Absence of qualitative genes controlling interspecific pairing in rye B chromosomes

Summary The meiotic behaviour of hybrids between Secale cereale carrying B chromosomes and S. vavilovii has been studied in order to estimate the effects of B chromosomes on hybrid meiotic pairing. The possible effect of Bs on the meiotic pairing of the offspring from backcrosses with S. vavilovii has been studied also. The results obtained clearly indicate that no detectable differences existed in chromosome pairing of hybrids with or without B chromosomes. The hypothetical existence of epistatic genes on cereale genome masking the effect of Bs has been rejected after the results obtained in backcrosses. Therefore, lack of qualitative genes controlling interspecific pairing on rye B chromosomes has been concluded. A quantitative effect of B chromosomes was detected only when they were in alien cytoplasm.     

Patterns of meiotic double-strand breakage on native and artificial yeast chromosomes

The preferred positions for meiotic double-strand breakage were mapped on Saccharomyces cerevisiae chromosomes I and VI, and on a number of yeast artificial chromosomes carrying human DNA inserts. Each chromosome had strong and weak double-strand break (DSB) sites. On average one DSB-prone region was detected by pulsed-field gel electrophoresis per 25 kb of DNA, but each chromosome had a unique distribution of DSB sites. There were no preferred meiotic DSB sites near the telomeres. DSB-prone regions were associated with all of the known ”hot spots” for meiotic recombination on chromosomes I, III and VI. Received: 19 March 1996; in revised form: 26 July 1996 / Accepted: 18 August 1996     

Atm-dependent interactions of a mammalian Chk1 homolog with meiotic chromosomes

Background: Checkpoint pathways prevent cell-cycle progression in the event of DNA lesions. Checkpoints are well defined in mitosis, where lesions can be the result of extrinsic damage, and they are critical in meiosis, where DNA breaks are a programmed step in meiotic recombination. In mitotic yeast cells, the Chk1 protein couples DNA repair to the cell-cycle machinery. The Atm and Atr proteins are mitotic cell-cycle proteins that also associate with chromatin during meiotic prophase I. The genetic and regulatory interaction between Atm and mammalian Chk1 appears to be important for integrating DNA-damage repair with cell-cycle arrest.Results: We have identified structural homologs of yeast Chk1 in human and mouse. Chk1^Hu/Mo has protein kinase activity and is expressed in the testis. Chk1 accumulates in late zygotene and pachytene spermatocytes and is present along synapsed meiotic chromosomes. Chk1 localizes along the unsynapsed axes of X and Y chromosomes in pachytene spermatocytes. The association of Chk1 with meiotic chromosomes and levels of Chk1 protein depend upon a functional Atm gene product, but Chk1 is not dependent upon p53 for meiosis I functions. Mapping of CHK1 to human chromosomes indicates that the gene is located at 11q22–23, a region marked by frequent deletions and loss of heterozygosity in human tumors.Conclusions: The Atm-dependent presence of Chk1 in mouse cells and along meiotic chromosomes, and the late pachynema co-localization of Atr and Chk1 on the unsynapsed axes of the paired X and Y chromosomes, suggest that Chk1 acts as an integrator for Atm and Atr signals and may be involved in monitoring the processing of meiotic recombination. Furthermore, mapping of the CHK1 gene to a region of frequent loss of heterozygosity in human tumors at 11q22–23 indicates that the CHK1 gene is a candidate tumor suppressor gene.     

