Biochemical aspects of colchicine action on meiotic cells

DNA topology in heterochromatin (a hypothesis)

A hypothesis explaining the known heterochromatin features--a compact DNA packaging, transcriptional inactivity, propensity to aggregate (stickiness) and position effect variegation-is described. The hypothesis is based on the assumption that DNA molecules in heterochromatin are topologically open and contain single-strand breaks in the regions with identical or similar primary sequences.

The leptotene-zygotene transition of meiosis

The leptotene/zygotene transition of meiosis, as defined by classical cytological studies, is the period when homologous chromosomes, already being discernible individualized entities, begin to be close together or touching over portions of their lengths. This period also includes the bouquet stage: Chromosome ends, which have already become integral components of the inner nuclear membrane, move into a polarized configuration, along with other nuclear envelope components. Chromosome movements, active or passive, also occur. The detailed nature of interhomologue interactions during this period, with special emphasis on the involvement of chromosome ends, and the overall role for meiosis and recombination of chromosome movement and, especially, the bouquet stage are discussed.

Nucleotypic influences on chromosome-specific chiasma variation inCrepis capillaris. I. Responses to early colchicine treatment and chromosome doubling

Additive and multiplicative effects of colchicine treatment at the seedling stage and of chromosome doubling on chromosome-specific chiasma frequency at metaphase I have been measured in comparisons between C0and untreated diploids and between C0autotetraploids and C0diploids. Early colchicine treatment increases the frequency of chromosome C univalents to 1.8% but has no similar effect on chromosomes A and D. Colchicine treatment has little net effect on mean chiasma frequency, deducting an average of 0.204 chiasmata per set but otherwise multiplying the mean by a factor of 1.182. These additive and multiplicative effects represent averages of six phenotypes. Chromosome doubling in a tetraploid–diploid chimaera subtracts an average of 0.265 chiasmata per set but otherwise doubles the numbers of chiasmata at the diploid level (× 2.134). Comparison of six diploids and tetraploids reveals modest average additive (+ 1.103) and multiplicative effects (× 1.190). The implications of these findings are discussed in the light of new analyses of previously published data. Key words : chiasmata, Crepis, colchicine, meiosis, polyploidy.

The perinuclear microtubule-organizing center and the synaptonemal complex of higher plants share a common antigen: its putative transfer and role in meiotic chromosomal ordering

Recognition of homologous chromosomes during meiotic prophase is associated in most cases with the formation of the synaptonemal complex along the length of the chromosome. Telomeres, located at the nuclear periphery, are preferential initiation sites for the assembly of the synaptonemal complex. In most eukaryotic cells, telomeres cluster in a restricted area, leading to the "bouquet" configuration in leptotene-zygotene, while this typical organization progressively disappears in late zygotene-pachytene. We wondered whether such striking changes in the intranuclear ordering and pairing of meiotic chromosomes during the progression of prophase I could be correlated with activity of the centrosome and/or microtubule-organizing center (MTOC). Plant cells may be used as a model of special interest for this study as the whole nuclear surface acts as an MTOC, unlike other cell types where MTOCs are restricted to centrosomes or spindle pole bodies. Using a monoclonal antibody (mAb 6C6) raised against isolated calf centrosomes we found that the 6C6 antigen is present over the entire surface of the plant meiotic nucleus, in early prophase I, before chromosomal pairing. At zygotene, short fragments of chromosomes become stained near the nuclear envelope and within the nucleus. At pachytene, after complete synapsis, the labeling specifically concentrates within the synaptonemal complexes, although the nuclear surface is no longer reactive. Ultrastructural localization using immunogold labeling indicates that the 6C6 antigen is colocalized with the synaptonemal complex structures. Later in metaphase I, the antigen is found at the kinetochores. Our data favor the idea that the 6C6 antigen may function as a particular "chromosomal passenger-like" protein. These observations shed new light on the molecular organization of the plant synaptonemal complex and on the redistribution of cytoskeleton-related antigens during initiation of meiosis. They suggest that antigens of MTOCs are relocated to chromosomes during the synapsis process starting at telomeres and contribute to the spatial arrangement of meiotic chromosomes. Such cytoskeleton-related antigens may acquire different functions depending on their localization, which is cell-cycle regulated.

