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PubMed2006

Histone modifications and the chromatin scaffold for meiotic chromosome architecture.

Chromosomes are capable of remarkable structural adaptability that enables their diverse functions. Histone modifications play pivotal roles in conferring structural diversity to chromosomes by influencing the compactness of chromatin. Several multi-protein complexes bind to chromatin and affect chromosome dynamics, including cohesin, condensin, the chromosome passenger complex, and the synaptonemal complex. The roles of these complexes in promoting chromosome functions include cohesion, condensation and synapsis. It is now crucial to define the relationship between the protein complexes that affect chromosome architecture and the underlying state of the chromatin. During meiosis chromosomes undergo striking morphological changes, including alignment of homologous chromosomes, double-strand break formation and repair, and establishment of meiosis-specific chromosome structures. These dynamic chromosome arrangements are accompanied by the recruitment and expulsion of multi-protein complexes from chromatin. Meiotic chromosome dynamics ensure proper chromosome segregation and production of healthy gametes. Meiosis thus affords an excellent opportunity to determine how histone modifications impact higher order chromosome dynamics by affecting localization and function of chromosome protein complexes. A meiotic mutation in the Drosophila histone kinase, NHK-1, uncovered a critical requirement for histone modifications in chromosome architecture, underscoring the power of this approach.

Animals↗

PubMed1990

Meiosis in asynaptic yeast.

The Saccharomyces cerevisiae red1 mutant fails to assemble synaptonemal complex during meiotic prophase. This mutant displays locus-specific reductions in interchromosomal gene conversion and a moderate reduction in crossing over. The occurrence of a significant amount of meiotically induced recombination in the red1 mutant indicates that the synaptonemal complex is not absolutely required for meiotic exchange. The RED1 gene product is required for intrachromosomal recombination in some assays but not others. Chromosomes that have undergone reciprocal exchange nevertheless nondisjoin in red1 mutants, indicating that crossovers are not sufficient for disjunction. Epistasis studies reveal that HOP1 is epistatic to RED1, and that RED1 acts in an independent pathway from MER1. A model for the function of the RED1 gene product in chromosome synapsis is discussed.

Alleles↗

PubMed1990

Pachytene pairing and oocyte numbers in mice with two single Robertsonian translocations and the male-sterile compound with monobrachial homology.

Oocyte numbers and synaptonemal complexes were studied in two Robertsonian translocations, Rb(6.15)1Ald and Rb(4.6)2Bnr, and their male-sterile compound. Oocyte numbers in the compound were lower than those of either parent, and there was a marked difference between reciprocal crosses. Synaptonemal complexes of homozygous females appeared as 19 bivalents, those of single heterozygotes as 18 bivalents and a trivalent, and those of compound heterozygotes as 17 bivalents and a quadrivalent. Most trivalents were fully paired, whereas the majority of quadrivalents exhibited terminal asynapsis. About one-half of all oocytes had other pairing abnormalities, probably reflecting reduced survivability. Whereas all fully paired quadrivalents were present in cells not showing any pairing anomalies, one-half of the quadrivalents with terminal asynapsis were seen in oocytes with other anomalies. It is suggested that in oocytes destined for atresia, there is a predisposition to synaptic failure of translocation configurations. Additional oocytes are likely to break down because of the deleterious effect of the compound translocation on gametogenesis. This effect seems to be more pronounced in Rb1Ald/Rb2Bnr spermatocytes than in oocytes.

Animals↗

PubMed1988

G-band position effects on meiotic synapsis and crossing over.

An examination of synaptic data from a series of X-autosome translocations and crossover data from an extensive series of autosome-autosome translocations and autosomal inversions in mice has lead to the development of a hypothesis which predicts synaptic and recombinational behavior of chromosomal aberrations during meiosis. This hypothesis predicts that in heterozygotes for chromosomal rearrangements that meiotically align G-light chromatin with G-light chromatin lack of homology will be recognized. If homologous synapsis cannot proceed, synaptonemal complex formation will cease and there will be no physical suppression of crossing over in such rearrangements. However, if a chromosomal rearrangement aligns G-light chromatin with G-dark chromatin at the time of synapsis, lack of homology will not be recognized and synaptonemal complex formation will proceed nonhomologously through the G-dark chromatin. Crossing over will be physically suppressed in this region and this suppression of crossing over will be confined to the chromosome in which the G-light chromatin is nonhomologously synapsed with G-dark chromatin. When G-light chromatin is once again aligned with G-light chromatin, lack of homology again will be recognized and either homologous synapsis will be reinitiated (as in an inversion loop), or will cease altogether (as in some translocations). Unlike the previously described "synaptic adjustment", this nonhomologous synapsis of G-light with G-dark chromatin appears to compete with homologous synapsis during early pachynema.

