Premeiotic DNA Synthesis and the Time of Chromosome Pairing in Locusta migratoria

Labeled spermatocytes from locusts killed 0, 1, 2, 3, 4, and 6 days after exposure to tritiated thymidine were examined with the electron microscope to determine their meiotic stage. It was found that tritiated thymidine uptake in young spermatocyte nuclei is completed some 24 hours before synaptinemal complexes are formed. It is concluded that in the locust, premeiotic DNA synthesis is completed well in advance of pairing of homologous chromosomes as marked by the formation of synaptinemal complexes. The G-1 and the S-phase are in the order of 24 hours each, while meiotic prophase lasts four to five days under the conditions used.

Sex-specific crossover patterns in Zebrafish (Danio rerio)

Some species display intersex variation in their rate of meiotic recombination, where recombination is usually suppressed in the heterogametic sex. Although no heteromorphic sex chromosomes have been detected in zebrafish (Danio rerio), genetic analysis has indicated a lower frequency of recombination in males relative to females. Our study of the meiotic recombination pattern in female zebrafish indicates that adult females have only a few meiotic oocytes that are found in groups in the ventral zone of the ovarian surface. We used antibody staining of human mutL homolog 1 (MLH1) protein to mark the sites of putative chiasmata to seek a physical basis for the pattern of recombination and its relative frequency in both sexes. We report that MLH1 foci are found mostly in distal regions of the synaptonemal complexes (SCs) in males, but tend to be more evenly distributed in females. Our cytological analysis yields a ratio of MLH1 foci per chromosome in males versus females of 1:1.55. This lower level of recombination in males is in general agreement with previously published results from linkage map analysis. However, the similar ratio of MLH1 foci per unit length of SCs in both sexes demonstrates a correlation between SC length and the frequency of recombination rather than a mechanism that suppresses recombination in males. Thus, chiasma interference seems to provide similar expression in males and females in agreement with the situation in humans, where oocytes with longer SCs display a higher level of recombination that is not a consequence of more closely spaced crossovers along the SCs.

Male mouse meiotic chromosome cores deficient in structural proteins SYCP3 and SYCP2 align by homology but fail to synapse and have possible impaired specificity of chromatin loop attachment

The targeted deletion of the meiotic chromosome core component MmSYCP3 results in chromosome synaptic failure at male meiotic prophase, extended meiotic chromosomes, male sterility, oocyte aneuploidy and absence of the MmSYCP2 chromosome core component. To test the functions of SYCP2 and SYCP3 proteins in the cores, we determined the effect of their deletion on homology recognition by whole chromosome painting and the effect on chromatin loop attachment to the cores with endogenous and exogenous sequences. Because we observed that the alignment of cores is between homologs, it suggested that alignment is not a function of the chromosome core components but might be mediated by chromatin-chromatin interactions. The alignment function therefore appears to be separate from intimate synapsis function of homologous cores that is observed to be defective in the SYCP3-/- males. To examine the functions of the SYCP2 and 3 core proteins in chromatin loop attachment, we measured the loop sizes of the centromeric major satellite chromatin and the organization of an exogenous transgene in SYCP3+/+ and SYCP3-/- males. We observed that these satellite chromatin loops have a normal appearance in SYCP3-/- males, but the loop regulation of a 2-Mb exogenous lambda phage insert appears to be altered. Normally the insert fails to attach to the core except by flanking endogenous sequences, but in the absence of SYCP2 and SYCP3, there appears to be multiple attachments to the core. This suggests that the selective preference for the attachment of mouse sequences to the chromosome core in the wild-type male is impaired in the SYCP3-/- male. Apparently the SYCP2 and SYCP3 proteins function in the specificity of chromatin attachment to the chromosome core.

CLIP-50 immunolocalization during mouse spermiogenesis suggests a role in shaping the sperm nucleus

The spermatid nucleus and cytoplasm undergo dramatic morphological modifications during spermatid differentiation into mature sperm. Some of the external force causing this nuclear shaping is generated by a microtubular structure termed the manchette, which attaches to the perinuclear ring of the spermatid. Here, we report the isolation and characterization of a protein component of this perinuclear ring in an immunological screening of a mouse testis cDNA library. We termed this protein CLIP-50 because of its high similarity at the amino acid level to the C-terminal region of the microtubule-binding protein CLIP-170/restin. CLIP-50 lacks the characteristic microtubule-binding motif, but retains a portion of the predicted coiled-coiled domain and the metal-binding motif. The CLIP-50 transcript and protein are abundant in testis. The protein is also expressed in heart, lung, kidney, and skin. A distinct size variant exists in brain. In the spermatids, CLIP-50 protein localizes specifically to the centriolar region where the sperm tail originates and to the perinuclear ring from which the manchette emerges. CLIP-50 staining is retained in the ring throughout its migration over the surface of the nucleus which accompanies the nuclear shaping into its characteristic sperm configuration. This localization pattern indicates a very specific function for this novel CLIP derivative during mouse spermiogenesis.

Characterization of a novel human SMC heterodimer homologous to the Schizosaccharomyces pombe Rad18/Spr18 complex

The structural maintenance of chromosomes (SMC) protein encoded by the fission yeast rad18 gene is involved in several DNA repair processes and has an essential function in DNA replication and mitotic control. It has a heterodimeric partner SMC protein, Spr18, with which it forms the core of a multiprotein complex. We have now isolated the human orthologues of rad18 and spr18 and designated them hSMC6 and hSMC5. Both proteins are about 1100 amino acids in length and are 27-28% identical to their fission yeast orthologues, with much greater identity within their N- and C-terminal globular domains. The hSMC6 and hSMC5 proteins interact to form a tight complex analogous to the yeast Rad18/Spr18 heterodimer. In proliferating human cells the proteins are bound to both chromatin and the nucleoskeleton. In addition, we have detected a phosphorylated form of hSMC6 that localizes to interchromatin granule clusters. Both the total level of hSMC6 and its phosphorylated form remain constant through the cell cycle. Both hSMC5 and hSMC6 proteins are expressed at extremely high levels in the testis and associate with the sex chromosomes in the late stages of meiotic prophase, suggesting a possible role for these proteins in meiosis.

