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PubMed1979

Recombination nodules and synaptonemal complex in recombination-defective females of Drosophila melanogaster.

The cytological effects of mutant alleles of the mei-9, mei-218, and mei-41 loci during prophase I have been examined by electron microscopy. None of these mutants affect synaptonemal complex structure, continuity, or temporal behavior. Both the precondition-defective mutants mei-218 and mei-41 affect both number and morphology of spherical recombination nodules and apparently affect at least the numbers of ellipsoidal recombination nodules, whereas in the exchange-defective mutant mei-9 the numbers and morphologies of both ellipsoidal and spherical recombination nodules are normal. The parallel effects of mei-218 and mei-41 on meiotic recombination and on recombination nodules indicate that spherical recombination nodules at least mark the site of exchange events; the effects of these mutants on nodule morphology suggest that the nodule performs an active role in the recombination process. The nodule phenotype of mei-9 indicates that spherical nodules are present, and presumably functioning, well before the concluding stages of the recombination event. The parallel effects of all 3 mutants on ellipsoidal and spherical nodules indicate that these are indeed related structures but does not ellucidate the nature of the relationship. It is suggested that all aspects of meiotic recombination are under the aegis of recombination nodules.

Animals↗

PubMed1984

The synaptonemal complexes of Caenorhabditis elegans: pachytene karyotype analysis of the rad-4 radiation-sensitive mutant.

In the rad-4 mutant of C. elegans there is a specific increase in the number of 'Disjunction Regulator Regions' (DDR) present on the synaptonemal complexes (SC) in pachytene nuclei. These DRRs either promote disjunction or inhibit nondisjunction of the X-chromosome as evidenced by the 10-fold decrease in the rate of X-chromosome nondisjunction as compared to the wild-type. The structure of the tripartite SC is normal, thus, the decrease in the rate of X-chromosome nondisjunction in the rad-4 mutant is not related to the structure of the SC but may be related to the number of DRRs. Other changes are also associated with the sensitivity to irradiation, i.e. the pachytene nuclear morphology is altered such that nuclei and nucleoli are 50% the size of wild-type. In addition, the autosomal: X-chromosome size ratio is reduced in the rad-4 mutant. That there are six SCs confirms n = 6 in this mutant and of these six SCs three can be identified: (1) the XX bivalent, SC No. 1, is the shortest and pairs synchronously with the autosomes; (2) the longest bivalent, SC No. 6, carries the nucleolar organizer region at one extreme end; and (3) SC No. 5 has two DRRs located approximately one micron apart from each other.

Animals↗

PubMed1982

Synaptonemal complex analysis of mouse chromosomal rearrangements. IV. Synapsis and synaptic adjustment in two paracentric inversions.

Two paracentric inversions in the mouse, In (1) 1 Rk and In (2) 5 Rk, have been studied in surface microspreads of spermatocytes from heterozygotes. At zytogene, synaptic initiation occurs independently in three regions: within the inversion, and without, on either side. Synaptonemal complex (SC) formation is restricted to homologous regions, resulting in inversion loops in all early pachytene spermatocytes. An adjusting phase then occurs during pachytene in which the inversion loop is reduced by desynapsis of homologously synapsed SC, followed immediately by non-homologous synapsis with the alternate pairing partner, progressing from the ends toward the middle. Adjustment occurs during the first half of pachytene, but is not closely synchronized with sub-stage. It is complete by late pachytene, the loop having been eliminated in all cases and replaced by "straight" SCs in which the inverted region is heterosynapsis. Synapsis in the adjustment phase is evidently permitted only after the homosynaptic phase, and is indifferent to homology. It may lead to hetersynapsis, as in the inversion region, or to synapsis of homologous regions not synapsed at zytogene. The anaphase bridge frequency, a measure of crossing over within the inversion, is about 34% for both inversions studied, indicating that such crossovers do not block adjustment, that crossing over probably occurs before or during the adjustment period, and that there is some crossover suppression. The last could be the consequence of blocking by desynapsis/heterosynapsis. Synaptic adjustment appears to be a general phenomenon that occurs to varying extents in different forms. A hypothetical scheme for two phases of synapsis is proposed: at zytogene, a basic propensity for indifferent SC formation is limited by a restricting condition to synapsis between homologous regions, Subsequently, the restriction is lifted, whereupon synaptic instability is resolved by desynapsis, followed by resynapsis that is indifferent to homology, but that results in a topologically more stable structure.

Animals↗

PubMed1981

A light microscopic study of the development and behaviour of the synaptonemal complex in spermatocytes of the mouse.

A method is presented for the sequential analysis of male meiosis using hydroxyurea (HU). HU produces a gap in the spermatogenic line. The front of surviving cells behind the gap was examined day by day using silverstained whole mount spreads on glass slides. With this method it was possible to study the development and behaviour of the synaptonemal complex (SC) in mouse spermatocytes by the light microscope. At zygotene no unpaired axial elements could be seen. Unpaired axial elements were found to be specific for the diplotene stage. The axes of the XY pair could be recognized from late zygotene up to diplotene.

