Chinese hamster spermatocyte nucleus

The question of chromosome distribution in the mammalian nucleus is addressed, and data are provided in support of the ordered arrangement of chromosomes in the Chinese hamster spermatocyte. Testicular cells were dispersed and air-dried without prior fixation, then stained and karyotyped. The position of chromosome telomeres in 217 pachytene spermatocytes was determined in relation to four concentric rings which equally divided the nuclear area. The distribution of telomeres showed a progressive decline from the central to the peripheral rings. This was particularly pronounced for chromosomes 1-7, but was reversed for the XY chromosomes. The distribution of the total as well as of the individual chromosomes was significantly different from that expected on the basis of random distribution. The only exceptions to this were chromosomes 8-10, which exhibited random distribution. Thus, while chromosomes 1-7 had a central position, the XY pair had a peripheral localization. The mean ring position appeared to be related to chromosome length, except for the XY chromosomes, suggesting that chromosome length may determine chromosome position.

Eukaryotic genome: model considerations

The paper presents a new model of chromosome structure based on the assumption that multiple circular subunits of DNA exist. The essential difference with previously described models is the circular DNA unit forms a central chromosome axis. Chromosome configurations during various phases of the cell cycle depend on the various conformations of this central integrating unit. The described model can be generalized for all haploid set of eukaryotic nucleus. Some aspects of the chromosome structure and their functions have been discussed.

Organization of DNA sequences and replication origins at yeast telomeres

We have shown that the DNA sequences adjacent to the telomeres of Saccharomyces cerevisiae chromosomes are highly conserved and contain a high density of replication origins. The salient features of these telomeres can be summarized as follows. There are three moderately repetitive elements present at the telomeres: the 131 sequence (1 to 1.5 kb), the highly conserved Y sequence (5.2 kb), and the less conserved X sequence (0.3 to 3.75 kb). There is a high density of replication origins spaced about 6.7 kb apart at the telomeres. These replication origins are part of the X or the Y sequences. Some of the 131-Y repetitive units are tandemly arranged. The terminal sequence T (about 0.33 to 0.6 kb) is different from the 131, X, or Y sequences and is heterogeneous in length. The order of these sequences from the telomeric end towards the centromere is T-(Y-131)n-X-, where n ranges from 1 to no more than 4. Although these telomeric sequences are conserved among S. cerevisiae strains, they show striking divergence in certain closely related yeast species.

Chinese hamster nuclear proteins. An electrophoretic analysis of interphase, metaphase and nuclear matrix preparations

A comparison, by two-dimensional gel electrophoresis, of total interphase nuclear, metaphase chromosomal and nuclear matrix proteins from Chinese hamster V-79 cells was undertaken to examine the distribution of these proteins during mitosis. We have found a number of differences among these populations, although the two-dimensional gel patterns are generally similar. The most striking observation is that a loose cluster of six interphase nuclear polypeptides, with isoelectric points in urea between 5.7 and 6.7 and molecular masses ranging from 53 to 75 kDa, is greatly enriched in chromosome preparations. Each of these species is prominent also in the nuclear matrix. Preliminary evidence suggests that one of these polypeptides is the intermediate filament protein, vimentin. In addition, two major polypeptides of interphase nuclear preparations, a basic 94-kDa species and an approximately 65-kDa species, are absent from chromosomes. The latter polypeptide is the nuclear pore-lamina complex lamin B. Actin is present in all of these fractions, but tubulin has not been observed. hnRNP particle polypeptides are major components of the nuclear matrix, but are markedly reduced in metaphase chromosomes. The intermediate and basic 65-75-kDa nuclear matrix polypeptides we have previously demonstrated to be major components of rat liver nuclear matrix, are reduced in Chinese hamster matrix preparations and at least one of these species, a minor, basic, 68-kDa polypeptide, is missing entirely from metaphase chromosomes. These results are discussed in relation to nuclear and chromosome structure and the possibility of contamination of nuclear protein preparations from cultured cell lines with intermediate filaments.

The karyotype of tuleen 346 barley

Tuleen 346 barley is a triple homozygote for 3 unequal interchanges, T1-5v, T2-6y and T3-7d, which were induced independently in the variety, 'Bonus'. It has great potential value in studying differences in behaviour and position among chromosomes within the same cell in barley and its hybrids since at least 5 and usually all 7 chromosome types can be identified in Feulgen preparations, compared with only 3 types in normal barley. Measurement of chromosome arm and satellite lengths in 10 haploid root-tip metaphases showed that in all comparisons except of the longest with the next longest, the 7 chromosome types were distinct in total length (P<0.001-0.05) and in 8 of the 10 cells, the longest chromosome had a smaller arm ratio than the next longest. In these preparations, each of the 5 shortest chromosomes was easily identifiable by size and morphology. The longest chromosome was about 2.2 times as long as the shortest. C-banding showed that each of the 7 chromosome types had a clearly unique band pattern. The C-band patterns of interchange chromosomes confirmed and increased the precision of previously published breakpoint locations, viz. the short arms of chromosomes 1, 2, 3 and 7 and the long arms of chromosomes 5 and 6.