Checkpoint and DNA-repair proteins are associated with the cores of mammalian meiotic chromosomes

Meiotic checkpoints are manifested through protein complexes capable of detecting an abnormality in chromosome metabolism and signaling it to effector molecules that subsequently delay or arrest the progression of meiosis. Some checkpoints act during the first meiotic prophase to monitor the repair of chromosomal DSBs, predominantly by meiotic recombination, or to ensure the correct establishment of synapsis and its well-timed dissolution. In mammals, a number of checkpoint and repair proteins localize to the meiotic chromosomal cores, sometimes in the context of the synaptonemal complex (SC). Here we discuss possible functions of these proteins in the accomplishment of meiotic recombination and normal progression of the meiotic pathway. Also, we present arguments for a structural role of cores and SCs in the assembly of the repair and checkpoint protein complexes on the chromosomes.

The genomic structure of SYCP3, a meiosis-specific gene encoding a protein of the chromosome core

SYCP3 localizes to the lateral elements of the synaptonemal complex and is essential for male meiosis. The genomic structure of SYCP3 consists of nine exons spanning approximately 14 kb. In mouse and rat, but not in hamster, the putative translation start of SYCP3 is present in the first exon. The putative promoter of SYCP3 was also cloned and shown to drive transcription of a reporter gene in somatic cells.

Identification of the mouse beta'-COP Golgi component as a spermatocyte autoantigen in scleroderma and mapping of its gene Copb2 to mouse chromosome 9

In an immunological screening of a mouse testicular cDNA library with a human CREST serum we isolated five overlapping cDNA clones encoding the mouse homolog of a Golgi coatomer complex protein (accession number AF043120), designated beta'-COP in bovine and p102 in humans. We generated antibodies against this protein which specifically recognize the Golgi apparatus of mouse spermatocytes. FISH analyses assigned the beta'-COP gene Copb2 to mouse Chromosome 9, region E3-F1. Our results demonstrate that CREST sera can contain antibody components against Golgi proteins as well as against nuclear proteins.

Expression and nuclear localization of BLM, a chromosome stability protein mutated in Bloom's syndrome, suggest a role in recombination during meiotic prophase

Bloom's syndrome (BS) is a recessive human genetic disorder characterized by short stature, immunodeficiency and elevated risk of malignancy. BS cells have genomic instability and an increased frequency of sister chromatid exchange. The gene mutated in BS, BLM, encodes a 3′-5′ helicase (BLM) with homology to bacterial recombination factor, RecQ. Human males homozygous for BLM mutations are infertile and heterozygous individuals display increased frequencies of structural chromosome abnormalities in their spermatozoa. Also, mutations in the Saccharomyces cerevisiae homolog of BLM, Sgs1, cause a delay in meiotic nuclear division and a reduction in spore viability. These observations suggest that BLM may play a role during meiosis. Our antibodies raised against the C terminus of the human protein specifically recognize both mouse and human BLM in western blots of cell lines and in successive developmental stages of spermatocytes, but fail to detect BLM protein in a cell line with a C-terminally truncated protein. BLM protein expression and location are detected by immunofluorescence and immunoelectron microscopy as discrete foci that are sparsely present on early meiotic prophase chromosome cores, later found abundantly on synapsed cores, frequently in combination with the recombinases RAD51 and DMC1, and eventually as pure BLM foci. The colocalization of RAD51/DMC1 with BLM and the statistically significant excess of BLM signals in the synapsed pseudoautosomal region of the X-Y chromosomes, which is a recombinational hot spot, provide indications that BLM protein may function in the meiotic recombination process.

The association of ATR protein with mouse meiotic chromosome cores

The ATR (ataxia telangiectasia- and RAD3-related) protein is present on meiotic prophase chromosome cores and paired cores (synaptonemal complexes, SCs). Its striking characteristic is that the protein forms dense aggregates on the cores and SCs of the last chromosomes to pair at the zygotene-pachytene transition. It would appear that the ATR protein either signals delays in pairing or it is directly involved in the completion of the pairing phase. Atm-deficient spermatocytes, which are defective in the chromosome pairing phase, accumulate large amounts of ATR. The behaviour of ATR at meiotic prophase sets it apart from the distribution of the RAD51/DMC1 recombinase complex and our electron microscope observations confirm that they do not co-localize. We failed to detect ATM in association with cores/SCs and we have reported elsewhere that RAD1 protein does not co-localize with DMC1 foci. The expectation that putative DNA-damage checkpoint proteins. ATR, ATM and RAD1, are associated with RAD51/DMC1 recombination sites where DNA breaks are expected to be present, is therefore not supported by our observations.