Factors affecting formation of micronucleus-like structures after colchicine treatment of Trichoderma reesei

Micronucleus-like structures were produced in Trichoderma reesei only when 0.1% colchicine treatment was used to enhance nuclear division. The average DNA content of these 'small nuclei' was 30% that of the normal nuclei, indicating that they were aneuploid nuclei. Such small nuclei may be useful in transferring small amounts of DNA into protoplasts.

Synaptonemal complex damage induced by clastogenic and anti-mitotic chemicals: implications for non-disjunction and aneuploidy

Mice were treated with mitomycin C, cyclophosphamide, amsacrine, colchicine, or vinblastine sulfate, and meiotic prophase cells analyzed for synaptonemal complex (SC) damage. All test agents caused synaptonemal complex breakage and synapsis irregularities, although propensities for inducing specific types of damage at S-phase or prophase stages varied among the chemicals. The data indicate that SC analysis can reveal chemical-specific alterations to meiotic homologue pairing/synapsis which have not generally been recognized, and which theoretically may be implicated in non-disjunction.

Mechanistic aspects on chemical induction of spindle disturbances and abnormal chromosome numbers

Work on the chemical induction of spindle disturbances and abnormal chromosome numbers, and work on the composition and biochemistry of the spindle are reviewed. Some early investigations have shown that there is an unspecific mechanism for chemical induction of spindle disturbances. This mechanism is based on the interaction of compounds with cellular hydrophobic compartments. Some compounds act differently and are more active than predicted from their lipophilic character. Selected compounds of that kind and their possible mechanisms of action are discussed. Changes in sulfhydryl and ATP levels, oxidative damage of membranes and impaired control of cytoplasmic Ca2+ levels are discussed in this context.

Effect of the antitubulin drug nocodazole on meiosis and postmeiotic development in Tetrahymena thermophila. Induction of achiasmatic meiosis

Nocodazole (ND), a potent antitubulin drug, can be used to dissect the steps of meiosis in Tetrahymena, presumably by interfering with the assembly of microtubules. Its effects depend upon the time during conjugation at which the drug is applied. When applied prior to the elongation of the micronucleus into the characteristic 'crescent' configuration, no crescent is formed and the chromosomes of prepachytene and pachytene condense into spherical nuclei. If ND is applied after micronuclear elongation has begun, but before it is fully elongated, the chromosomes fail to synapse and appear in metaphase I as unpaired monovalents. In contrast, the metaphase I chromosomes appear as bivalents when ND is applied later, during or after the crescent has reached its maximum elongation. Still later, application of ND inhibits chromosome movements during anaphase and telophase of either meiotic division, but does not prevent separation of kinetochores. In some of the blocked restitutive nuclei an additional round of chromosome replication occurs, corresponding to the third pregamic division in normal conjugation. The hyperploid micronuclei produced by such treatment may be useful in certain genetic manipulations and in studying the regulation of nuclear DNA content.

Replication and nicking of zygotene DNA sequences. Control by a meiosis-specific protein

A protein having a molecular weight of 73,000 daltons has been purified from the nuclear membranes of preleptotene, leptotene, and zygotene cells. It has been named the leptotene protein (L-protein) because of its role in suppressing the replication of zygotene DNA sequences through leptotene until the initiation of zygotene DNA synthesis. The protein has been found to be highly specific in its inhibitory activity. S-phase replication in somatic nuclei and in microspore nuclei are unaffected by the protein. Only zygotene DNA sequences appear to be affected. L-protein binds specifically to zygotene DNA. The binding is limited to a relatively short DNA segment, probably no longer than 90 base pairs (bp). Chloroplast and mitochondrial DNA do not bind to the protein, but a low level of binding is displayed by DNA from several other eukaryotic species. The L-protein also has the capacity to nick the bound DNA in the presence of ATP. Nicking does not occur in the absence of binding. Using supercoiled plasmids with zygotene DNA inserts as substrates, the nicking has been found to be confined to a small region of the plasmid and to occur in only one of the strands. The L-protein is considered to be one of the principal factors responsible for the irreversible commitment of cells to meiosis at the end of the preleptotene S-phase. It is also proposed that its endonucleolytic activity plays a role in the initiation of synapsis.

