Synaptonemal Complex Structure Research Articles

The effect on meiotic synapsis of a recombination modulator in Petunia hybrida

In Petunia hybrida, a major gene called Rm1 is able to induce a considerable increase in the meiotic recombination rates. To monitor the effect of Rm1 on meiotic synapsis, synaptonemal complex (SC) formation was investigated in pollen mother cells of Rm1 Rm1 and rm1 rm1 plants. Three-dimensional serial-section reconstruction showed no morphological difference in their SC structure. In two-dimensional spreading nuclei, although extensive SC formation was observed in the absence of Rm1, our observations strongly suggest that SC formation is more regular and more efficient in the presence of Rm1. SC analysis showed that the reduced chiasma frequency observed in rm1 rm1 plants corresponds to a partial failure of synapsis involving chromosome ends, pericentric regions, and may also extend to the whole length of short chromosome arms. It is proposed that Rm1 probably corresponds to a regulatory system with multiple effects, including the course of synapsis and the extent and quality of SC formation.Key words: synaptonemal complex, synapsis, chiasma, recombination modulator gene, petunia.

The effect of tequila in the synaptonemal complex structure of mouse spermatocytes

The effect of tequila in the synaptonemal complex (SC) of mouse spermatocytes was determined. We tested 3 dosages (2.1, 4.2 and 8.4 g/kg) administered in a single intraperitoneal inoculation. The frequency of SC alterations was established in pachytenic nuclei 5 days after the administration using a silver impregnation technique. Three types of alterations were observed (desynapses, breaks and multiaxials) and the rate of each alteration was compared with that obtained with appropriate controls, including cyclophosphamide (CP) (150 mg/kg). The results showed a significant increase induced by tequila only in the frequency of desynapses. This damage began at the second highest dose (4.2 g/kg). The other SC alterations were in the control range. CP, however, induced a significant increase in all 3 types of SC alterations.

Temporal comparison of recombination and synaptonemal complex formation during meiosis in S. cerevisiae

In synchronous cultures of S. cerevisiae undergoing meiosis, an early event in the meiotic recombination pathway, site-specific double strand breaks (DSBs), occurs early in prophase, in some instances well before tripartite synaptonemal complex (SC) begins to form. This observation, together with previous results, supports the view that events involving DSBs are required for SC formation. We discuss the possibility that the mitotic pathway for recombinational repair of DSBs served as the primordial mechanism for connecting homologous chromosomes during the evolution of meiosis. DSBs disappear during the period when tripartite SC structure is forming and elongating (zygotene); presumably, they are converted to another type of recombination intermediate. Neither DSBs nor mature recombinant molecules are present when SCs are full length (pachytene). Mature reciprocally recombinant molecules arise at the end of or just after pachytene. We suggest that the SC might coordinate recombinant maturation with other events of meiosis.

Ultrastructure of meiotic pairing in B chromosomes of Crepis capillaris

The meiotic pairing behaviour of four B isochromosomes of Crepis capillaris was studied by synaptonemal complex (SC) surface spreading of pollen mother cells. The four B chromosomes form a tightly associated group, separate from the standard chromosomes, throughout zygotene and pachytene. All four B chromosomes are also folded around their axis of symmetry, the centromere, and the eight homologous arms are closely aligned from the earliest prophase I stages. A high frequency of multivalent pairing of the four B chromosomes is observed at pachytene, in excess of 90%, mirroring the situation observed at metaphase I but exceeding the frequency expected (76.2%) on the assumption of random pairing among the eight B isochromosome arms with a single distal pairing initiation site per arm. The higher than expected frequency of multivalents is due to the occurrence of multiple pairing initiations along the B isochromosome arms, resulting in high frequencies of pairing partner switches. Pairing of the standard chromosome set is frequently incomplete in the presence of four B chromosomes, and abnormalities of SC structure such as thickening and splitting of axes and lateral elements are also frequently seen. Similarly, B chromosomes show partial pairing failure, the extent of which is correlated with pairing failure in the standard chromosome set. The B chromosomes themselves also show abnormalities of SC structure. Both standard and B chromosomes show non-homologous foldback pairing of regions that have failed to pair homologously.