Meiotic activation of rat pachytene spermatocytes with okadaic acid: the behaviour of synaptonemal complex components SYN1/SCP1 and COR1/SCP3

The phosphatase inhibitor okadaic acid accelerates meiotic events in rodent germ cells in culture. Isolated pachytene spermatocytes treated with okadaic acid proceed to a metaphase I arrest in a few hours as opposed to the similar process in vivo, which requires several days. Leptotene/zygotene spermatocytes cannot be activated in this way, suggesting that okadaic acid enables cells to bypass a sensor of the meiotic progression, which is pachytene specific. We monitored the chromosome behaviour accompanying the transition to metaphase I in rat spermatocytes with antibodies against COR1/SCP3, a component of the meiotic chromosome cores, and against the synaptic protein, SYN1/SCP1. Okadaic acid induced a rapid synaptonemal complex dissolution and bivalent separation, followed by chromosome condensation and chiasmata formation, similar to the succession of events in untreated cells. The similarity between meiosis I induced with okadaic acid and the meiosis I events in vivo extends to the dissolution of the nuclear membrane and the disappearance of the microtubule network at the onset of metaphase I. This cell culture system provides a model for the in vivo transition from pachytene to metaphase I and therefore can be used in the study of this transition at the molecular level. The effect of okadaic acid is most likely mediated by the activation of tyrosine kinases, as addition of genistein, a general tyrosine kinase inhibitor, completely abolishes the observed effect of okadaic acid on chromosome metabolism. The okadaic acid-induced progression to the metaphase I arrest is not affected by the inhibition of protein synthesis. However, pachytene spermatocytes incubated in the presence of protein synthesis inhibitors for 6 hours show loss of synapsis which is abnormal in that it is not accompanied by chiasmata formation. The two meiosis-specific proteins, SYN1/SCP1 and COR1/SCP3, are efficiently phosphorylated in vitro by extracts from isolated pachytene cells. Extracts from cells that have reached metaphase I upon okadaic acid treatment, with concomitant displacement of SYN1/SCP1 and COR1/SCP3 from their chromosomes, do not have this capability. These data support the hypothesis that phosphorylation of SYN1/SCP1 and COR1/SCP3 targets their removal from the chromosomes and that activity of the kinases involved correlates with the presence of these two proteins on the chromosomes.

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.

Human and mouse homologs of Schizosaccharomyces pombe rad1(+) and Saccharomyces cerevisiae RAD17: linkage to checkpoint control and mammalian meiosis

Preventing or delaying progress through the cell cycle in response to DNA damage is crucial for eukaryotic cells to allow the damage to be repaired and not incorporated irrevocably into daughter cells. Several genes involved in this process have been discovered in fission and budding yeast. Here, we report the identification of human and mouse homologs of the Schizosaccharomyces pombe DNA damage checkpoint control gene rad1(+) and its Saccharomyces cerevisiae homolog RAD17. The human gene HRAD1 is located on chromosome 5p13 and is most homologous to S. pombe rad1(+). This gene encodes a 382-amino-acid residue protein that is localized mainly in the nucleus and is expressed at high levels in proliferative tissues. This human gene significantly complements the sensitivity to UV light of a S. pombe strain mutated in rad1(+). Moreover, HRAD1 complements the checkpoint control defect of this strain after UV exposure. In addition to functioning in DNA repair checkpoints, S. cerevisiae RAD17 plays a role during meiosis to prevent progress through prophase I when recombination is interrupted. Consistent with a similar role in mammals, Rad1 protein is abundant in testis, and is associated with both synapsed and unsynapsed chromosomes during meiotic prophase I of spermatogenesis, with a staining pattern distinct from that of the recombination proteins Rad51 and Dmc1. Together, these data imply an important role for hRad1 both in the mitotic DNA damage checkpoint and in meiotic checkpoint mechanisms, and suggest that these events are highly conserved from yeast to humans.

Partial rescue of the prophase I defects of Atm-deficient mice by p53 and p21 null alleles

Patients with the human disorder ataxia-telangiectasia (A-T; refs 1,2) and Atm-deficient mice have a pleiotropic phenotype that includes infertility. Here we demonstrate that male gametogenesis is severely disrupted in Atm-deficient mice in the earliest stages of meiotic prophase I, resulting in apoptotic degeneration. Atm is required for proper assembly of Rad51 onto the chromosomal axial elements during meiosis. In addition, p53, p21 and Bax are elevated in testes from Atm-deficient mice. To determine whether these elevated protein levels are important factors in the meiotic disruption of Atm-deficient mice, we analysed the meiotic phenotype of Atm/p53 or Atm/p21 double mutants. In these double mutants, meiosis progressed to later stages but was only partly rescued. Assembly of Rad51 foci on axial elements remained defective, and gametogenesis proceeded only to pachytene of prophase I. Previous results demonstrated that mice homozygous for a null mutation in Rad51 (ref. 6) display an early embryonic lethal phenotype that can be partly rescued by removing p53 and/or p21. Because Atm-deficient mice are viable but completely infertile, our studies suggest that the Rad51 assembly defects and elevated levels of p53, p21 and Bax represent tissue-specific responses to the absence of Atm.

Chromosome cores and chromatin at meiotic prophase

We review the synaptonemal complex, SC, of the synapsed homologous chromosomes at meiotic prophase in insects and mammals in terms of its formation, and the association of specific chromatin elements with the synaptonemal complexes. The focus is: (1) The SC as visualized with a variety of techniques; (2) The nature of the chromatin loops where they are associated with the SCs--the bases of the loops may be instrumental in recombinant events judging from the presence of Rad51 protein and late recombination nodules at the SCs; (3) Differences in DNA content of similarly sized loops; (4) Requirements for chromatin attachment to the chromosome cores, requirements that are apparently lacking in foreign DNA inserts; (5) Regulation of loop size by the position along the chromosome; (6) The structural correlates of recombination at the SCs--these comments are based on studies of SC structure, DNA-core protein associations, fluorescent in situ hybridization to visualize specific DNA segments, and fluorescent immunocytology to visualize the chromosome core proteins.