Colchicine induced quadrivalent formation in Helianthus: evidence of ancient polyploidy

The premeiotic treatment of microsporocytes of Helianthus annuus and H. laciniatus (x= 17) results in the formation of multivalent chromosome configurations at meiosis in these 'diploid' species. This is interpreted to mean that colchicine has disrupted the normal attachment of genomes to the nuclear membrane and allowed synapsis of the ancestral genomes, indicating the polyploid origin of these 'diploid' species. This technique should be of importance in generating intergenomal recombination in classical allopolyploid agronomic species because it could produce new genotypic combinations normally impossible to obtain.

Tubulin from cultured tobacco cells: isolation and identification based on similarities to brain tubulin

The microtubule protein, tubulin, was isolated from most other proteins of cell suspension cultures of Nicotiana tabacum L. by its copolymerization with cow-brain tubulin. Cow-brain tubulin was added to the soluble protein fraction of extract from (35)S-labeled tobacco cells and subjected to two cycles of temperature-dependent assembly-disassembly (copolymerization). When analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) about 70% of the radioactivity in the twice copolymerized protein was found in a prominent doublet migrating close to the doublet of brain tubulin. When analyzed by two-dimensional isoelectric-focusing-SDS-PAGE the radioactive doublet behaved like the doublet of brain tubulin. Limited proteolysis of the individual polypeptides of the coublets showed that, while the peptide maps of the leading radioactive band and of the β-subunit of brain tubulin were virtually indistinguishable, the maps of the trailing radioactive band and of the α-subunit of brain tubulin, though similar, were not identical. Most of the copolymerized (35)S-labeled protein also behaved like brain tubulin during gel filtration and ion-exchange chromatography. It is concluded that the doublet of radioactive polypeptides isolated by copolymerization with brain tubulin are tobacco tubulin polypeptides that have, in their native as well as denatured forms, properties very similar to, but not identical with, cow brain tubulin. Apparently, tubulin has been highly conserved during evolution.

A colcemid-sensitive mechanism involved in regulation of chromosome movements during meiotic pairing

Active movements of the chromosomes may be needed in the process, where homologous chromosomes find each other during the meiotic pairing. Because the components of the cytoskeleton are generally believed to be responsible for all movements in living nonmuscle cells, we have analyzed the regulation of the movements of zygotene chromosomes in the male rat by using specific inhibitors of the assembly of the various components of the cytoskeleton. --Colcemid, an inhibitor of microtubule formation, completely inhibited the chromosome movements in vitro at a concentration of 1 microgram/ml. This was associated with a damage of the nuclear envelope revealed by the electron microscopic analysis. Another inhibitor of microtubule formation, vinblastine, was ineffective below the level of general toxicity (100 microgram/ml). A specific microfilament inhibitor, cytochalasin B was similarly ineffective. --The findings suggest the presence of a specific colcemid-sensitive mechanism in the nuclear envelope of the zygotene spermatocytes, which regulates the movements of the chromosomes during meiotic pairing.