Animals↗

PubMed2000

Unusual triploid males in a microchromosome-carrying clone of the Amazon molly, Poecilia formosa.

The Amazon molly, Poecilia formosa, is an all-female fish of hybrid origin which reproduces by gynogenesis, i.e. it depends on sperm of males of closely related species to trigger parthenogenetic development of the embryo. Therefore the offspring is clonal and identical to the mother. In rare cases the exclusion mechanism fails and paternal introgression occurs. This may result either in triploid offspring - if the whole haploid chromosome set of the sperm fuses with the diploid egg nucleus - or in siblings with microchromosomes - if only subgenomic amounts of paternal DNA are included. In one of our diploid, microchromosome-carrying laboratory stocks we observed eight triploid individuals which all developed into males. We investigated the mitotic and meiotic chromosomes, the synaptonemal complex (SC), and sperm production of these males, and compared them to males of the gonochoristic parental species (P. latipinna and P. mexicana) and their hybrids. This comparison revealed that P. formosa males are functional males with reduced effective fertility. They show a deviation from the typical 23 bivalents in the synaptonemal complexes as well as in diakinesis due to the triploid state. They produced offspring but only with gynogenetic Amazon molly females. This shows that the probably aneuploid sperm from P. formosa males can trigger parthenogenetic development of unreduced eggs.

Aneuploidy↗

PubMed2003

Temporally and spatially selective loss of Rec8 protein from meiotic chromosomes during mammalian meiosis.

Sister chromatid cohesion is maintained from DNA replication to metaphase-to-anaphase transition by multisubunit protein complexes called cohesin, which include at least four proteins, SMC1alpha, SMC3, Rad21 and either SA1 or SA2, in mammalian somatic cells. We report here the first evidence of the involvement of Rec8 protein, a mammalian homolog of yeast Rec8p, in meiosis-specific chromosome behavior in mammals. In immunoblotting and immunohistochemical analysis using specific antibodies against mouse Rec8, we found that Rec8 was expressed in the testis but not in the kidney or liver; more precisely, it was expressed in spermatocytes and spermatids but not in spermatogonia or other somatic cells. We also found that Rec8 is present in both phosphorylated and dephosphorylated states in vivo. Immunoprecipitation analyses revealed that Rec8 associates with other cohesin proteins, SMC1beta (meiosis-specific protein) and SMC3 and with a component of synaptonemal complexes, SCP3, but not with SMC1alpha. In meiotic chromosome spreads, Rec8 was localized along the axial/lateral elements of the synaptonemal complexes in meiotic prophase from the leptotene to diplotene stages. At later stages, diakinesis and metaphase I, Rec8 was localized along the interstitial axes of chromosomes, including both centromere and arm regions of chromosomes. However, concomitantly with separation of homologous chromosomes in anaphase I, Rec8 was no longer detected along the arm regions, while it persisted on centromere regions up to metaphase II. In anaphase II, the centromeric signals were diminished. We propose from these results that mammalian Rec8 protein, in association with SMC3 and SMC1beta but not SMC1alpha, is involved in meiosis-specific chromosome behavior, and that homologous chromosome separation is triggered by selective loss of Rec8 from chromosome arms in meiosis I, while sister chromatid cohesion is maintained until metaphase II/anaphase II transition by centromeric Rec8 during mammalian meiosis.

Anaphase↗

PubMed1998

Atm deficiency results in severe meiotic disruption as early as leptonema of prophase I.

Infertility is a common feature of the human disorder ataxia-telangiectasia and Atm-deficient mice are completely infertile. To gain further insight into the role of ATM in meiosis, we examined meiotic cells in Atm-deficient mice during development. Spermatocyte degeneration begins between postnatal days 8 and 16.5, soon after entry into prophase I of meiosis, while oocytes degenerate late in embryogenesis prior to dictyate arrest. Using electron microscopy and immunolocalization of meiotic proteins in mutant adult spermatocytes, we found that male and female gametogenesis is severely disrupted in Atm-deficient mice as early as leptonema of prophase I, resulting in apoptotic degeneration. A small number of mutant cells progress into later stages of meiosis, but no cells proceed beyond prophase I. ATR, a protein related to ATM, DMC1, a RAD51 family member, and RAD51 are mislocalized to chromatin and have reduced localization to developing synaptonemal complexes in spermatocytes from Atm-deficient mice, suggesting dysregulation of the orderly progression of meiotic events. ATM protein is normally present at high levels primarily in ova cytoplasm of developing ovarian follicles, and in the nucleus of spermatogonia and to a lesser extent in spermatoctyes, but without localization to the synaptonemal complex. We propose a model in which ATM acts to monitor meiosis by participation in the regulation or surveillance of meiotic progression, similar to its role as a monitor of mitotic cell cycle progression.