Identical megabase transgenes on mouse chromosomes 3 and 4 do not promote ectopic pairing or synapsis at meiosis

To investigate ectopic interactions at the chromatin level, we examined the meiotic organization of 1-2 mb phage lambda transgenes on mouse chromosomes 3 and 4 by fluorescence in situ hybridization in combination with immunocytology of meiotic chromosomes. At early meiotic prophase, the transgenes are sufficiently dispersed in the nuclear volume to permit potential DNA-DNA interactions, but no synaptonemal complexes form between the sites of transgenes residing on different chromosomes. At later stages, when the chromatin is more condensed, the transgenes on different chromosomes are not preferentially associated as they are when they are on the same chromosome. At diplotene and metaphase I, no formations were observed that could be interpreted as reciprocal crossovers or chiasmata between the transgenes located on chromosomes 3 and 4. It appears that in normal fertile mice, a I- to 2-mb homology is insufficient to initiate synapsis between nonhomologs, and it is concluded that homology is assessed within the broader context of the chromosome to initiate synapsis at meiotic prophase.

Protein-protein interactions in the synaptonemal complex

In mammalian systems, an approximately M(r) 30,000 Cor1 protein has been identified as a major component of the meiotic prophase chromosome cores, and a M(r) 125,000 Syn1 protein is present between homologue cores where they are synapsed and form the synaptonemal complex (SC). Immunolocalization of these proteins during meiosis suggests possible homo- and heterotypic interactions between the two as well as possible interactions with yet unrecognized proteins. We used the two-hybrid system in the yeast Saccharomyces cerevisiae to detect possible protein-protein associations. Segments of hamsters Cor1 and Syn1 proteins were tested in various combinations for homo- and heterotypic interactions. In the cause of Cor1, homotypic interactions involve regions capable of coiled-coil formation, observation confirmed by in vitro affinity coprecipitation experiments. The two-hybrid assay detects no interaction of Cor1 protein with central and C-terminal fragments of Syn1 protein and no homotypic interactions involving these fragments of Syn1. Hamster Cor1 and Syn1 proteins both associate with the human ubiquitin-conjugation enzyme Hsubc9 as well as with the hamster Ubc9 homologue. The interactions between SC proteins and the Ubc9 protein may be significant for SC disassembly, which coincides with the repulsion of homologs by late prophase I, and also for the termination of sister centromere cohesiveness at anaphase II.

FISH technology in chromosome and genome research

Fluorescent in situ hybridization technology is one of the most exciting and versatile research tools to be developed in recent years. It has enabled research to progress at a phenomenal rate in diverse areas of basic research as well as in clinical medicine. Fluorescent in situ hybridization has applications in physical mapping, the study of nuclear architecture and chromatin packaging, and the investigation of fundamental principles of biology such as DNA replication, RNA processing, gene amplification, gene integration and chromatin elimination. This review highlights some of these areas and provides source material for the reader who seeks more information on a specific field.

Regulation of meiotic chromatin loop size by chromosomal position

At meiotic prophase, chromatin loops around a proteinaceous core, with the sizes of these loops varying between species. Comparison of the morphology of sequence-related inserts at different sites in transgenic mice demonstrates that loop size also varies with chromosomal geography. Similarly, chromatin loop lengths differ dramatically for interstitially and terminally located hamster telomeric sequences. Sequences, telomeric or otherwise, located at chromosome termini, closely associate with the meiotic proteinaceous core, forming shorter loops than identical interstitial sequences. Thus, we present evidence that different chromatin packaging mechanisms exist for interstitial versus terminal chromosomal regions, which act separately from those operating at the level of the DNA sequence. Chromosomal position plays the dominant role in chromatin packaging.

Histones H1 and H4 of surface-spread meiotic chromosomes

The chromatin conformation of somatic and meiotic chromosomes is, at least in part, a function of electrostatic nucleosome interactions that are mediated by transient acetylation of the histone H4 N-terminal domain and phosphorylation of histone H1. The distribution of those histones in the chromatin of meiotic chromosomes is reported here. Antibodies to testis-specific histone 1, H1t, detect H1t in the chromatin of mouse meiotic prophase chromosomes only after synapsis and synaptonemal complex (SC) assembly is completed and before core separation is initiated. The H1t protein is evenly distributed over euchromatin, heterochromatin and the SC. Antibodies to acetylated lysine residues 5, 12 or 16 of histone H4, indicate that the euchromatin is more acetylated than the centromeric heterochromatin. The pattern is most pronounced for acetylated residue 5 and least for 16. Antibodies to phosphorylated H1 epitopes do not react with chromatin but, instead, recognize the chromosome cores and SCs. Possibly these are not phosphorylated histone H1 epitopes, but SC proteins with similar potentially phosphorylatable sequences such as KTPTK of the synaptic protein Syn1.

Immunocytology of chiasmata and chromosomal disjunction at mouse meiosis

Immunocytological and in situ hybridization evidence supports the hypothesis that at meiosis of chiasmate organisms, chromosomal disjunction and reductional segregation of sister centromeres are integrated with synaptonemal complex functions. The Mr 125,000 synaptic protein, Syn1, present between cores of paired homologous chromosomes during pachytene of meiotic prophase, is lost from synaptonemal complexes coordinately with homolog separation at diplotene. Separation is constrained by exchanges between non-sister chromatids, the chiasmata. We show that the Mr 30,000 chromosomal core protein, Cor1, associated with sister chromatid pairs, remains an axial component of post-pachytene chromosomes until metaphase I. We demonstrate that at this time the chromatin loops are still attached to their cores. A reciprocal exchange event between two homologous non-sister chromatids is therefore immobilized by anchorage of sister chromatids to their respective cores. Cores thus contribute to the sister chromatid cohesiveness required for maintenance of chiasmata and proper chromosomal disjunction. Cor1 protein accumulates in juxtaposition to pairs of sister centromeres during metaphase I. Presumably, independent movement of sister centromeres at anaphase I is restricted by Cor1 anchorage. That reductional separation of sister centromeres is mediated by Cor1, is supported by the dissociation of Cor1 from separating sister centromeres at anaphase II and by its absence from mitotic anaphases.