Microtubule-membrane interactions in cilia. I. Isolation and characterization of ciliary membranes from Tetrahymena pyriformis

Tetrahymena ciliary membranes were prepared by four different techniques, and their protein composition was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), electron microscopy, and two-dimensional thin-layer peptide mapping. Extraction of the isolated cilia by nonionic detergent solubilized the ciliary membranes but left the axonemal microtubules and dyneine arms intact, as determined by quantitative electron microscopy. The proteins solubilized by detergent included a major 55,000-dalton protein, 1-3 high molecular weight proteins that comigrated, on SDS-PAGE, with the axonemal dynein, as well as several other proteins of 45,000-50,000 daltons. Each of the major proteins contained a small amount of carbohydrate, as determined by PAS-staining; no PAS-positive material was detected in the detergent-extracted axonemes. The major 55,000-dalton protein has proteins quite similar to those of tubulin, based on SDS-PAGE using three different buffer systems as well as two-dimensional maps of tryptic peptides from the isolated 55,000-dalton protein. To determine whether this tubulin-like protein was associated with the membrane or whether it was an axonemal or matrix protein released by detergent treatment, three different methods to isolate ciliary membrane vesicles were developed. The protein composition of each of these differetn vesicle preparations was the same as that of the detergent-solubilized material. These results suggest that a major ciliary membrane protein has properties similar to those of tubulin.

Colchicine-induced resistance to antibiotic and amino-acid analogue in plant cell cultures

Colchicine-resistant plant cell strains have been isolated from cell suspensions of carrot and sycamore. In the same way as colchicine-resistant animal cell strains, they show an increased resistance towards streptomycin and N-methylalanine. Cultivation under non selective conditions leads to a slow progressive loss of the resistance.

Meiotic and radiation studies in four oligochiasmatic men

The meiotic findings in four oligochiasmatic males are described. Radiation studies on the somatic cells of three of them failed to provide evidence for a reduced facility to repair DNA which might also have accounted for the observed failure of chiasma formation at meiosis. The data support the idea that the 'low chiasma count' condition in sterile men is of mixed aetiology.

Biochemical analysis of genetic recombination in eukaryotes

Recent studies concerning molecular mechanisms of genetic recombination in eukaryotes are reviewed. Since many of these studies have focused on the testable predictions arising from the hybrid DNA theory of genetic recombination, this theory is summarised. Experiments to determine the time of meiotic crossing-over and the structure of the synaptonemal complex which facilitates meiotic crossing-over are described. Investigations of DNA nicking and repair events implicated in recombination are discussed. Properties of proteins which may facilitate hybrid DNA formation, and biochemical evidence for hybrid DNA formation are presented. Finally, a nuclease which has been implicated in gene conversion is described.

FUSION OF CHROMOCENTERS IN PREMEIOTIC INTERPHASE OF SECALE CEREALE AND ITS POSSIBLE RELATIONSHIP TO CHROMOSOME PAIRING

The premeiotic interphase of rye has been found to exhibit a synchronized fusion of chromocenters. This results in a solid knot of heterochromatin localized to one side of the nucleus. Colchicine injected into the immature inflorescence of diploid rye has been found to induce univalent and multivalent formation at metaphase I. The early premeiotic interphase was the phase of development most sensitive to colchicine. Colchicine was also found to have a similar disruptive effect on fusion of chromocenters in the premeiotic interphase. The relationship of these observations to chromosome pairing is discussed.

THE EFFECT OF COLCHICINE ON CHROMOSOME PAIRING

Beginning at 120 hours prior to first metaphase of meiosis (MI) a 0.03% aqueous solution of colchicine was injected into the boot of pentaploid (hexaploid triticale × tetraploid wheat) hybrids developing at 20 °C ± 1° under continuous illumination. Colchicine applied 40 h or less prior to MI had no effect on chromosome pairing, while its application 40 h or more prior to MI induced a steady decline, culminating in a 40% reduction in chromosome pairing at about 80 h from MI. Between 48 and 35 h before MI (late premeiotic interphase to early zygotene) meiocytes underwent a period of active nucleolar fusion. The time, therefore, at which the colchicine sensitive aspects of chromosome pairing were completed coincided with the completion of nucleolar fusion. From comparison with other findings it was concluded that there is a colchicine sensitive bouquet stage which appears in leptotene and early zygotene; this bouquet is responsible for active nucleolar fusion and final close association between homologous chromosomes.