Animals↗

PubMed1984

Sterile mutants in Caenorhabditis elegans: the synaptonemal complex as an indicator of the stage-specific effect of the mutation.

Two sterile mutants of Caenorhabditis elegans hermaphrodites have been examined using the electron microscope and serial section analysis. The F4 and F80 mutants were described previously ( Mounier and Brun , 1980), and they were shown to be blocked at the start of oogenesis. In the F4 mutant, normal sperm are produced and the pachytene nuclei contain tripartite synaptonemal complexes (SC) between the homologously paired chromosomes. In the F80 asynaptic mutant, only a few sperm are produced and they have abnormal morphology. Whereas SCs and SC associated structures (termed 'SC knobs') are present in the F4, these structures are absent from the F80 . The mutation in F80 affects gametogenesis prior to the pachytene stage of meiosis and pairing of homologous chromosomes apparently does not occur. The SC knobs may influence the regulation of the disjunction of the chromosomes. For that reason, these structures are now termed 'disjunction regulator regions'.

Animals↗

PubMed1985

Defective pairing and synaptonemal complex formation in a Sordaria mutant (spo44) with a translocated segment of the nucleolar organizer.

The recessive meiotic mutant spo44 of Sordaria macrospora, with 90% ascospore abortion, exhibits striking effects on recombination (67% decrease), irregular segregation of the almost unpaired homologues, and a decrease in chiasma frequency in the few cases where bivalents are formed. Three-dimensional reconstructions of ten prophase nuclei indicate that pairing, as judged by the absence of fully formed synaptonemal complexes (SC), is not achieved although lateral elements (LE) assemble. The pairing failure is attributable to defects in the alignment of homologous chromosomes. The leptotene alignment seen in the wild type before SC formation was not observed in the spo44 nuclei. Dense material, considered to be precursor of SC central elements, was found scattered among the LE in two nuclei. The behaviour of spo44 substantiates the hypothesis that chromosome matching and SC formation are separable events. - The total length of the LE in the mutant is the same as in the wild type, but due to variable numbers and length of the individual LE, homologues cannot be lined up. Light microscopic observations indicate that the irregular length and number of LE is due to extensive chromosome breakage. The wild-type function corresponding to spo44 is required for both LE integrity and chromosome matching. Reconstructions of heterozygous nuclei reveal the presence of a supernumerary nucleolar organizer in one arm of chromosome 7. It is suggested that rDNA has been inserted into a gene whose function is involved in pairing or into a controlling sequence that interacts with the pairing process.

Ascomycota↗

PubMed1980

Synaptonemal complexes at premeiotic interphase in the mouse spermatocyte.

Male mice were injected intraperitoneally with 125 microCi (1 Ci = 3.7 X 10(10) becquerels) of [3H]thymidine at 1-hr intervals and killed 1 hr after the second injection. Testes were prepared for bright-field and electron microscopic autoradiography. Primary spermatocytes, identified by light microscopy to be at the premeiotic interphase stage, were found to be heavily labeled. Electron microscopic examination disclosed the coincidental occurrence of synaptonemal complexes and label within the nuclei of premeiotic interphase spermatocytes, indicating synapses of homologues had begun during the S phase. The significance of this finding for the traditional view of meiosis is discussed.

Animals↗

PubMed1976

The nucleolus organizer and the synaptonemal complex in Endymion non-scriptus (L.).

Stages of meiosis from the bluebell Endymion non-scriptus (L.) were studied by electron microscopy. The nucleolus went through the process of segregation at the beginning of meiosis with the movement to its surface of a pale-staining region. This region was shown to be the same as that called the 'L zone' or lacunae of nucleoli. Its chromosomal nature was strongly suggested by the presence of the synaptonemal complex within it. This demonstrated that the pale-staining region of nucleoli is the nucleolus organizer and almost certainly the chromosome region containing the ribosomal cistrons, and justifies the use of these terms to describe the structure when seen inside the nucleolus. The relationship between this zone and the heterochromatic knob called the nucleolar organizing body in maize by other workers is discussed.

Cell Nucleolus↗

PubMed1984

Heterochromatin, the synaptonemal complex and crossing over.

A combined light- and electron-microscopic examination of chromosomes from two angiospermous plants, Plantago ovata and Lycopersicon esculentum, and a mammal, Mus musculus, was performed. From this investigation three observations have been made that may be relevant to the observed lack of crossing over in heterochromatin. (1) Differential staining indicates that heterochromatin represents a smaller fraction of the length of pachytene chromosomes than it represents in the length of mitotic metaphase chromosomes. Since the synaptonemal complex (SC) runs throughout the length of these pachytene chromosomes, it is under-represented in heterochromatin. Considering the evidence for a rough correlation between the length of SC and the amount of crossing over, this could result in less crossing over in heterochromatin than expected on the basis of its length in mitotic metaphase chromosomes. (2) Electron microscopy indicates that, unlike the SC in euchromatin, the SC in heterochromatin is densely ensheathed in highly compact chromatin. If crossing over occurs in the SC or even in the surrounding chromatin, the compaction of the chromatin may prevent the penetration of enzymes needed in recombination. (3) Finally, a difference in the structure of SCs in euchromatin versus heterochromatin was observed that could be associated with the lack of crossing over in heterochromatin.

