Differential elongation of autosomal pachytene bivalents related to their DNA content in human spermatocytes

Genome-wide variability in recombination activity is associated with meiotic chromatin organization

Recombination enables reciprocal exchange of genomic information between parental chromosomes and successful segregation of homologous chromosomes during meiosis. Errors in this process lead to negative health outcomes, whereas variability in recombination rate affects genome evolution. In mammals, most crossovers occur in hotspots defined by PRDM9 motifs, although PRDM9 binding peaks are not all equally hot. We hypothesize that dynamic patterns of meiotic genome folding are linked to recombination activity. We apply an integrative bioinformatics approach to analyze how three-dimensional (3D) chromosomal organization during meiosis relates to rates of double-strand-break (DSB) and crossover (CO) formation at PRDM9 binding peaks. We show that active, spatially accessible genomic regions during meiotic prophase are associated with DSB-favored loci, which further adopt a transient locally active configuration in early prophase. Conversely, crossover formation is depleted among DSBs in spatially accessible regions during meiotic prophase, particularly within gene bodies. We also find evidence that active chromatin regions have smaller average loop sizes in mammalian meiosis. Collectively, these findings establish that differences in chromatin architecture along chromosomal axes are associated with variable recombination activity. We propose an updated framework describing how 3D organization of brush-loop chromosomes during meiosis may modulate recombination.

Use of meiotic pachytene stage of spermatocytes for karyotypic studies in insects

Coleopterans represent by far the largest animal group, with more than 300,000 identified species. Only little progress in their chromosome analysis has been accomplished during recent decades, compared with that made in vertebrate cytogenetics. Both the small size of their genome and the difficulty of obtaining mitotic cells with nice chromosomes have limited the application of conventional techniques, such as chromosome banding. A method for obtaining chromosome banding on well-spread bivalents from the pachytene stage of the meiotic prophase, the most frequent stage in young imagines, is described. It makes possible the identification of all bivalents and the establishment of the karyotype with greater ease and accuracy than with mitotic cells. In addition, it gives some insight into chromosome organization at a stage when autosomes are assumed to undergo an intense transcriptional activity. The results of the technique, which was successfully applied to many species, are described here in two of them, Cetonia aurata and Adesmia montana as examples.

Zygotene-pachytene substaging and synaptonemal complex karyotyping of boar spermatocytes

Synaptonemal complex analysis, by electron microscopy of spread spermatocytes after phosphotungstic acid staining, made possible description of the chromosomal synaptic patterns and the synaptonemal complex karyotype of the pig (Sus scrofa domestica). The autosomal synaptic pattern in conjunction with the sex chromosome morphology and pairing behaviour can serve as a reference for the meiotic cell progression from the zygotene stage to the pachytene. The autosomes started terminal synapsis at early zygotene (Z1) and at mid-zygotene (Z2) some of the small bivalents were completely paired. The extension of pairing between the X and the Y chromosome, and the differentiation of their axes, disclosed seven types of sex bivalent (Types 0-VI). The Type 0 sex bivalent occurred in late zygotene (Z3), at which the X and Y axes began terminal synapsis by their short arms. Each one of the pachytene substages, early, mid-, and late, included two sex bivalent types. By early pachytene (P1-P2) the entire Y chromosome was paired with the X axis. Subsequently, progressive desynapsis and differentiation of the sex chromosome axes defined the mid- (P3-P4) and late pachytene substages (P5-P6). At mid-pachytene, the unpaired XY segments were associated end-to-end and showed differing degrees of complexity (thickening, splitting, despiralization and strandedness). These axial complexities were replaced at late pachytene by fine excrescences along the sex chromosome axes, which still formed a short SC stretch. Additionally, an attempt to construct an SC karyotype for the pig is presented. There was general agreement between the autosomal SC karyotype and the autosomal somatic karyotype when comparisons were made, considering the relative lengths and arm ratios of individual chromosome pairs.