Organization of heterologous DNA inserts on the mouse meiotic chromosome core

With simultaneous immunofluorescence and fluorescent in situ hybridization, we have determined the organization of native and heterologous DNA sequences relative to the cores of meiotic prophase chromosomes. The normal chromatin organization is demonstrated with probes of mouse sequences: a cosmid probe that identifies unique sequences and a 720 kb yeast artificial chromosome (YAC) probe that recognizes a specific region of the chromatin domain. The heterologous DNA consists of a 1.8 Mb insertion of 40 tandem head-to-tail phage lambda LIZ vectors and of 11.4 Mb of bacterial/mouse DNA repeats. The lengthy lambda insert is unusual in that it is not contained in the chromatin domain of chromosome 4 and in that it fails to form direct attachments to the chromosome core. The ends are attached indirectly, probably by means of the flanking mouse sequences. At late stages of meiotic prophase, while the terminal attachments remain the same, the lambda DNA becomes highly compacted. Apparently, higher order condensation and core attachment are independent processes. The condensed inserts relax precociously at metaphase I. In the mouse heterozygous for the insert, the two sister inserts are usually merged, as are all four inserts in the homozygous mouse. Evidently chromatin loops with identical sequences can become associated during meiotic prophase. Mouse sequences within a heterologous DNA insert (repeats of bacterial plasmid pBR322 with a mouse beta-globin insert) were observed to restore some degree of core attachment.

Synaptonemal complex proteins: occurrence, epitope mapping and chromosome disjunction

We have used polyclonal antibodies against fusion proteins produced from cDNA fragments of a meiotic chromosome core protein, Cor1, and a protein present only in the synapsed portions of the cores, Syn1, to detect the occurrence and the locations of these proteins in rodent meiotic prophase chromosomes. The 234 amino acid Cor1 protein is present in early unpaired cores, in the lateral domains of the synaptonemal complex and in the chromosome cores when they separate at diplotene. A novel observation showed the presence of Cor1 axial to the metaphase I chromosomes and substantial amounts of Cor1 in association with pairs of sister centromeres. The centromere-associated Cor1 protein becomes dissociated from the centromeres at anaphase II and it is not found in mitotic metaphase centromeres. The extended presence of Cor1 suggests that it may have a role in chromosome disjunction by fastening chiasmata at metaphase I and by joining sister kinetochores, which ensures co-segregation at anaphase I. Two-colour immunofluorescence of Cor1 and Syn1 demonstrates that synapsis between homologous cores is initiated at few sites but advances rapidly relative to the establishment of new initiation sites. If the rapid advance of synapsis deters additional initiation sites between pairs of homologues, it may provide a mechanism for positive recombination interference. Immunogold epitope mapping of antibodies to four Syn1 fusion proteins places the amino terminus of Syn1 towards the centre of the synaptonemal complex while the carboxyl terminus extends well into the lateral domain of the synaptonemal complex. The Syn1 fusion proteins have a non-specific DNA binding capacity. Immunogold labelling of Cor1 antigens indicates that the lateral domain of the synaptonemal complex is about twice as wide as the apparent width of lateral elements when stained with electron-dense metal ions. Electron microscopy of shadow-cast surface-spread SCs confirms the greater width of the lateral domain. The implication of these dimensions is that the proteins that comprise the synaptic domain overlap with the protein constituents of the lateral domains of the synaptonemal complex more than was apparent from earlier observations. This arrangement suggests that direct interactions might be expected between some of the synaptonemal complex proteins.

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.

Isolation and characterization of a cDNA encoding a synaptonemal complex protein

A gene encoding a 65-kilodalton antigen of the rat synaptonemal complex, SC65, has been cloned by screening rat testis λgt11 and λZAPII cDNA expression libraries using polyclonal antibodies against rat synaptonemal complex proteins. The longest open reading frame, initiating at an ATG codon in the cDNA, encodes a protein of 431 amino acids, with a relative molecular mass of 50 000. Immunological analysis locates the SC65 gene product on the synaptonemal complex between the pairing faces of the parallel aligned cores of homologous chromosomes in spermatocytes. Of the rat tissues examined, the SC65 gene is transcribed in testis, brain, and heart at similar levels, and in the liver at a much lower level. The DNA sequence extending about 80 base pairs downstream of the translation termination codon has 93% similarity to the identifier sequence present in the rat genome in 1 × 105 – 1.5 × 105 copies and in cDNA clones of precursors of brain-specific mRNAs. The amino acid sequence encoded by the SC65 gene contains an acidic region in the C-terminal domain of the protein, potential glycosylation sites, and at least one possible phosphorylation site. The protein shows no overall similarity to proteins of known function, nor is there similarity to protein sequences present in GenBank or EMBL data bases.Key words: meiosis, synaptonemal complex, antibody, rat testis cDNA library, molecular cloning.