An interphase model for mitotic chromosome organization in eukaryota

A model for the spatial relationship of the arrangement of the chromosomes in the nucleus in eukaryota is presented. Evidence is derived from light and electron microscopic studies, application of autoradiographic and banding techniques; on the organization, structure and behaviour of chromosomes during interphase and other stages of cell cycle. This model visualizes the entire chromosomal DNA as a single uninemic multirepliconic continuum where the chromosomes with their centromeres and telomeres have a predetermined arrangement among themselves as well as in relation to the nucleolus and nuclear membrane. This orderly arrangement is presumably maintained through interchromosomal connections. The impact of this model on the interpretation of various cytogenetic phenomena is discussed.

Arrangement of chromosomes in the interphase nucleus of plants

Chromosomal arrangement in the interphase nucleus has two main aspects: (1) arrangement of chromosomes with respect to nuclear polarity and to other nuclear components, and (2) arrangement of chromosomes with respect to one another. The latter aspect consists of two main types of spatial relationships; (1) relationships between different members of one chromosomal set, (b) relationships between different chromosomal sets. Data concerning various aspects of chromosomal arrangement in the interphase nucleus are presented and discussed and the genetic control as well as subcellular mechanisms which are involved in nuclear organization, are elucidated. Evidence is presented indicating that, in common wheat, the gene system that determines the specific pattern of chromosomal arrangement in the nucleus is operating via the microtubular elements of the spindle system. The significance of ordered arrangement of chromosomes in the nucleus for the regularity of genetic activity and chromosomal behavior, is pointed out.

Meiotic chromosome pairing and synaptonemal complex transformation in Culex pipiens oocytes

The synaptonemal complexes of the oocytes of the mosquito Culex pipiens quinquefasciatus have been reconstructed from serial sections. A diffuse structure, probably a chromocenter composed of centromeric heterochromatin, was present during pachytene. As no synaptonemal complexes were visible inside the chromocenter the continuity of the 2 arms of a bivalent was lost. The telomeric ends were clustered on a small area of the nuclear membrane in a bouquet arrangement; they were associated in pairs, and sometimes joined through a special structure. One pair was composed of the 2 telomeres of the shortest bivalent and a ring configuration was thus formed. The other 2 chromosomes may form one or two rings. During a short transitional stage, after the disappearence of the synaptonemal complexes, several thousand annuli, 1200-1500 A in diameter, were present in the nuclei. The annuli disappeared as material originating mainly from the transverse filaments of the synaptonemal complexes formed a "capsule" around the chromosomes during diplotene.

Linear differentation of cereal chromosomes : III. Rye, triticale and 'Aurora' variety

The BSG test was used in an investigation of the linear differentiation in rye variety 'Zhitkinskaya', common wheat variety 'Aurora' and two secondary Triticale namely AD-196 and F-1239.Chromosomes of 'Aurora' variety and wheat chromosomes within Triticale may be easily divided into "constant" and 'variable' chromosomes as described previously (lordansky et al. 1977; Zurabishvili et al 1977).It is necessary to emphasize that the diversity of "variable" chromosomes underlies karyotypical polymorphism within wheat and Triticale species. The polymorphism observed exists in parallel with strict homomorphism of homologous chromosomes.In IB chromosomes of 'Aurora' variety, the short arm is substituted by the rye chromosome arm. The karyotype of Triticale AD-196 consists of six pairs of rye chromosomes and fifteen pairs of wheat chromosomes.

Distribution of nuclear pores and perinuclear dense substances in spermatocytes I of some Oedionychina fleabeetles

The nuclear envelope of growing postpachytene spermatocyte I differs notably in structure between the fleabeetles Omophoita cyanipennis and Oedionychus bicolor. The former species shows a more conventional structure with an even and probably random distribution of nuclear pores, and a strongly electron-opaque layer of fibrogranular material (FM) separated from the outer nuclear membrane by an intermediate layer of about 40 nm thickness. Peripherally from the FM layer, a continuous corona of granular dense material (GM) is accumulated around the nucleus. In Oedionychus, all nuclear pores are clustered in "nuclear sieves", i.e., shallow, cup-like indentations of the nuclear envelope. The sieves are filled with an electron-opaque substance resembling the FM of the Omophoita spermatocytes. This substance is kept at a distance of about 40 nm from the outer nuclear membrane. GM is produced only at the sives, and is thus discontinuous. The sieves with their contents are called nuclear sieve complexes (NSC).