Adenosine Triphosphatases↗

PubMed1990

Prophase of meiosis in human spermatocytes analysed by EM microspreading in infertile men and their controls and comparisons with human oocytes.

Observations at the electron microscope (EM) level have been made on 1883 primary spermatocytes from 40 chromosomally normal subfertile men and 566 spermatocytes from 10 fertile controls, using the technique of microspreading. Spermatocytes of infertile men in general showed greater indications of degeneration including higher levels of background silver deposition, nucleolar organising region - XY associations, fragmentation of synaptonemal complexes and overproduction of XY excrescences. A few oligospermic men also showed an immature morphology of the XY pair and/or a reduced extent of XY synapsis. Dissociation of the sex chromosome axes at prophase was found to occur with a much lower frequency than that recorded for separated X and Y chromosomes at metaphase I. In a single spermatocyte, synaptonemal complex formation was observed between Xqter and Yqter, a situation that could enable rare XqYq interchange. A proteinaceous stalked body exists on the Y axis towards its non-pairing end; this structure might have a functional relationship with the gene for spermatogenesis, (AZF), located at the euchromatin/heterochromatin interface. Compared with human oocytes, spermatocytes show fewer anomalies of synapsis, i.e. asynapsed segments or whole axes, non-homologous associations, interchanges, interlocks. These latter data agree well with findings from the mouse.

Adult↗

PubMed1992

Equalization of Z and W axes in chicken and quail oocytes.

The different morphological types of ZW pairs have been classified in three main types according to the relative extension of the free segment of the Z axis: 1, "long asynaptic segment;" 2, "medium asynaptic segment;" and 3, "equalized." Pre- and post-pairing types have also been defined. Frequencies of each type were determined at day 20 and day 21 of incubation, and one and three days after hatching. The changing frequencies and the morphological transitions observed show a definite sequence of ZW types that can be used as a timetable for pachytene substaging. Measurements made on each ZW type show that the Z axis of the chicken shortens from 20.6 microns to 13.1 microns. This shortening occurs both in the free segment (at a higher rate) and in the paired segment (at a lower rate). The synaptonemal complex becomes elongated while adjustment occurs. The equalized Z axis makes many twists around the W axis. However, a segment 1 micron long from the synaptic terminus is free from twists and is assumed to be the homologously paired region. The ZW pair of the quail shows a similar behavior but equalization of the Z and W axes ends earlier and forms a straighter synaptonemal complex as compared with the chicken. In both species a recombination nodule is strictly localized near the synaptic terminus. In the ZW pair of the quail the average location of this nodule is 0.14 microns from the synaptic terminus. The meiotic behavior of ZW pairs in birds may be conserved.

Animals↗

PubMed1989

Synaptic adjustment in Peromyscus beatae (Rodentia: Cricetidae) heterozygous for interstitial heterochromatin.

Chromosomal pairing and chiasma formation were studied two individuals of Peromyscus beatae heterozygous for the presence of a large block of interstitial heterochromatin. Although the modified chromosome was of medium size, analysis of C-banded diakinetic configurations revealed that it was the homolog of one of the smallest autosomes. Analysis of silver stained synaptonemal complexes indicated that synapsis was either unidirectional from initiation at one set of telomeres or was bidirectional from initiation at both sets of telomeres. Each pattern resulted in characteristic heteromorphic pairing configurations (interstitial asynapsis or terminally positioned unpaired segments) in early pachynema. These configurations underwent synaptic adjustment and, by mid-pachynema, the lateral elements of the polymorphic bivalent either appeared typical of homomorphic bivalents or exhibited regional heteropycnosis in one or both axes. Synaptonemal complex data for Peromyscus and many other mammalian species reflect an apparent need for fully paired, linear bivalents prior to the end of pachynema.

Animals↗

PubMed1997

Cyclophosphamide-induced aberrations of chromosome pairing in pachytene oocytes.