Analysis of wild-type and rad50 mutants of yeast suggests an intimate relationship between meiotic chromosome synapsis and recombination

The RAD50 gene of S. cerevisiae is required during meiosis for both recombination and chromosome synapsis and is also required for repair of double strand breaks during vegetative growth. We present below the isolation and analysis of several types of rad50 mutants. We show that null mutations block both meiotic recombination and formation of synaptonemal complex (SC) at early stages, while nonnull mutations block both processes at intermediate stages. These observations suggest that recombination and SC formation involve a series of intimately related events. Furthermore, all rad50 mutants block formation of tripartite SC structure but permit other aspects of SC development, i.e., formation of axial cores. In light of this and other observations, the meiotic and mitotic defects of rad50 mutants can be accounted for economically by the proposal that meiotic recombination, meiotic chromosome pairing, and vegetative DNA repair all use a common chromosomal homology search that involves RAD50 function.

Two-dimensional spreads of synaptonemal complexes from solanaceous plants. IV. Synaptic irregularities

Irregularities in the structure of synaptonemal complexes have been reported in a wide variety of organisms, but so far there has been no study concentrating on synaptic irregularities per se. We have used a hypotonic bursting technique to spread synaptonemal complexes from autotetraploid Solanum tuberosum (potato) and diploid, tetraploid, and trisomic Lycopersicon esculentum (tomato). In this study we observed most of the synaptic irregularities that have been reported in other organisms as well as an irregularity that has not been illustrated previously (lateral element buckles containing additional synaptonemal complex components). These observations have been used to provide partial answers to the following questions about the formation of the synaptonemal complex. (i) In what way does homology affect synapsis? (ii) What controls multiple synapsis? (iii) What is the relationship of synaptonemal complex components to chromatin and to each other? (iv) Do lateral elements have structural or functional polarity?Key words: synaptonemal complexes, synapsis, synaptic irregularities, solanaceous plants.

X‐Y chromosome univalency in the testes of hyperthermic mice: II. Light and electron microscopic analysis of synaptonemal complexes

AbstractHyperthermia‐induced X‐Y dissociation has been observed in diakinesis‐metaphase I sper‐matocytes but not in pachytene spermatocytes, which have been implicated as highly susceptible to heat stress. To determine X‐Y dissociation in pachytene spermatocytes and to compare levels of dissociation between pachytene and diakinesis‐metaphase I spermatocytes male ICR mice were exposed to 35°C ± 0.07°C and 65% ± 0.14% relative humidity for 2 or 4 days. Control mice were housed at 24°C ± 0.04°C and 43% ± 0.58% relative humidity. Mice were killed 0, 3, 5, 8, or 10 days after stress and the testes processed for meiotic chromosome display at diakinesis‐metaphase I and synaptonemal complex display at pachynema. Twenty‐five to thirty cells per mouse at both stages of meiosis were observed with light microscopy, and pachytene spreads with transmission electron microscopy to determine heat‐stress effects on synaptonemal complex structure. Statistical analyses revealed significant linear increases in X‐Y dissociation with prolonged exposure to heat at pachynema and diakinesis‐metaphase I. Levels of pachytene dissociation were one‐half the levels of dissociation at diakinesis‐metaphase I. The resolvable structure of the lateral elements of the synaptonemal complex was not affected by heat stress.

Ultrastructure of primary spermatocyte in fish (Tilapia: Oreochromis niloticus): The synaptonemal complex

The Primary Spermatocytes represent a stage in spermatogenesis when the first meiotic cell division occurs. They are derived from Spermatogonium or Stem cell through mitotic division. At the zygotene phase of meiotic prophase the Synaptonemal complex appears in these cells in the space between the paired homologous chromosomes. Spermatogenesis and sperm structure in fish have been studied at the electron microscope level in a few species? However, no work has yet been reported on ultrastructure of tilapia, O. niloticus, spermatozoa and spermatogenetic process. In this short communication we are reporting the Ultrastructure of Primary Spermatocytes in tilapia, O. niloticus, and the fine structure of synaptonemal complexes seen in the spermatocyte nuclei.

The synaptonemal complexes of Caenorhabditis elegans: Pachytene karyotype analysis of hermaphrodites from the recessive him-5 and him-7 mutants

The him-5 and him-7 mutants (high incidence of males) of Caenorhabditis elegans both showed increased rates of X chromosome non-disjunction (16% and 3%, respectively) but him-7 also had a high frequency of autosomal non-disjunction (34%). Synaptonemal complex (SC) karyotype analysis revealed a haploid chromosome number of six in each strain. Alterations in him-7 nuclear morphology were observed but there were no aberrations in SC structure that could account for the increased frequency of autosomal non-disjunction. However, the frequency of X-chromosome non-disjunction occurred at predicted rates on the basis of the number of disjunction regulator regions (DRRs) present on the SCs. The observation that the levels of X-chromosome non-disjunction were not influenced by the increase in the frequency of autosomal non-disjunction supports the notion that the X chromosome is subject to separate controls during meiosis. The him-7 mutant is nested within the rad-4 map region on linkage group V, however, SC analysis did not reveal the physical position on the chromosome because of synaptic adjustment.