Animals↗

PubMed1979

Synaptonemal complex and recombination nodules in wild-type Drosophila melanogaster females.

Electron microscope serial section reconstruction analysis of all zygotene-pachytene nuclei of meiotic cells from three wild-type germaria (a subunit of the ovary containing the early meiotic stages arrayed in temporal developmental sequence) of Drosophila melanogaster females corroborates and extends earlier observations (Carpenter 1975a) on the nature and sequence of ultrastructural events occurring during the time of meiotic recombination. Emphasis has been placed on (1) the time of appearance and disappearance of the synaptonemal complex (SC) and the changes in its dimensions that accompany a cell's progression through pachytene, and (2) the appearance, disappearance, number and chromosomal locations of recombination nodules (Carpenter 1975b). For both the SC and the recombination nodule the availability of several developmental series has provided an estimate of the biological variability in the properties of these recombination-associated structures. The much more extensive data presented here substantiate the earlier hypothesis that recombination nodules occur at sites where reciprocal meiotic recombination will occur, has occurred, or is occurring. A second morphological type of recombination nodule is reported; it is suggested that the presence of the latter type of nodule may correlate with sites of gene conversion. The hypothesis that there may be two types of meiotic recombination processes is discussed.

Animals↗

PubMed1982

Heterochromatin, synaptonemal complex, and NOR activity in the somatic and germ cells of a male domestic dog, Canis familiaris (Mammalia, Canidae).

C-banding and silver staining of the somatic and germ cells of the male domestic dog. Canis familiaris, have shown that: (1) the amount of C-banding is small compared to most other mammalian species, (2) three pairs of autosomes have nucleolus organizer regions (NORs) at the terminal ends of their long arms, whereas the Y chromosome has an NOR on the terminal end of the short arm, (3) the organization of the synaptonemal complex (SC) is similar to that of other mammalian species, (4) a distinct SC is formed between the long arm of the Y chromosome and probably the short arm of the X chromosome, and (5) the differential axes of both sex chromosomes do not demonstrate fusiform thickenings nor do they stain darkly with silver as do the XY bivalents in many other mammalian species.

Animals↗

PubMed1981

Synaptonemal complex of the sex-autosome trivalent in a male indian muntjac.

Bright-field microscopy of silver-stained pachytene spermatocytes of a male Indian muntjac, Muntiacus muntjak revealed that (a) the synapsis between the autosomal homologs, including the long arm of the X and Y2, was normal, (b) the nucleolus organizer regions were present in both the No. 1 bivalent and the long arm of the X and Y2, (c) the accessory structures of the X chromosome short arm in the forms of light and dark thickenings and the hairpin-like bend were present despite the X-autosome translocation, (d) a short synaptonemal complex was present between the Y1 (real Y) and the short arm of the X chromosome, and (e) the centromeric orientation of the Y1 and Y2 chromosomes was in Cis configuration as opposed to the X chromosome.

Animals↗

PubMed1992

Effects of dimethyl sulfone (DMSO2) on early gametogenesis in Caenorhabditis elegans: ultrastructural aberrations and loss of synaptonemal complexes from pachytene nuclei.

The free-living nematode Caenorhabditis elegans has been used extensively for studies in developmental and reproductive genetics. Recently, toxicologic studies have been initiated using specific sex chromosome mutations. In the present study, high incidence of male (him) mutants, him-5 and him-8, were treated with dimethyl sulfone (DMSO2), the primary metabolite of dimethyl sulfoxide (DMSO). In addition to differential effects on X-chromosome nondisjunction, loss of viability and fertility were observed. Much lower concentrations of DMSO2 were required to elicit the same aberrational effects characteristic of DMSO (1); thus, the toxicity of the former was significantly more potent. The observed decrease in life span was associated with senescent morphology of meiotic prophase nuclei, such that nuclei from young and old specimens could not be differentiated. Aging in oocytes at pachytene is characterized by nucleo-cytoplasmic aberrations, increased density of the nucleoplasm and cytoplasm, and decrease in numbers of mitochondria. Increasing concentrations of DMSO2 resulted in a corresponding decrease in fertility and increased production of abnormal gametes. At DMSO2 concentrations higher than 1.0%, synaptonemal complexes (SC) were absent from pachytene nuclei; thus, effective pairing and segregation of homologous chromosomes was prohibited. Since the SC is essential for regulating pairing and subsequent separation of bivalents, the lack of an SC explains the loss of fertility, due to the production of unbalanced gametes, observed in DMSO2-treated specimens.

Animals↗

PubMed2003