The aim of this study was to characterise damage to synaptonemal complexes in oocytes following cyclophosphamide exposure. Pregnant mice were treated with three different doses of cyclophosphamide (10, 30 and 50 mg/kg body weight) at day 13 of gestation, when oogonia and very early meiotic cells in the female fetuses are found. Primary oocytes were analysed by light- and electron microscopy at gestational day 17 to reveal effects of the alkylating agent on the chromosomal pairing behaviour. Our pachytene analysis demonstrated that the fraction of cells with lesions of synaptonemal complexes, partial asynapsis and desynaptic bivalents were significantly increased over the levels in the control group. The frequency of alterations was similar at doses of 30 and 50 mg/kg. In addition, a significant increase in frequency of univalents over the base level became evident, showing the highest incidence on a dose of 50 mg/kg.

Animals↗

PubMed2005

DNA double-strand breaks and homology search: inferences from a species with incomplete pairing and synapsis.

The relationship between meiotic recombination events and different patterns of pairing and synapsis has been analysed in prophase I spermatocytes of the grasshopper Stethophyma grossum, which exhibit very unusual meiotic characteristics, namely (1) the three shortest bivalents achieve full synapsis and do not show chiasma localisation; (2) the remaining eight bivalents show restricted synapsis and proximal chiasma localisation, and (3) the X chromosome remains unsynapsed. We have studied by means of immunofluorescence the localisation of the phosphorylated histone H2AX (gamma-H2AX), which marks the sites of double-strand breaks; the SMC3 cohesin subunit, which is thought to have a close relationship to the development of the axial element (a synaptonemal complex component); and the recombinase RAD51. We observed a marked nuclear polarization of both the maturation of SMC3 cohesin axis and the ulterior appearance of gamma-H2AX and RAD51 foci, these being exclusively restricted to those chromosomal regions that first form cohesin axis stretches. This polarised distribution of recombination events is maintained throughout prophase I over those autosomal regions that are undergoing, or about to undergo, synapsis. We propose that the restricted distribution of recombination events along the chromosomal axes in the spermatocytes is responsible for the incomplete presynaptic homologous alignment and, hence, for the partial synaptonemal complex formation displayed by most bivalents.

Animals↗

PubMed1991

XY-trivalent association and synaptic anomalies in a male carrier of a Robertsonian t(13;14) translocation.

Synaptonemal complexes were analysed in 91 pachytenes from a patient carrier of a Robertsonian translocation 45XY, t(13;14). Electron microscopic studies were carried out in 36 pachytenes. In some cases, sequential light microscopy/electron microscopic images were obtained (16/36). As the resolution of the light microscope does not allow an accurate characterization of anomalies, only EM images have been taken into account. Association of the trivalent and the sex vesicle was detected in 8 of 36 pachytenes (22.2%). Synaptic anomalies (25%), fragmentation of synaptonemal complexes (11.1%) or both (2.8%) were seen in 14 of 36 nuclei. According to previous studies, synaptic anomalies may produce a spermatogenic arrest. However, it has been suggested that synaptic anomalies may be the result of a process of cell degeneration, and not its cause. The non-random relationship between the sex vesicle and the translocation trivalent may result from the tendency of unpaired segments to associate with each other after homology search is relaxed and heterologous synaptic adjustment is allowed.

Adult↗

PubMed1994

Molecular perspectives of chromosome pairing at meiosis.

Ideas about the mechanisms that regulate chromosome pairing, recombination, and segregation during meiosis have gained in molecular detail over the last few years. The purpose of this article is to survey briefly the shifts in paradigms and experiments that have generated new perspectives. It has never been very clear what it is that brings together the homologous chromosomes at meiotic prophase. For a while it appeared that the synaptonemal complex might be the nuclear organelle responsible for synapsis, but the supporting evidence has not been entirely convincing. Whatever the mechanism, it has always been assumed that homologous synapsis creates the opportunity for homologous DNA sequences to initiate recombination. At present, alternative ideas are developing. Attractive is the concept that double strand DNA repair mechanisms, that find and use the undamaged homologue for repair, have evolved into a meiotic mechanism for the recognition and pairing of homologous sequences. Subsequent intimate synapsis of homologous chromosomes in the context of the synaptonemal complex may serve later functions in the regulation of interference and segregation at first anaphase. A number of areas that are being tested at present and some that may be investigated in the future are discussed at the end of the review.

Animals↗

PubMed1994

Saccharomyces cerevisiae cells lacking the homologous pairing protein p175SEP1 arrest at pachytene during meiotic prophase.