SUPPRESSION OF SEX-RATIO MEIOTIC DRIVE AND THE MAINTENANCE OF Y-CHROMOSOME POLYMORPHISM IN DROSOPHILA

Like several other species of Drosophila, D. quinaria is polymorphic for X-chromosome meiotic drive; matings involving males that carry a “sex-ratio” X chromosome (X^SR) result in the production of strongly female-biased offspring sex ratios (Jaenike 1996). A survey of isofemale lines of D. quinaria from several populations reveals that there is genetic variation for partial suppression of this meiotic drive. Crossing experiments show that there is Y-linked, and probably autosomal, variation for suppression of drive. Y-linked suppressors of X-chromosome drive have now been described in several species of Diptera. I develop a simple model for the maintenance of Y-chromosome polymorphism in species polymorphic for X-linked meiotic drive. One interesting feature of this model is that, if there is a stable Y-chromosome polymorphism, then the equilibrium frequency of the standard and sex-ratio X chromosomes is determined solely by Y-chromosome parameters, not by the fitness effects of the different X chromosomes on their carriers. This model suggests that Y-chromosome polymorphism may be easier to maintain than previously thought, and I hypothesize that karyotypic variation in Y chromosomes will be found to be associated with suppression of sex-ratio meiotic drive in other species of Drosophila.     

Meiotic pairing of the amphiploid Hordeum chilense X Triticum turgidum conv. durum studied by means of Giemsa C-banding technique

Summary The meiotic behaviour of the amphiploid Hordeum chilense X Triticum turgidum conv. durum using a C-banding staining method is studied. Nine pairs of chromosomes at metaphase-1 (4A, 7A and the seven of the B genome) were identified and the remaining wheat chromosomes (1A, 2A, 3A, 5A and 6A) and seven of the chilense (1 to 7 H ^ch chromosomes) were assigned to its particular genome. A similar mean number of univalents from parental genomes (wheat and wild barley) were found. No meiotic pairing between chilense and turgidum chromosomes was detected. Differences in the meiotic behaviour per chromosome and amongst genomes are explained on the basis of cytomorphological and heterochromatin characteristics.     

Cytological evidence for preferences of identical over homologous but not-identical meiotic pairing

A spontaneous tetraploid/diploid chimera involving meiotic cells of a male individual of Euchorthippus pulvinatus gallicus was heterozygous for the C-banding pattern in chromosome pair 8. This allowed the study of the possible existence of competition in meiotic pairing between identical and homologous but not-identical chromosomes. The results suggest the existence of such a competition. An excess of bivalents formed by identical chromosomes was observed. It is suggested that during the pairing process slight specificity or activity differences between chromosomes with a high degree of resemblance would be responsible for the pairing preferences found.     

Analyse des caryotypes métaphasiques méiotiques chez le mâle dePleurodeles waltlii (Amphibien,Urodèle) après coloration des chromosomes par l'argent ammoniacal

Analysis of metaphasic meiotic karyotypes in male of Pleurode les waltlii(Amphibia, Urodela) after silver ammoniacal staining of chromosomes. In the newtPleurodeles waltlii, the ammoniacal silver staining technique was applied to the male meiotic chromosomes at metaphase I and II. A specific staining of paracentromeric heterochromatin and of the centromere is observed on each chromosome of the complements. The two karyotypes are analysed and the homology between meiotic and mitotic chromosomes is established.

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Analyse des caryotypes métaphasiques méiotiques chez le male dePleurodeles waltlii (Amphibien, Urodèle) après coloration des chromosomes par l'argent ammoniacal

     

The condensin complexes play distinct roles to ensure normal chromosome morphogenesis during meiotic division in Arabidopsis

Meiosis is a specialized cell division essential for sexual reproduction. During meiosis the chromosomes are highly organized, and correct chromosome architecture is required for faithful segregation of chromosomes at anaphase I and II. Condensin is involved in chromosome organization during meiotic and mitotic cell divisions. Three condensin subunits, AtSMC4 and the condensin I and II specific subunits AtCAP‐D2 and AtCAP‐D3, respectively, have been studied for their role in meiosis. This has revealed that both the condensin I and condensin II complexes are required to maintain normal structural integrity of the meiotic chromosomes during the two nuclear divisions. Their roles appear functionally distinct in that condensin I is required to maintain normal compaction of the centromeric repeats and 45S rDNA, whereas loss of condensin II was associated with extensive interchromosome connections at metaphase I. Depletion of condensin is also associated with a slight reduction in crossover formation, suggesting a role during meiotic prophase I.     