Synaptic process in the rat (Rattus norvegicus): Influence of methodology on results

Synaptonemal complex (SC) analysis is a widely used method for assessing the effects of genotoxic agents in germ cells. Although the evolution of the SCs and their related annexed structures, such as nucleoli, has been well established, sometimes it is difficult to assess whether the abnormal features observed correspond to genotoxic effects or to an artefact related to the method used to obtain the SC preparations. In this article, we describe a new method of obtaining SC preparations for electron microscopy, as well as the results of a study of the first meiotic prophase in oocytes and spermatocytes of the rat (Rattus norvegicus Sprague Dawley) in which we analysed how the methodology used can influence the results. Besides important sex-specific differences, mainly during desynapsis (diplotene), a relationship between several bivalents and nucleolar structures, that in some cases could disturb the synaptic process, was observed in oocytes. At the same time, the characteristic SC fragmentation in oocytes was verified, but this fragmentation, in addition to a sex-specific component, was influenced by the method itself. By reducing to a minimum the artefacts produced by the method, it is possible to optimise the analysis of SCs as a method of testing genotoxic effects in the germ line.

Male mouse recombination maps for each autosome identified by chromosome painting

Linkage maps constructed from genetic analysis of gene order and crossover frequency provide few clues to the basis of genomewide distribution of meiotic recombination, such as chromosome structure, that influences meiotic recombination. To bridge this gap, we have generated the first cytological recombination map that identifies individual autosomes in the male mouse. We prepared meiotic chromosome (synaptonemal complex [SC]) spreads from 110 mouse spermatocytes, identified each autosome by multicolor fluorescence in situ hybridization of chromosome-specific DNA libraries, and mapped >2,000 sites of recombination along individual autosomes, using immunolocalization of MLH1, a mismatch repair protein that marks crossover sites. We show that SC length is strongly correlated with crossover frequency and distribution. Although the length of most SCs corresponds to that predicted from their mitotic chromosome length rank, several SCs are longer or shorter than expected, with corresponding increases and decreases in MLH1 frequency. Although all bivalents share certain general recombination features, such as few crossovers near the centromeres and a high rate of distal recombination, individual bivalents have unique patterns of crossover distribution along their length. In addition to SC length, other, as-yet-unidentified, factors influence crossover distribution leading to hot regions on individual chromosomes, with recombination frequencies as much as six times higher than average, as well as cold spots with no recombination. By reprobing the SC spreads with genetically mapped BACs, we demonstrate a robust strategy for integrating genetic linkage and physical contig maps with mitotic and meiotic chromosome structure.

Meiotic chromosomes: integrating structure and function

Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, a feature which greatly facilitates the study of chromosome behavior. The complex events involved in homolog juxtaposition necessitate prolongation of prophase, thus permitting resolution of events that are temporally compressed in the mitotic cycle. Furthermore, once homologs are paired, the chromosomes are connected by a specific structure: the synaptonemal complex. Finally, interaction of homologs includes recombination at the DNA level, which is intimately linked to structural features of the chromosomes. In consequence, recombination-related events report on diverse aspects of chromosome morphogenesis, notably relationships between sisters, development of axial structure, and variations in chromatin status. The current article reviews recent information on these topics in an historical context. This juxtaposition has suggested new relationships between structure and function. Additional issues were addressed in a previous chapter (551).

Aspects of three-dimensional chromosome reorganization during the onset of human male meiotic prophase