Deficiency of X and Y chromosomal pairing at meiotic prophase in spermatocytes of sterile interspecific hybrids between laboratory mice (Mus domesticus) and Mus spretus

The normal association between the X and Y chromosomes at metaphase I of meiosis, as seen in air-dried light microscope preparations of mouse spermatocytes, is frequently lacking in the spermatocytes of the sterile interspecific hybrid between the laboratory mouse strains C57BL/6 and Mus spretus. The purpose of this work is to determine whether the separate X and Y chromosomes in the hybrid are asynaptic, caused by failure to pair, or desynaptic, caused by precocious dissociation. Unpaired X-Y chromosomes were observed in air-dried preparations at diakinesis, just prior to metaphase I. Furthermore, immunocytology and electron microscopy studies of surface-spread pachytene spermatocytes indicate that the X and Y chromosomes frequently fail to initiate synapsis as judged by the failure to form a synaptonemal complex between the pairing regions of the X and Y chromosomes. Several additional chromosomal abnormalities were observed in the hybrid. These include fold-backs of the unpaired X or Y cores, associations between the autosome and sex chromosome cores, and autosomal univalents. The occurrence of abnormal autosomal and XY-autosomal associations was also correlated with cell degeneration during meiotic prophase. The primary breakdown in hybrid spermatogenesis occurs at metaphase I (MI), with the appearance of degenerated cells at late MI. In those cells, the X and Y are decondensed rather than condensed as they are in normal mouse MI spermatocytes. These results, in combination with the previous genetic analysis of spermatogenesis in hybrids and backcrosses with fertile female hybrids, suggest that the spermatogenic breakdown in the interspecific hybrid is primarily correlated with the failure of XY pairing at meiotic prophase, asynapsis, followed by the degeneration of spermatocytes at metaphase I. Secondarily, the failure of XY pairing can be accompanied by failure of autosomal pairing, which appears to involve an abnormal sex vesicle and degeneration at pachytene or diplotene.

Synaptonemal complexes from DNase-treated rat pachytene chromosomes contain (GT)n and LINE/SINE sequences

Purified chromosome cores (synaptonemal complexes) of rat pachytene chromosomes, from which the chromatin is removed by extensive DNase II digestion, retain a residual class of DNA, presumably the bases of chromatin loops. This synaptonemal complex-associated DNA, isolated by proteinase digestion and phenol extraction of purified DNase-treated synaptonemal complexes, and cloned in plasmid vector pEMBL18, has a length distribution of 50-500 bp. From a library of these fragments, 21 fragments were sequenced. Present in this sample are short 40-200-bp segments with greater than 80% identity to "long" and "short" interspersed repeated elements (LINE/SINEs), an excess of GT/CA tandem repeats and a number of unidentified sequences. The LINE/SINE segments may play a role in homology vs. nonhomology recognition during meiosis and the alternating purine-pyrimidine sequences have been implicated in genetic recombination. Their enrichment in synaptonemal complexes may be related to the synapsis and recombination functions of meiosis.

Searching for synaptonemal complex proteins and their genes

As an alternative to the production and use of monoclonal antisynaptonemal complex (SC) antibodies to isolate SC genes, we have explored the use of polyclonal anti-SC antibodies to identify SC genes from a cDNA expression library. The method proved relatively simple, reliable, and fast and has yielded two SC genes. A homologue of one of these genes from a different species has previously been isolated in another laboratory.

Telomere and centromere DNA are associated with the cores of meiotic prophase chromosomes

Mouse (Mus musculus) whole-mount, surface-spread, meiotic prophase chromosomes have an axial core structure, the synaptonemal complex, SC, from which extend chromatin loops. This arrangement permits a novel approach to the analysis of chromosome structure. Using in situ hybridization, the types of DNA sequences preferentially associated with the SC and the types located primarily in the chromatin loops can be determined. With biotinylated probes, detected by avidin conjugated to FITC, we present evidence for differential chromatin-SC interaction. The telomere sequence (TTAGGG)n is associated exclusively with the two ends of each autosomal SC rather than with the chromatin loops. The minor satellite DNA sequences are predominantly localized to the centromeric region of the SC, as defined by CREST serum anti-centromere antibodies. In contrast, the major satellite DNA probe hybridizes to the chromatin loops of the centromeric heterochromatin, and a probe containing a LINE sequence hybridizes to chromatin loops in general with no obvious preference for the SC. These observations demonstrate that, depending on the type of DNA sequence, the chromatin has different properties in regard to its association with the SC.

In situ DNA sequence mapping with surface-spread mouse pachytene chromosomes

Surface-spread pachytene chromosomes are several times the length of metaphase chromosomes and the decondensed chromatin loops are attached to a well-defined axis (Weith and Traut, 1980). This arrangement permits detailed DNA sequence localization by in situ hybridization. We show that two probes to low-frequency repeated sequences (20 to 50 copies) which locate the centromere proximal in the mouse X metaphase chromosome between bands A1 and A3 (Disteche et al., 1985) and which map 5.5 cM apart (Disteche et al., 1989), hybridize to two distinct chromatin regions 3 to 5 microns apart on a 25 microns long pachytene X chromosome core.

Anti-topoisomerase II recognizes meiotic chromosome cores

At meiotic prophase the chromatin becomes arranged in loops on newly formed chromosome cores. The cores of homologous chromosomes become aligned in parallel and thus form the synaptonemal complex (SC), a structure found in the meiocytes of nearly all recombinationally competent, sexually reproducing organisms. We report that two polyclonal antibodies against topoisomerase II (topo II), which recognize the mitotic metaphase chromosome scaffold give, at pachytene, a positive immunocytological reaction with the chromatin and, predominantly, with the cores and centromeric regions of the paired chromosomes. It therefore appears that during meiotic prophase, topo II - a DNA-binding enzyme implicated in transient double-strand breaks, chromosome condensation, and anaphase separation - is associated with the chromatin and SCs of the pachytene and diplotene chromosomes.