Orientation of interphase chromosomes as detected by Giemsa C-bands

The orientation of Giemsa C-bands has been studied in mitotic and interphase cells of Allium cepa. A sativum and of Aloe vera. The C-bands in these three species are located at the telomeres, secondary constriction region of the nucleolar chromosomes and the centromeric regions, respectively. Observations in A. cepa and Aloe indicate clearly that the interphase chromosomes are non-random in their orientation and possibly maintain their telophase configuration through the attachment of telomeres and perhaps of kinetochores with the nuclear membrane. Electron micrographs of onion cells also reveal that certain heterochromatic segments are associated with the nuclear membrane.--The nucleolar interstitial C-bands in A. sativum remain free in the nucleoplasm and may come close to each other due to heterochromatic attraction. Such a heterochromatic attraction is also evident between telomeric regions and between centromeres. However, a two by two attachment could not be noticed. A diagrammatic representation of the orientation of interphase chromosomes has been presented.

Synaptonemal complex karyotyping in spermatocytes of the Chinese hamster (Cricetulus griseus). III. Quantitative evaluation

Relative length is a constant and distinctive characteristic for each autosomal SC, despite variations in absolute length from cell to cell. Arm ratio is distinctive for each SC except for two of the three sub-acrocentrics, and serves, together with relative length, for identification. The constancy of relative length and arm ratios indicates biological stability and lack of physical distortion in these spread preparations. There is a 1:1 relationship between relative lengths of autosomal SCs and mitotic autosomes: their arm ratios are similar. These close parallels provide strikingly similar SC and somatic karyotypes. Variability was observed in sub-acrocentric arm ratios and in lengths of unpaired X and Y axes, correlated with the presence of constitutive heterochromatin. - Utilizing progressive differentiations of the X and Y chromosomes for staging, it is demonstrated that autosomal SCs decrease in length from late zygotene to mid-pachytene, and then increase at late pachytene. Within a nucleus, synchrony of length changes is maintained. It is concluded that the factors governing autosomal SC length are regular for any given bivalent from cell to cell and may be related to those that control somatic autosome length relationships. - The X and Y axes differ quantitatively as well as qualitatively from autosomal SCs. The SC portion of the X and Y is constant in length through most of pachytene; the unpaired axes shorten and lengthen, but not in proportion to autosomal SCs. X and Y relative lengths ard arm ratios vary throughout pachytene and do not maintain proportionality with somatic values. The evidence suggests, but does not prove, that the long arm of the X is paired with the short arm of the Y.-Twists occur in autosomal SCs at increasing frequencies throughout pachytene but cannot account for length changes. The number of twists per SC is directly proportional to SC length. Intertwining of SCs is random and proportional to SC length. End-to-end associations of autosomal SCs appear to be random; however, the ends of the X and Y are less often involved in such connections. - The length of axial material in all chromosomes at pachytene, expressed as an equivalent length of DNA double helix, represents 0.013% of the diploid DNA complement.

Synaptonemal complex karyotyping in spermatocytes of the Chinese hamster (Cricetulus griseus). I. Morphology of the autosomal complement in spread preparations

Using the Counce-Meyer spreading technique, in over 70 spermatocytes it was possible consistently to obtain whole, flattened nuclei containing complete sets of pachytene SCs. The SCs are visible in both the phase and electron microscopes. Each SC is morphologically intact, preferentially stained, and attached to the nuclear envelope by a dense, terminal plaque. It is thus possible to trace each SC for its entire length. Also, a structure representing the kinetochore is clearly visible in each autosomal SC. Karyotypes comparable to the somatic karyotype can be constructed by arranging SCs according to length and kinetochore position. The observed regularity of SC morphology implies structural stability sufficient to withstand the stresses imposed by the procedure.--A coarse network of closely packed nuclear annuli connecting SC attachment plaques often provides end-to-end associations and may tend to immobilize SCs during processing.--Three kinds of perturbation of SC structure are encountered. Twists in the SC frequently occur, but no regular pattern or correspondence with chiasma distribution is observed. SCs occasionally hook around each other without disruption, but in two instances the unpaired axis of the X apparently was interlocked within an autosomal SC. Streching of the SC is infrequent; it is conspicous when it occurs and is usaully associated with other obvious distortions of the nucleus.--Distinctive morphologies of the X and Y chromosomes facilitate their identification inall preparations.--During zygotene, autosomal synapsis, i.e., the formation of SCs from the pairing of single axial elements, initiates at distal ends and terminates at the kinetochore region; neither initiation nor termination is synchronous among all autosomes.