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 DDRs 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 DDRs. 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 DDRs located approximately on μm apart from each other.

Heterochromatin and nucleolus-organizer-region behaviour at male pachytene of Sus scrofa domestica

In the domestic pig (2n = 38) two types of constitutive heterochromatin can be differentiated by fluorescence counterstaining techniques. All 24 biarmed autosomes and the X chromosome have chromomycin A3-positive centromeric C-bands, whereas all 12 acrocentric chromosomes exhibit DA-DAPI-positive centromeric heterochromatin. Fluorescence analysis of male pachytene nuclei revealed that the DA-DAPI-positive C-bands form one or two large chromocentres per cell, while the chromomycin A3-bright C-material is well scattered. Hence, the bivalents formed by the acrocentric chromosome pairs are centromerically associated, whilst the submetacentric bivalents are not. Counce-Meyer spreading techniques were used to study the structure of synaptonemal complexes (SCs) both by light and electron microscopy. In general, the SCs of the domestic pig resemble those described for other mammals. The SC formed by the X and the Y may include up to 94.5% of the Y chromosome. In silver-stained microspreads each of the bivalents (nos. 8 and 10) bearing the nucleolus-organizer-regions (NORs) is connected to a pair of nucleoli, indicating that all four NORs are active during early meiotic stages. By contrast, in the majority of mitotic metaphases of phytohaemagglutinin-stimulated lymphocytes only one pair (no. 10) exhibited Ag-NOR staining. The significance of the chromosome disposition in the pachytene nucleus is discussed with regard to heterochromatin composition and karyotype evolution.

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.

The synaptonemal complex as part of the nuclear matrix of the flour moth, Ephestia kuehniella

A nuclear matrix fraction was prepared from ovaries of the achiasmatic flour moth, Ephestia kuehniella, by removal of the chromatin, using detergent treatment of homogenized ovaries or dissected ovary tips followed by DNase digestion and high salt extraction. Removal of DNA and histones from the nuclei was demonstrated by Feulgen staining and polyacrylamide gel electrophoresis (PAGE), respectively. By light microscopy, ribbon-like structures similar in dimension to the synaptonemal complex were observed in the oocyte after digestion of the chromosomes. Electron microscopic examination of matrix preparations of pachytene cells showed a defined synaptonemal complex structure with both lateral and central elements. Such structures were not found in either the fully differentiated nurse cells or in follicle cells which were exposed to the same preparative technique concurrently. However, in early post-pachytene nurse cells the typical polycomplex structures, formed in these cells from the synaptonemal complex, were found in nuclear matrix preparations. The results suggest an association of synaptonemal complexes with the nuclear matrix.

The synaptonemal complex in higher plants

The basic structure of the synaptonemal complex and its components will be outlined, and placed in the‐context of current understanding of meiotic prophase events. The range of plants in which synaptonemal complexes have been studied will be reviewed, and variations in synaptonemal complex structure noted. This will be followed by an examination of the behavior of the synaptonemal complex during meitoic pairing and crossing over in plants, and a survey of experiments which have attempted to study the structure and biochemistry of synaptonemal complex formation and function. The occurrence, structure, composition and possible functions, of recombination nodules will also be explored. Finally, there will be a discussion of the application of newer techniques to the study of the synaptonemal complex in plants.

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.