Saccharomyces cerevisiae cells containing null mutations in the SEP1 gene, which encodes the homologous pairing and strand exchange protein p175SEP1, enter pachytene with a delay. They arrest uniformly at this stage of meiotic prophase, probably revealing a checkpoint in the transition from pachytene to meiosis I. At the arrest point, the cells remain largely viable and are cytologically characterized by the duplicated but unseparated spindle pole bodies of equal size and by the persistence of the synaptonemal complex, a cytological marker for pachytene. In addition, fluorescence in situ hybridization revealed that in arrested mutant cells maximal chromatin condensation and normal homolog pairing is achieved, typical for pachytene in wild type. A hallmark of meiosis is the high level of homologous recombination, which was analyzed both genetically and physically. Formation and processing of the double-strand break intermediate in meiotic recombination is achieved prior to arrest. Physical intragenic (conversion) and intergenic (crossover) products are formed just prior to, or directly at, the arrest point. Structural deficits in synaptonemal complex morphology, failure to separate spindle pole bodies, and/or defects in prophase DNA metabolism might be responsible for triggering the observed arrest. The pachytene arrest in sep1 cells is likely to be regulatory, but is clearly different from the RAD9 checkpoint in meiotic prophase, which occurs prior to the pachytene stage.

Chromosomes, Fungal↗

PubMed1985

Meiotic chromosome pairing in the normal human female.

The synaptonemal complexes of oocytes from 16-22 week human fetuses were spread using detergent and silver-stained for examination by light microscopy. Zygotene chromosome synapsis generally begins at the telomeres, without obvious prealignment, and proceeds towards the centromeres. Synapsis is not synchronous and longer bivalents may sometimes be completely paired before shorter ones. At pachytene, when pairing is usually complete, some regions presumed to correspond to the heterochromatic blocks of chromosomes 1.9 and 16 may remain unpaired. Residual univalents are uncommon, and little interlocking is evident at this stage. Desynapsis indicating the beginning of diplotene frequently begins at the telomeres, although there is a general relaxation of pairing throughout the bivalents which become increasingly diffuse as diplotene proceeds. The total synaptonemal complex complement length at pachytene in the female is 519 micron, which is about twice that found in the human male. The implications of these results for genetic mapping are discussed.

Chromosome Mapping↗

PubMed2005

XY chromosomal bivalent: nucleolar attraction.

Nucleolar organization by autosomal bivalents occurs during male meiotic prophase in mammalian species. During late leptotene-early zygotene stages, several autosomal bivalents are engaged in ribosomal RNA synthesis. At pachytene stage, nucleolar masses detach from the sites of primary autosomal origin, relocate close to the XY chromosomal pair, and nucleolar components become segregated. In early pachytene, an extensive synaptonemal complex at the pseudoautosomal region, links X and Y chromosomes in close juxtaposition along most of the length of the Y chromosome, except for a terminal region of the Y that diverges from the pairing region. As meiotic prophase advances, X and Y chromosomes progressively desynapse and, at diplotene, the XY pair is associated end-to-end. Xmr (Xlr-related, meiosis regulated) is a protein component of the nucleolus associated to the XY pair and of the asynapsed portions of the X and Y axial cores. Xmr, like SCP3, is a component of the lateral element of the synaptonemal complex. Both share structural homology in their C-terminal region. This region contains several putative coiled-coil domains known to mediate heterodimeric protein-protein interactions and to provide binding sites to regulatory proteins. Like Xmr, the tumor repressor protein BRCA1 is present along the unsynapsed cores of the XY bivalent. Both Xmr and BRCA1 have been implicated in a mechanism leading to chromatin condensation and transcription inactivation of the XY bivalent. The BRCA1-ATR kinase complex, as recent research suggests, triggers the phosphorylation of histone H2AX, which predominates in the condensed chromatin of the XY chromosomal pair. Xmr is not present in the XY bivalent when the expression of histone H2AX is deficient. The role of Xmr in chromatin condensation of the XY bivalent has not been determined. The partial structural homology of SCP3 and Xmr, their distribution along the unsynapsed axial cores of the X and Y chromosomes, and the presence of Xmr in the XY pair-associated nucleolus raises the possibility that Xmr, and other proteins including protein kinases, may be recruited to the nucleolus to perform functions related to chromosomal synapsis, chromatin condensation and recombination processes, as well as cell cycle progression.

Animals↗

PubMedDate not supplied

Robertsonian heterozygosity and male sterility.

Meiotic and synaptonemal complex studies using electron microscopy were carried out on infertile man with a 13/14 translocation. Synaptonemal complex analysis showed a typical trivalent with incomplete pairing of the acrocentric elements. The sterilizing effect caused by the failure of synapsis is discussed. This pairing failure has a diagnostic value and a poor prognosis.

Adult↗