Satellite DNA I in chromatin loops of rat pachytene chromosomes and in spermatids

Biotinylated rat satellite DNA I probe p93-50 was used to visualize the chromatin of surface-spread rat pachytene chromosomes. Fluorescein isothiocyanate (FITC)-conjugated avidin produces a beaded fluorescence pattern along the chromatin loops that insert in the centromeric region of the synaptonemal complex (SC), the paired cores of homologous chromosomes. The number of fluorescent beads ranges from zero for centromeres without satellite DNA I homologous to probe p93-50, to several hundred for satellite-rich centromeric regions. For the chromosomes that can be identified, the relative amount of satellite DNA is chromosome specific. No satellite DNA I was detected at the non-centromeric ends of the chromosomes or interstitially. DNase-digested nuclei or isolated SCs did not have detectable amounts of satellite DNA in the centromeric regions of the chromosomes or in the residual SCs. The fate of the satellite DNA was followed during spermiogenesis. In the round spermatid the centromeric regions, which appear to be attached to the nuclear envelope, are still distinct and have converging loops of fluorescent chromatin. At later stages there are fewer but still bright fluorescent patches. Satellite DNA I is still detectable in the mature sperm head. These results demonstrate the organization of satellite DNA I in the chromatin loops at the centromeric regions, and they forecast the analysis of chromosome organization in unprecedented detail with a variety of probes in surface spreads of meiotic prophase chromosomes.

Localized recombination nodules and sex chromosome behavior in the male mole cricket, Neocurtilla hexadactyla

During meiotic prophase, the 10 metacentric autosomal bivalents of the mole cricket, Neocurtilla hexadactyla Perty, formed synaptonemal complexes only at their ends. These complexes were of similar morphology to those of other species. Virtually all of these distal synaptonemal complex segments contained one or more recombination nodules. There was complete correlation between the locations of the synaptonemal complex segments at pachytene and chiasmata at diplotene. The sex chromosomal bivalent X2 and Y, formed a synaptonemal complex at one end only. While no apparent physical or spatial connection was found during prophase between the X2Y bivalent and the third sex chromosome, X1, electron-dense material covered the centromeres of X1 and Y and to a lesser extent X2, thus differentiating the centromeres of the sex chromosomes from those of the autosomes.Key words: localised pairing, recombination nodules, chiasmata, sex chromosomes.

Synaptonemal complex proteins

Synaptonemal complexes were isolated from rate spermatocytes for the purpose of biochemical and morphological analysis. Several monoclonal antibodies were elicited against purified synaptonemal complexes to study the composition and assembly of these structures. Four classes of antibodies could be discriminated according to the polypeptides that they recognize on Western blots of purified synaptonemal complexes, namely antibodies recognizing (i) a 190-kDa polypeptide; (ii) a 30- and a 33-kDa polypeptide; (iii) two polypeptides with molecular weights of about 120 kDa; and (iv) polypeptides with molecular weights of 66-55 kDa. The localization of these antigens within spermatocytes was analyzed light microscopically, by means of the immunoperoxidase technique and ultrastructurally, by immunogold labelling of surface-spread spermatocytes. The 66- to 55-kDa polypeptides are not confined to synaptonemal complexes; rather, these polypeptides appear to be chromosomal components. The 190-, 30-, and 33-kDa polypeptides make part of the lateral elements of paired as well as unpaired segments of synaptonemal complexes. The 120-kDa polypeptides were localized on the inner edge of the lateral elements, specifically in paired segments of synaptonemal complexes. The distribution of the 190-, 120-, 30-, and 33-kDa polypeptides within the testis was analyzed by immunofluorescence staining of cryostat sections. All these polypeptides turned out to be specific for nuclei of zygotene up to and including diplotene spermatocytes. Only in some early spermatids could the 190-, 30-, and 33-kDa polypeptides be detected, presumably in remnants of synaptonemal complexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptonemal complex antigen location and conservation

The axial cores of chromosomes in the meiotic prophase nuclei of most sexually reproducing organisms play a pivotal role in the arrangement of chromatin, in the synapsis of homologous chromosomes, in the process of genetic recombination, and in the disjunction of chromosomes. We report an immunogold analysis of the axial cores and the synaptonemal complexes (SC) using two mouse monoclonal antibodies raised against isolated rat SCs. In Western blots of purified SCs, antibody II52F10 recognizes a 30- and a 33-kD peptide (Heyting, C., P. B. Moens, W. van Raamsdonk, A. J. J. Dietrich, A. C. G. Vink, and E. J. W. Redeker, 1987, Eur. J. Cell Biol., 43: 148-154). In spreads of rat spermatocyte nuclei it produces gold grains over the cores of autosomal and sex chromosomes. The cores label lightly during the chromosome pairing stage (zygotene) of early meiotic prophase and they become more intensely labeled when they are parallel aligned as the lateral elements of the SC during pachytene (55 grains/micron SC). Statistical analysis of electronically recorded gold grain positions shows that the two means of the bimodal gold grain distribution coincide with the centers of the lateral elements. At diplotene, when the cores separate, the antigen is still detected along the length of the core and the enlarged ends are heavily labeled. Shadow-cast SC preparations show that recombination nodules are not labeled. The continued presence suggests that the antigens serve a continuing function in the cores, such as chromatin binding, and/or structural integrity. Antibody III15B8, which does not recognize the 30- and 33-kD peptides, produces gold grains predominantly between the lateral elements. The grain distribution is bimodal with the mean of each peak just inside the pairing face of the lateral element. The antigen is present where and while the cores of the homologous chromosomes are paired. From the location and the timing, it is assumed that the antigen recognized by III15B8 functions in chromosome pairing at meiotic prophase. The two anti-rat SC antibodies label rat and mouse SCs but not rabbit or dog SCs. A positive control using human CREST (calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) anti-centromere serum gives equivalent labeling of SC centromeres in the rat, mouse, rabbit, and dog. It is concluded that the SC antigens recognized by II52F10 and III15B8 are not widely conserved. The two antibodies do not bind to cellular or nuclear components of somatic cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of two major components of the lateral elements of synaptonemal complexes of the rat

