The assembly of the synaptinemal complex

The assembly of the synaptinemal complex in the ascomycete Neottiella was studied by three-dimensional reconstruction of a late zygotene nucleus. A single banded lateral component is formed between the two sister chromatids of each homologous chromosome prior to their pairing. The central regions are pre-assembled in organized form in folds of the granular part of the nucleolus and then converted into an amorphous transport form. The latter appears to move through the nucleoplasm to sites between the lateral components of synapsing homologous chromosomes. The central region material is reorganized into blocks with a recognizable central component and attached to one lateral component. The last step in the completion of the synaptinemal complex is the association of the free surface of the organized central region with the corresponding segment of the homologous lateral component. The findings are discussed in relation to mechanisms of chromosome pairing and chiasma formation.

The synaptinemal complex in Rhoeo spathacea

The synaptinemal complex in meiocytes of Rhoeo spathacea is described. Unpaired zygotene chromosomes do not exhibit well defined axial cores under the ordinary fixations of electron microscopy and appear diffuse. However, the axial core is defined by ethanolic phosphotungstic acid (PTA) although it does not respond to uranyl-EDTA-lead. Thus the core appears to contain histone but not RNA and presents a condition which is modified later in pairing when lateral elements of the synaptinemal complex respond positively to both tests. The total number of attachments of synaptinemal complexes to the nuclear envelope was determined in several nuclei from serial sections. Eleven of the twelve possible attachments were found in one nucleus. It thus seems certain that all must be so attached. In the same manner all chromosomes can be seen to have an attachment to a chromocentre. Chromocentres are often very large and compound in that two kinds of heterochromatin can be distinguished. These states of chromatin within the chromocentre are considered to be a function of the degree of condensation. Segments of synaptinemal complexes are distributed randomly through sections of pachytene nuclei and long uncoiled segments of complexes are frequently found in or near the centres of median nuclear sections. Synaptinemal complexes are also found in chromocentres. Our findings suggest that on completion of pairing, which begins distally, homologous chromosomes in Rhoeo are paired throughout their entire lengths, rather than in small terminal segments only.

Corresponding-site interference, synaptinemal complex structure, and 8+:0m and 7+:1m octads from wild-type × mutant crosses of Ascobolus immersus

SUMMARY‘Wider ratio’ octads (8:0, 0:8, 7:1 and 1:7) regularly occurred in wild-type(+) × white ascospore(w) crosses of the Pasadena strains of Ascobolus. Control crosses showed that phenocopies and false octad clusters were absent or rare; no reversion from w to + occurred, but mutation from + to w was found at a number of loci, with nearly all 0+:8w and many 2+:6w octads in + × w crosses arising from mutation, not conversion. Nearly all 8+:0w, 7+:1w and 6+:2w octads appeared to arise by conversion.The finding of genuine wider ratio octads implies hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids in the same bivalent, which conflicts with models of the synaptinemal complex requiring that only two of the four chromatids pair intimately at any point. Octad types arising from hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids were described and formulae were derived for their frequencies. The lack of genuine wider ratio octads in several other Ascobolus studies was shown to be explicable quantitatively in terms of their conversion frequencies.‘Corresponding-site interference’ is defined as interference between the two pairs of non-sister chromatids of a bivalent in hybrid-DNA formation at exactly corresponding sites. Formulae based on observed octad frequencies were derived for calculating coincidence values for this kind of interference. Corresponding-site interference was found to be weak, with coincidence values differing between crosses with high and with low conversion frequencies.

Ultrastructure of Meiosis and Centriole Behaviour in Ulva Mutabilis Føyn

The present work deals with the ultrastructure of the meiotic processes leading to the formation of zoospores. The formation of an exit pore on the outer surface of the sporangium is the first visible sign that the cell will undergo meiosis and not just a somatic division. Prophase I nuclei differ from nuclei in mitotic prophase by having one or several invaginations of the nuclear membrane. Electron-dense lines probably representing the lateral elements of the synaptinemal complex are observed during prophase I. The central element of the complex has, however, never been observed. The fate of the centrioles during meiosis is described in detail. The number of duplications of the centrioles is found to be the same at meiotic and mitotic zooid formation. The number of cell divisions is, however, different in the 2 cases.

Ultrastructure of Nuclear Division in Paramecium Aurelia III. Meiosis in the Micronucleus During Conjugation

Ultrastructural features of micronuclear meiosis occurring during conjugation of the ciliate P. aurelia are described. Following pair formation the micronuclei enter prophase. The core of condensed chromatin in the micronucleus fragments, and almost the whole nucleus becomes filled with coiled and twisted chromatin elements. After about 1� 5 hr of pairing regions of condensed chromatin appear linked into strands 1-2 p;m long. Somewhat later, ill-defined, synaptinemal, complex-like material lies between paired masses of chromatin. Only a few synaptinemal complex-like structures can be seen in the nucleus. A short while later, synaptinemal complex-like material may be seen free in the nucleoplasm. After about 2�5 hr of pairing, the micronucleus expands into the crescent stage. Microtubules appear in the nucleus, which expands rapidly to a length of about 20 /Lm, usually oriented along the long axis of the cell. Concurrently with expansion the chromatin becomes diffuse and spreads along the length of the nucleus. This stage probably corresponds to diplotene. The nuoleus then shrinks, miorotubules reappear, and metaphase of the first meiotic division oocurs. A large number of chromosomes, with indistinct kinetochores, can be seen. At telophase the nucleus is 15-20 /Lm long and dumbbell-shaped. It breaks into two parts, there being no separation spindle, in contrast to mioronuclear mitosis. The seoond meiotic division occurs immediately. Ultrastructurally it resembles the first with respeot to metaphase, anaphase, and telophase. Since P. aurelia has two micronuclei, there are eight haploid meiotic products. The one nearest the paroral cone survives and the other seven disintegrate and are resorbed. The remaining nucleus undergoes a modified mitotic division. One of the two final produots becomes a stationary ("female") pronucleus, while the other becomes a migratory ("male") pronuoleus. Male pronuclei are exchanged aoross the fused paroral oone regions of the partners. The male pronucleus contains microtubules during its migration. After fusion to produce the synkaryon, the conjugants separate.