Cytochemical and ultrastructural studies on the synaptonemal complex of rat spermatocytes

The effects of several dehydration treatments on the synaptonemal complex (SC), histone solubility in 2.0 M NaCl, and histone-DNA interaction in unfixed rat spermatocytes were evaluated. Freeze substitution with ethanol or dehydration with polyethylene glygol resulted in loss of the SC, preservation of histone solubility and DNA-histone salt linkages. Dehydration with ethylene gylcol or hexylene glycol resulted in preservation of SC with a clear delineation of attachment of the chromatin fibrils to the lateral elements, but a loss of histone solubility and histone-DNA linkages. Dehydration to a fifty percent concentration with glycerol with completion of dehydration with ethylene glycol had the same effect but also resulted in an even distribution of chromatin fibrils. Dehydration with glycerol alone resulted in clumping of chromatin and loss of SC structure, histone solubility and histone-DNA linkages. Partial dehydration to a fifty percent concentration with these three solvents followed by freeze substitution with ethanol resulted in the loss of SC structure and histone solubility but the preservation of histone-DNA linkages. It is likely that these nonaqueous solvents affected the histone hydrophobic groups and thereby altered histone conformation and interactions. These alterations, depending on the treatment used, resulted in the loss or preservation of SC, histone solubility and histone-DNA interactions thereby indicating that the hydrophobic interactions of the histones are crucial for the preservation of these feature of meiotic chromosomes. These results also demonstrate that neither does the preservation of the histone-DNA salt linkages suffice for the preservation of the SC nor does their disruption necessarily result in its loss. The lysine-rich histones, particularly that one unique to meiotic cells, may through their interactions play a crucial role in SC structure.

Positive contrast staining and protected drying of surface spreads: Electron microscopy of the synaptonemal complex by a new method

A new procedure, protected drying, provides a simple rapid means for maintaining three-dimensional structural fidelity in whole mount preparations during drying. As demonstrated on surface spread meiotic chromosomes after fixation and positive staining, the preparation is air-dried in a protective matrix consisting of an inert, water-soluble, biologically compatible, organic compound, such as ficoll, polyvinyl pyrrolidone, or dextran, which dries as a homogeneous electron-lucent film that supports the structure in three dimensions and protects it from damage during drying. Details of synaptonemal complex structure are comparable to those established in thin sections.

Biochemical controls of meiosis.

This review is addressed to the biochemical events that underlie the early phases of the meiotic cycle. Late activities such as chromosome disjunction and special processes involved in gametogenesis are excluded from this article. Of primary concern are the prophase stages of meiosis during which homologous chromo­ somes pair and presumably undergo crossing-over. These two events are fundamental to genetic recombination and constitute a major and universal feature of meiosis. In reviewing the field from the standpoint of biochemical mechanisms we have one general concern which is best to state at the outset. The sources of evidence on biochemical activities during meiosis are few. Biochemi­ cal analyses of meiosis in yeast are just beginning and will not be reviewed here (20). The bulk of our information comes from analyses of liliaceous plants, but their study has been pursued in very few laboratories. The credibility of the conclusions drawn from biochemical studies depends to a large extent upon the degree to which they are consistent with the information provided by genetics and cytology. Genetic approaches to intragenic recombination in yeast were surveyed two years ago (20). The cytology and fine structure of meiotic cells from various sources have been fully reviewed in the previous volume (68) and genetic aspects of meiosis will be discussed in the succeeding one. Formulation of biochemical questions concerning the process of crossing-over is guided by genetic data on transmission of traits and by cytological data on pairing and chiasma formation. These classical disciplines continue to define the course of meiotic analyses. Additional pointers to possible mechanisms are provided by studies on the fine structure of the synaptonemal complex. The simplest picture of meiosis that may be drawn from these disciplines and that can serve as a basis for biochemical investigation is that chromosomes synapse during the zygotene stage and undergo crossing-over during the pachytene stage.

Synaptonemal complex and chromosome structure in the achiasmatic spermatogenesis of Panorpa communis (Mecoptera)

Synaptonemal complex and chromosome structure in the achiasmatic spermatogenesis of Panorpa communis (Mecoptera)

Light and electron microscope studies of chromosome pairing in relation to chiasma localisation in Stethophyma grossum (Orthoptera: Acrididae).

The relationship of chromosome pairing to chiasma localisation in the grasshopper Stethophyma grossum was investigated by a combined light and electron microscope study. Observations on the extent of synaptonemal complex formation in spermatocytes suggest that pairing is complete in all chromosome regions and that localised chiasmata do not therefore follow from localised pairing of homologues. This study also revealed an unprecedented variant of synaptonemal complex structure. Each bivalent was found to contain an asymmetrical region within which one lateral element was enormously enlarged while the other lateral element retained normal dimensions. This asymmetrical region is probably confined to one end of each bivalent and seems to extend constantly about 4 μ from the nuclear membrane attachment site. The possible significance of this variant of synaptonemal complex structure is briefly discussed.