This paper describes the identification of two major components of the lateral elements of synaptonemal complexes of the rat by immunocytochemical techniques. We prepared monoclonal antibodies against synaptonemal complexes (SCs) by immunization of mice with purified SCs. One of these antibodies, II52F10, reacts with a 30 and a 33 kDa polypeptide, which are major components of purified SCs. Using this antibody, we studied the localization of its antigens light microscopically, by means of the indirect immunoperoxidase technique, as well as ultrastructurally, by means of the immunogold labeling technique. The immunolocalization was carried out on whole-mount preparations of lysed spermatocytes. The antibody reacts with paired as well as unpaired segments of zygotene, pachytene and diplotene SCs. In light microscopic preparations, the attachment plaques, particularly those of late pachytene and diplotene SCs, also appear to react strongly. In electron micrographs the lateral elements in paired as well as unpaired segments could be seen to react. No reaction was observed in the attachment plaques; however, in late pachytene and diplotene SCs the swollen terminal segments of the lateral elements did react with the antibody. Thus, we conclude that a 30 and a 33 kDa polypeptide make part of the lateral elements of synaptonemal complexes of the rat.

Synaptonemal complexes of normal and mutant yeast chromosomes (Saccharomyces cerevisiae)

Synaptonemal complex (SC) analysis of six laboratory yeast strains showed the SC karyotypes to be repeatable within strains. Chromosomal differences were found between strains. In five of the strains, two SCs insert into the nucleolus. This represents a single bivalent with a nucleolar organizer in a medial position as is suggested by genetic data or two bivalents each with a terminal nucleolar organizer. In the first interpretation, n = 16; in the second, n = 17. Strain Tris has a single nucleolar SC and n = 17. In strains DCx374, DCx416 and x8366a the genetically determined rearrangements of linkage group III could not be identified. Presumably the short SC (0.33 micron) associated with linkage group III cannot accommodate an inversion loop or a translocation configuration. The strains however were found to harbour a reciprocal translocation involving the nucleolar chromosome. Trisomy for one of the longer chromosomes was observed in Tris and spo10. It is concluded that rearrangements of the medium and long but not short yeast chromosomes can be detected cytologically. Measurements of nuclear volumes show SC length to vary with artifactually induced swelling of the nucleus. Linear regression of SC length over nuclear radius indicates that actual SC length is only about one-half the observed length. As a result the DNA packing per SC unit length is higher than previously estimated.

The synaptonemal complex: does it have contractile proteins?

Actin and myosin are commonly involved in cell motility systems. This study attempts to identify these two proteins in the synaptonemal complex (SC), a nuclear organelle associated with chromosome pairing during meiotic prophase. The several experimental approaches reported here give no evidence of SC staining with either anti-myosin or anti-actin antibodies. These findings differ from those of other reportings.

Three-dimensional approaches to the residual structure of histone-depleted HeLa cell nuclei

Histone-depleted nuclei were prepared by high-salt extraction of interphase HeLa cell nuclei. A large amount of the nuclear DNA remained associated with a rapidly sedimenting residual nuclear structure including cytoplasmic (intermediate filament) and nuclear (matrix and lamina) proteins. Electron microscopy allowed detection in the insoluble structure of a residual nuclear envelope, nucleolar residues, and an intranuclear network whose correspondence with components of in situ fixed nuclei is discussed. Using three-dimensional electron microscopy, it is further demonstrated that the salt-insoluble structure remaining after histone depletion in 2 M NaCl is highly ordered. This is of the utmost importance when considering the roles reportedly ascribed to this structure in nuclear functions.

The ultrastructural meiotic phenotype of the radiation sensitive mutant rad 6-1 in yeast

An ultrastructural analysis of three yeast rad 6-1/rad 6-1 diploids on sporulation medium for 0, 6, 10, and 24 h shows that arrest occurs at meiotic prophase. Two strains, CL 139 and PU 6, fail to complete chromosome synapsis based on the continued presence of single chromosomal cores in arrested nuclei. A clone derived from CL 139, however, showed complete pairing as evident from the presence of 17 synaptonemal complexes. All three strains underwent spindle pole body duplication but the poles failed to form a proper metaphase I spindle. A revertant Rad 6+ isolated from CL 139 showed normal chromosome behaviour and normal kinetic functions. It is concluded that the absence of meiotic recombination in some Rad 6- strains may result from asynapsis, but that in other strains (e.g., CL 139s) recombination fails in spite of complete synapsis. In all cases the lack of sporulation is adequately explained by failure of the kinetic apparatus to form a metaphase I spindle.

Light and electron microscopy of conjugation in the yeast, Schizosaccharomyces octosporus

Conjugation in Schizosaccharomyces octosporus is described through the use of interference contrast microscopy, fluorescence microscopy, and electron microscopy of serial sections. At the light microscope level, mating was frequently observed to occur between cells of common ancestry. Fluorescent staining of the nuclei showed that nuclear migration occurs prior to karyogamy, and following diploidization the nucleus then migrates to the end of the cell. A brightly fluorescent spot was found at the apex of the migration nucleus. At the electron microscope level, the results showed that nuclear movement occurs in the presence of cytoplasmic microtubules that are associated with the spindle pole body, the conjugatory nuclei first fuse at or near the spindle pole bodies, and fusion of the spindle bodies occurs apparently by stacking one onto the other.

Heterozygote advantage in Tay-Sachs carriers?

Chi-square analyses of new data as well as data previously reported by Myrianthopoulos have shown that grandparents of Tay-Sachs carriers die from proportionally the same causes as grandparents of noncarriers. It is unlikely that there is any advantage to being a Tay-Sachs carrier insofar as resistance to tuberculosis is concerned. Our results are further evidence to support Fraikor's claim that the high carrier frequency of the allele in Ashkenazi Jews is probably caused by a combination of founder effect, genetic drift, and differential immigration patterns.