The role of chromosome ends during meiosis in Caenorhabditis elegans

Chromosome ends have been implicated in the meiotic processes of the nematode Caenorhabditis elegans. Cytological observations have shown that chromosome ends attach to the nuclear membrane and adopt kinetochore functions. In this organism, centromeric activity is highly regulated, switching from multiple spindle attachments all along the chromosome during mitotic division to a single attachment during meiosis. C. elegans chromosomes are functionally monocentric during meiosis. Earlier genetic studies demonstrated that the terminal regions of the chromosomes are not equivalent in their meiotic potentials. There are asymmetries in the abilities of the ends to recombine when duplicated or deleted. In addition, mutations in single genes have been identified that mimic the meiotic effects of a terminal truncation of the X chromosome. The recent cloning and characterization of the C. elegans telomeres has provided a starting point for the study of chromosomal elements mediating the meiotic process.     

A comparative study of male meiosis in Drosophila melanogaster and D. virilis

Male meiosis in D. melanogaster cytologically follows the usual pattern, whereas in D. melanogaster and in D. virilis oocytes the chromosomes clump into a karyosphere at early meiotic prophase and remain so up to metaphase I.Male meiosis in D. virilis spermatocytes has an intermediate character: a part of the chromatin clumps together in a karyosphere at early prophase, whereas the other part of the chromatin remains diffuse all through prophase. At the end of prophase, the diffuse chromatin becomes integrated into the karyosphere before metaphase I. During the meiotic divisions the chromosomes have the same clumped aspect as those in Drosophila oocytes and thus differ strikingly from the dividing chromosomes in D. melanogaster spermatocytes.In D. virilis spermatocytes the nucleolus exhibits changes during the meiotic prophase that may be related to synthetical activities. The DNA specific staining with the fluorochrome DAPI reveals the existence of extrachromosomal DNA in the later prophase. Other striking differences in meiotic events between the two Drosophila species concern the centrioles and spermiogenesis.     

Nuclear division in the red algae Chondria nidifica and Chondria tenuissima

Cytological investigations are reported for two Chondria species, the Pacific species Chondria nidifica Harvey and Chondria tenuissima (Goodenough et Woodward) C. A. Agardh from the shore of the Marmara Sea in Istanbul. Nuclear division during mitosis and meiosis has been followed in somatic cells and in tetrasporangial mother cells respectively of diploid tetrasporic plants. The spherical interphase nucleus stains densely, showing many chromatin granules. Mitotic nuclei in the apical groove show a large number of chromosomes at metaphase; the chromosome number has been estimated at diakinesis to be 40 in both C. nidifica and C. tenuissima. The meiotic nuclei of tetraspore mother cells in prophase contain several relatively large nucleolar-derivatives in both species. The nucleolar derivatives disappear completely before the chromosomes begin to differentiate. In meiotic prophase the tetraspore mother cell enlarges from its original diameter. The period of the second meiotic anaphase seems to be extremely short in comparison with other nuclear phases. When the chromosomes reach the poles, they spread and subsequently form a relatively compact mass at telophase. The spindle has not been observed in C. tenuissima. Photographs are presented of nucleoli and nucleolar-derivatives in mitotic and meiotic divisions.     