The three-dimensional morphology and distribution of human chromosomes 3 were studied in nuclei of spermatogonia and spermatocytes I from formaldehyde-fixed human testis sections. Chromosome arms, pericentromeres and telomeric regions were painted by a three-color, five-probe fluorescence in situ hybridization protocol. Light optical serial sections of premeiotic and meiotic nuclei obtained by confocal laser scanning microscopy revealed that premeiotic chromosomes 3 are separate from each other and occupy variably shaped territories, which are sectored in distinct 3 p- and q-arm domains. Three-dimensional reconstructions of the painted chromosome domains by a Voronoi tessellation approach showed that mean chromosome volumes did not differ significantly among the premeiotic and meiotic stages investigated. A significant increase in surface area and reduction of dimensionless ‘roundness factor’ estimates of arm domains indicated that the restructuring of spatially separate chromosome territories initiates during preleptotene. Telomeric regions, which in meiotic stem cells located predominantly in arm-domain chromatin, showed a redistribution towards the domain surface during this stage. At leptotene homologues were generally misaligned and displayed intimate intermingling of non-homologous chromatin. Pairing initiated at the ends of bent zygotene chromosomes, which displayed a complex surface structure with discernible sister chromatids. The results indicate that, in mammals, homology search is executed during leptotene, after remodeling of chromosome territories.

Presynaptic association of Rad51 protein with selected sites in meiotic chromatin

Eukaryotic homologs of Escherichia coli Rec-A protein have been shown to form nucleoprotein filaments with single-stranded DNA that recognize homologous sequences in duplex DNA. Several recent reports in four widely diverse species have demonstrated the association of RecA homologs with meiotic prophase chromatin. The current immunocytological study on mouse spermatocytes and oocytes shows that a eukaryotic homolog, Rad5l, associates with a subset of chromatin sites as early as premeiotic S phase, hours before either the appearance of precursors of synaptonemal complexes or the initiation of synapsis. When homologous chromosomes do begin to pair, the Rad5l-associated sequences are sites of initial contact between homologues and of localized DNA synthesis. Distribution of Rad5l foci on the chromatin of fully synapsed bivalents at early pachynema corresponds to an R-band pattern of mitotic chromosomes. R-bands are known to be preferred sites of both synaptic initiation and recombination. The time course of appearance of Rad51 association with chromatin, its distribution, and its interaction with other Rad5l-associated sequences suggests that it plays an important role preselection of sequences and synaptic initiation.

Chromosome pairing and recombination in mice heterozygous for different translocations in chromosomes 16 and 17

In order to clarify the relationship between meiotic pairing and recombination, an electron microscopic (EM) study of synaptonemal complexes (SC) and an analysis of chiasma frequency and distribution were made in male mice singly and doubly heterozygous for Robertsonian [Rb(16.17)7Bnr] and reciprocal [T(16:17)43H] translocations and also in tertiary trisomics for the proximal region of chromosome 17. In all these genotypes an extensive zone of asynapsis/desynapsis around the breakpoints was revealed. At the same time a high frequency of non-homologous pairing was observed in precentromeric regions of acrocentric chromosomes. The presence in the proximal region of chromosome 17 of the t haplotype did not affect the synaptic behaviour of this region. Chiasma frequency in the proximal region of chromosome 17 in the T(16:17)43H heterozygotes and trisomics was increased when compared with that in Robertsonian heterozygotes.

Heteropycnosis of an underreplicating chromosome

Drosophila nasutoides has an extraordinary genome since 62% of its DNA resides in chromosome 4. This element mainly consists of constitutive heterochromatin which does not polytenize. Earlier studies of heterochromatin attributed little attention to the fact that "condensed" chromosomes often vary in condensation. This paper reports that chromosomes of the same complement display different degrees and kinetics of condensation. In D. nasutoides, even sex specific differences can be observed. The results of a comparative microphotometric study on neuroblast metaphases in both sexes revealed the following picture. The process of chromosome condensation is not restricted to mitotic prophase but continues into the metaphase. The mean condensation is not equal for all chromosomes. In the metaphase of the female, Feulgen density increases from the X chromosome, via 3 and 2, to chromosome 4. In the male, the order is X, 2, 3, Y, and 4. During the metaphase of the male, chromosomes condense with similar kinetics. In contrast, chromosomes of the female display asynchrony as monitored by area and length determinations. The X chromosomes of the female probably have enhanced shortening during prophase. This would explain the metaphase of the female where the X chromosomes shorten less than the autosomes, and why each of the X chromosomes is 15% shorter than the X chromosome in the metaphase of the male. Further differences were observed in the longitudinal and lateral compaction of the chromosomes in males and females. The sex chromosomes and chromosome 3 condense by shortening, while chromosome 2 and 4 preferentially reduce their diameter. The large amount of DNA engaged in heteropycnosis and the isochromosome nature allow the identification of chromosome 4 during interphase. At this stage, a new category of extreme DNA packaging was detected. The interphase density of chromosome 4 can exceed that of metaphase by a factor of up to 8. Two events account for this high degree of condensation: (1) the homologues are particularly associated due to somatic pairing and (2) the arms are further tightened as a result of pericentric folding. The features of the isochromosome suggest that the interaction of chromatids during interphase is essentially caused by specific DNA sequences. The data confirm that heteropycnosis not only interferes with gene expression but also strongly inhibits DNA synthesis in endocycles.