The meotic prophase in Bombyx mori females analyzed by three dimensional reconstructions of synaptonemal complexes

Serial sectioning followed by three dimensional reconstruction of lateral componenets of the synaptoemal complex have been used to follow chromosome pairing during theprophase of the achiasmatic meiotic division in the silkworm. Bombyx mori. During leptotene and early zygotene, the lateral components become attached to the nuclear envelope at a specific region, thus forming a chromosome bouquet. The attachment of lateral componenets to the nuclear envelope precedes the completion of the components between their attachment points. Synapsis and synaptonemal complex formation start during the period of lateral component organization in the individual nucleus. Telomeric movements on the nuclear envelope occur at two stages of the prophase: the chromosome pairing appears to be initiated by an association of unpaired ends of homologus chromosomes, the nature of this primary attraction and recognition being unknown. Secondly, the paired chromosomes become dispersed in the nucleus by shifting of attachment sites of completed synaptonemal complexes at the end of zygotene. This movement is possible related to a membranes flow occuring during this stage. Membrane material is synthesized at the region of synaptonemal complex attachment. Later, the excess membrane material is shifted to the opposite pole where it protudes into the lumen of the nuclei thus forming vacuoles. Two previously undescribed features of chromosomes paring were revealed. In late zygotene, chromosome pairing and synaptonmal complex formation were frequently observed to be delayed or even prevented over s short distance by interlocking two bivalents, both being attached to the nuclear envelope. Such interlocking of bivalents was not found in pachytene...

Prophase chromosome movements in living house cricket spermatocytes and their relationship to prometaphase, anaphase and granule movements

Chromosome and granule movements in meiotic prophase and prometaphase have been studied by time-lapse cinemicrography in live spermatocytes of the house cricket, Acheta domesticus. Chromosome movements in prophase cells, up to one hour or more before breakdown of the nuclear envelope, are described. These movements are frequent but saltatory; are based mostly at chromosome ends but also at kinetochores; occur in very intimate association with the inside of the nuclear envelope; are directed towards and away from the extranuclear centres (centrioles); tend weakly to accumulate bivalents round the two centres and reach a velocity of 0.65 micron/sec. Saltatory movements in granules associated with extranuclear asters are remarkably similar to basic characteristics to the intranuclear chromosome movements. Surprisingly, the chromosome movements (and those granules) are reversably blocked by colcemid (but not lumi-colcemid), and yet occur in the apparent absence of an intranuclear envelope. However, kinetochore movements in very early prometaphase are similar in velocity and other respects to prophase movements; later prometaphase movements are clearly slower, and those of anaphase very much slower still. -The prophase movements suggest a two component model for motion: a non-microtubule, linear force producer together withrotubules with a skeletal, orientational role. Arguably, both these components are also necessary for chromosome movements in prometaphase and anaphase.

TELOMERIC ASSOCIATIONS OF GAMETIC AND SOMATIC CHROMOSOMES IN DIPLOID AND AUTOTETRAPLOID ORNITHOGALUM VIRENS

In acetocarmine root-tip squashes, diploid cells of Ornithogalum virens in prophase exhibit configurations resulting from end-to-end associations of the six chromosomes. Homologues lie opposite one another in a ring. Prophase chromosomes of the autotretraploid cells likewise associate end-to-end; however, four homogues instead of two generally lie adjacent to one another and four (or eight) ends are often connected instead of the two (or four) found in diploid cells. Prophase chromosomes in a haploid pollen grain of a diploid form an open chain of three chromosomes, whereas in pollen from the autotetraploid balanced gametes form a configuration in which homologous pairs lie adjacent to one another and are attached end-to-end to other homologous pairs of nonhomologous chromosomes to form a chain. These observations are discussed in terms of the role telomeric associations may play in recognition and pairing of homologous chromosomes during meiotic prophase. This recognition of homologues may occur as early as syngamy.