The analysis of exchanges in tritium-labelled meiotic chromosomes

The autoradiographic analysis of exchanges in tritium-labelled meiotic chromosomes is potentially a useful approach to the study of meiotic exchange events since this method differentially labels meiotic chromatids along their entire length. The main problem encountered in earlier autoradiographic studies is that of distinguishing label exchanges generated at chiasmata from label exchanges generated by sister chromatid exchange. This problem was overcome in the present study by the choice of a meiotic system (male meiosis of Stethophyma grossum) where chiasmata are limited to just one proximally localised chiasma in each bivalent. This system allows the positive identification of chiasma-generated label exchanges and demonstrates convincingly the origin of chiasmata through breakage and rejoining of homologous non-sister chromatids. Sister chromatid exchanges are also readily detected in labelled meiotic chromosomes of this species, where they occur with a mean frequency of 0.35 per chromosome. This frequency is similar to that found in mitotic spermatogonial cells and the exchanges are randomly distributed both within and between chromosomes. These features of meiotic sister chromatid exchanges suggest that they are unrelated to non-sister chiasmatic exchanges and they probably have no special meiotic significance.     

mei-2, a mutagen-sensitive mutant of Neurospora defective in chromosome pairing and meiotic recombination

Summary A Neurospora crassa mutation, mei-2, affecting meiosis and mutagen sensitivity, was characterized for its effect on meiotic recombination and chromosome pairing. Results from homozygous mei-2 crosses involving distant markers on the same chromosome demonstrated a drastic reduction in meiotic recombination. However, mitotic recombination continued to occur. Cytological observations indicated that pairing of homologous chromosomes in zygotene was greatly reduced or absent, resulting in aberrant segregation at anaphase I and often at subsequent divisions as well. The few mature ascospores produced were frequently disomic for one or more chromosomes.     

The karyotype of Nodipecten nodosus (Bivalvia: Pectinidae)

Earlier karyotypical work on Nodipecten nodosus embryos indicated that this species has a diploid number of 38, with six pairs respectively of metacentric and submetacentric chromosomes and seven pairs of subtelocentric chromosomes, although there were some difficulties in obtaining complete metaphases. The present work provides additional results on specific regions of the chromosomes in N. nodosus and, by meiotic studies, confirms the chromosome number with more reliability. Active nucleolar organizer regions (NOR), detected in mitotic metaphases from embryos, can be characterized in N. nodosus by a high level of heteromorphism of NOR-sites, indicating that these regions are not appropriate as chromosomal markers in this species. The procedure for detecting constitutive heterochromatin of chromosomes allowed us to observe most of the heterochromatic blocks at a pericentromeric position and some at telomeric and interstitial positions. The analysis of meiotic chromosomes from gonad tissue revealed the presence of 19 bivalents during metaphase I, all homomorphic and isopicnotic, confirming the previously described diploid chromosomal number of 38 for N. nodosus. From these results, some evolutionary aspects of the Pectinidae are briefly discussed.     

Double-strand breaks on artificial chromosomes in yeast

Yeast artificial chromosomes composed primarily of bacteriophage λ DNA exhibit very low levels of meiotic crossing over compared with similarly sized intervals of natural yeast DNA. When these recombinationally quiet chromosomes were augmented with a 12.5 kb insert of sequences from yeast chromosome VIII, genetic studies demonstrated that the artificial chromosomes had acquired recombination properties characteristic of this region of chromosome VIII. On authentic yeast chromosomes, most meiotic recombination events are initiated at sites where the DNA is cleaved to create a double-strand break (DSB). This report describes physical analyses that were carried out to examine the relationship between DSB sites and the recombination behavior of the artificial chromosomes. The results show that DSBs are rare on these artificial chromosomes, except for the 12.5 kb insert. Mapping of the DSB sites shows that their positions correlate with the previously determined positions of DSB sites on chromosome VIII. Deletion of two characterized chromosome VIII DSB sites from the 12.5 kb insert on the artificial chromosome resulted in the loss of the predicted DSB fragments and a reduction in crossing over between artificial chromosomes. Received: 15 May 1998; in revised form: 26 September 1999 / Accepted: 18 November 1999     

The effect of B chromosomes on meiotic and pre-meiotic spindles and chromosome pairing in Triticum/Aegilops hybrids