Transmission of three radiation-induced translations in the Syrian hamster. I. Chromosome studies of male meiosis: pachytene to metaphase II

Chromosome behaviour during meiosis in male Syrian hamsters heterozygous for one of three translocations was analysed as part of a study of the transmission of these structural changes. Synapsis was studied using preparations of synaptonemal complexes, and chiasmate associations and the results of anaphase I segregation were studied in air-dried preparations of metaphases I and II respectively. The main findings were: (i) that, at least in the two trivalent-forming translocations, there is no simple relationship between either the frequency or the extent of synapsis and chiasma formation between the chromosomes involved in the translocation; (ii) that the presence of a univalent in a substantial proportion of metaphase I cells does not necessarily lead to irregular segregation as judged by analysis of metaphase IIs; and (iii) conversely, that in translocation heterozygotes in which metaphase I contains the chromosomes involved in the translocation as a quadrivalent or as two bivalents, with no univalents or trivalents, unexpected numerical segregation can be found. The observations of meiotic chromosomes behaviour reported here show that it is not always possible to predict the effects of structural change, or to determine the basis of these effects, from an analysis of any stage of meiosis taken in isolation, or from an analysis of an apparently similar change.

Correlation between chromosomal breakpoint positions and synaptic behaviour in human males heterozygous for a pericentric inversion

The examination of synaptic data and localization of chromosomal breakpoints in a review of human pericentric inversions suggest that synaptic and recombination behaviour in rearranged chromosomes during meiosis can be predicted by determining the subband in which the breakpoint is located. According to this hypothesis, it can be postulated that loops in pericentric inversions are routinely formed only in cases when both breaks occur in G-light bands, with the genetic consequences of crossing-over. In other cases, heterosynapsis is accomplished without previous homosynapsis, thereby minimizing the production of unbalanced gametes.

Asymmetry in chromosome pairing: a major factor in de novo mutation and the production of genetic disease in man

At the outset of the meiotic pairing process in man, trial and error mismatching and misalignment, both within homologous pairs and between heterologues, can be observed cytologically. Pairing starts at early zygotene principally within subtelomeric regions where the synaptonemal complex initiates. In the present paper, evidence for the primary role in synaptic initiation of a GC rich minisatellite in the human XY pseudoautosomal segment is presented, and circumstantial evidence is provided to support the view that GC rich sequences (minisatellites and Alu repeats) function to promote pairing within autosomes. The known sequence hypervariability of proterminal human minisatellites, it is suggested, arises as a secondary consequence of unequal exchange after misalignment between tandem repeats at the outset of the pairing process. Unequal exchange within misaligned repeat sequences at early prophase of meiosis could make a major contribution to de novo germinal mutation (conversion, duplication, deficiency, inversion, translocation), with serious consequences in man for the production of hereditary disease. For somatic tissues, rare mispairing between G rich repeats followed by unequal exchange could be a key step in cancer progression. It might also explain somatic mosaicism in some non-neoplastic clinical conditions.