Diploid populations of Aegilops mutica and Aegilops speltoides containing B chromosomes have been used as male parents in crosses with aneuploid genotypes of Triticum aestivum to investigate the effect of B chromosomes on meiotic homologous and homoeologous chromosome pairing. F₁ hybrids of T. aestivum/Ae. mutica and T. aestivum/Ae. speltoides segregated into four classes with regard to the degree of meiotic chromosome pairing, irrespective of the presence of B chromosomes. The B chromosomes do not introduce factors altering the level of pairing other than that due to the natural allelic and gene variation occurring in the diploids. Similarly no reduction in pairing of homologous chromosomes was observed in genotypes in which pairs of homologues co-existed with B chromosomes. However, a significant drop in chiasma frequency was observed in F₁ hybrids of T. aestivum × Ae. mutica with B chromosomes and T. aestivum × Ae. mutica nullisomic for wheat chromosome 5D with B chromosomes, in temperature regimes of 12° C. No asynapsis occurred in similar hybrids in the absence of Mutica B chromosomes at low temperatures. The low-temperature sensitive phase lies early in the pre-meiotic interphase. In this instance the Mutica B chromosomes are interacting with specific gene loci of the A chromosomes. Synaptic pairing has been observed between A and B chromosomes in Ae. mutica. A high frequency of pollen mother cells with twice the number of chromosomes was observed in hybrids in the presence of Mutica B chromosomes due to failure of spindle formation at the last pre-meiotic mitosis. Meiotic spindle irregularities occurred in hybrids containing Speltoides B chromosomes. Hybrids of Ae. speltoides + B's X Ae. mutica + B's displayed the mitotic and meiotic spindle abnormalities introduced by the presence of the B chromosomes of each parent.     

Cytological studies of heterochromatin function in the Drosophila melanogaster male: autosomal meiotic pairing

In Drosophila melanogaster it is now documented that the different satellite DNA sequences make up the majority of the centromeric heterochromatin of all chromosomes. The most popular hypothesis on this class of DNA is that satellite DNA itself is important to the pairing processes of chromosomes. Evidence in support of such a hypothesis is, however, circumstantial. This hypothesis has been evaluated by direct cytological examination of the meiotic behaviour of heterochromatically and/or euchromatically rearranged autosomes in the male. It was found that neither substantial deletions nor rearrangements of the autosomal heterochromatin cause any disruption of meiotic pairing. Autosomal pairing depends on homologs retaining sufficient euchromatic homology. This is the first clear demonstration that the highly repeated satellite DNA sequences in the heterochromatin of the second, third and fourth chromosomes are not important in meiotic pairing, but rather that some euchromatic homology in the autosomes is essential to ensure a regular meiotic process. These results on the autosomes, when taken in conjunction with our previous studies on sex chromosome pairing, clearly indicate that satellite DNA is not crucial for male meiotic chromosome pairing of any member of the D. melanogaster genome.     

Mitotic and meiotic chromosomes of the American lobster Homarus americanus (Nephropidae, Decapoda)

The chromosomes of H. americanus have been characterised by C-banding, fluorochrome banding and restriction endonuclease banding. Thanks to these techniques, it has been possible to identify mitotic and meiotic figures clearly and to study the distribution and structure of heterochromatic regions. Moreover, we have identified small supernumerary chromosomes, variable in number and often asynaptic in first meiotic metaphase.     

Chromatin organization at meiosis

From 1956, when the complex ultrastructure of meiotic chromosomes was discovered, ¹ until 1985, when the isolation of meiotic chromosome cores was reported, knowledge of the molecular structure of the meiotic chromosome was at best a dream. The dissection of meiotic chromosome structures has become a realistic challenge through the arrival of isolated symptonemal complexes (SCs), monoclonal and polyclonal antibodies against SCs, the possibility for screening expression libraries for genes that encode SC proteins, the isolation of SC-associated DNA, and the development of techniques for the in situ recognition of DNA sequences in the context of the meiotic chromosome structure.