The analysis of exchanges in tritium-labelled meiotic chromosomes

The autoradiographic analysis of exchanges in tritium-labelled meiotic chromosomes is potentially a useful approach to the study of meiotic exchange events since this method differentially labels meiotic chromatids along their entire length. The main problem encountered in earlier autoradiographic studies is that of distinguishing label exchanges generated at chiasmata from label exchanges generated by sister chromatid exchange. This problem was overcome in the present study by the choice of a meiotic system (male meiosis of Stethophyma grossum) where chiasmata are limited to just one proximally localised chiasma in each bivalent. This system allows the positive identification of chiasma-generated label exchanges and demonstrates convincingly the origin of chiasmata through breakage and rejoining of homologous non-sister chromatids. Sister chromatid exchanges are also readily detected in labelled meiotic chromosomes of this species, where they occur with a mean frequency of 0.35 per chromosome. This frequency is similar to that found in mitotic spermatogonial cells and the exchanges are randomly distributed both within and between chromosomes. These features of meiotic sister chromatid exchanges suggest that they are unrelated to non-sister chiasmatic exchanges and they probably have no special meiotic significance.     

Normal and BrdC-substituted chromosomes during spermatogenesis with an endomeiotic chromosome reduplication in Carausius morosus Br.

Spermatogenesis involving an additional chromosome reduplication during zygotene in sporadic males and intersexes of the thelytokous phasmid Carausius morosus Br. has been examined using differential staining of chromatids after 5-bromodeoxycytidine incorporation. After reduplication autobivalents are formed by synapsis between identical sister chromosomes. Chiasmata are only formed after reduplication; they do not occur in constitutive heterochromatin, but can be formed in facultative heterochromatin, dependent on heteropycnosis and sex. Quadrivalents and U-type exchanges occur. In spermatogonia and spermatocytes the number of differentially stained chromosomes varies considerably; sister chromatid exchanges hardly appear. Sex bivalents with differentially stained chromosomes have a lower chiasma frequency than normally stained sex bivalents. Bivalents show reduced staining of all four, two outer, or one inner chromatid. Autobivalents arise in the same way as diplochromosomes; chromatids with the oldest DNA sub-units remain together during reduplication and are thus involved in sister chromosome pairing. The additional reduplication begins 7 days after the premeiotic S-phase, first metaphase after 19 days. Spermatogenesis is abnormal from first anaphase onwards.     

Analysis of exchanges in differentially stained meiotic chromosomes of Locusta migratoria after BrdU-substitution and FPG staining

Differential staining of the sister-chromatids of meiotic chromosomes of Locusta migratoria was achieved following abdominal implantation of BrdU tablets and fluorescent plus Giemsa (FPG) staining of fixed and squashed testicular follicles. This paper presents a detailed analysis of crossover exchanges between light and dark chromatids in monochiasmate bivalents. Approximately half the bivalents studied had visible exchanges of dark and light chromatids associated with the chiasmata, as expected if chiasmata originate by breakage and rejoining exchange events between randomly selected non-sister chromatids. In all the bivalents studied the visible crossover exchanges coincided exactly with chiasmata thus showing that chiasma movement (terminalisation) does not occur subsequent to crossing-over in Locusta migratoria, and that chiasmata are therefore accurate indicators of crossing over. It was noted that a proportion (9.5%) of chiasmata were associated with apparently anomalous exchanges of dark and light chromatids which could not be explained by conventional crossing-over. Various hypotheses for the origin of these anomalous exchanges are considered.     

Unequal Crossing Over Is the Principal Pathway of Homologous Recombination in Tandem Duplications of <Emphasis Type="Italic">Escherichia coli</Emphasis>

Homologous recombination between direct DNA repeats in tandem duplications usually leads to their dissociation. An even number of crossovers between two copies of a duplication should lead to the formation of diploid segregants, i.e., to the preservation of the duplication. However, in studies of the genotype of diploid segregants in heterozygous tandem duplications of Escherichia coli, it was shown that they arise by unequal exchanges between sister chromosomes rather than by intrachromosomal exchanges. Generally, these exchanges lead to the establishment of the homozygous state of (heterozygous) duplications. Since the available data suggest that the exchange between sister chromosomes may be coupled with DNA replication, it is supposed that unequal exchanges between direct DNA repeats occur in the process of DNA replication.__________Translated from Genetika, Vol. 41, No. 8, 2005, pp. 1038–1044.Original Russian Text Copyright © 2005 by Prokop’ev, Sukhodolets.     

Molecular mechanisms of production of symmetrical and asymmetrical chromosome exchanges

The formation of a heteroduplex is probably the first step leading to chromosome exchanges. Heteroduplexes occur by complementary association of two single DNA strands from different chromosomes. Therefore, repetitive DNA is the most common region involved in heteroduplex formation. DNA repeats are defined as polarized when they run the same, and antipolarized when they run opposite from centromere to telomere in two different chromosomes or chromosome arms. Paracentric inversions may easily account for the origin of antipolarized repeats. Palindromes are a special type of reversed repeats which always run the same from centromere to telomere independently of the existence or not of chromosomal rearrangements. Heteroduplexes leading to symmetrical exchanges can only occur by association of DNA strands with polarized repeats. On the other hand, antipolarized repeats are essential for the occurrence of asymmetrical rearrangements. Accordingly, the frequency of induced symmetrical and asymmetrical exchanges in a cell population may partially depend on the frequency of polarized and antipolarized repeats in the genome.     

Interspecies recombination, and phylogenetic distortions, within the glutamine synthetase and shikimate dehydrogenase genes of Neisseria meningitidis and commensal Neisseria species

Visual inspection showed clear evidence of a history of intraspecies recombinational exchanges within the neighbouring meningococcal shikimate dehydrogenase ( aroE  ) and glutamine synthetase ( glnA) genes, which was supported by the non-congruence of the trees constructed from the sequences of these genes from different meningococcal strains, and by statistical tests for mosaic structure. Many examples were also found of highly localized interspecies recombinational exchanges between the meningococcal aroE and glnA genes and those of commensal Neisseria species. These exchanges appear to have inflated the sequence variation at these loci, and have resulted in major distortions of the phylogenetic trees constructed from the sequences of the aroE and glnA genes of human pathogenic and commensal Neisseria species. Statistical tests for sequence mosaicism, and for anomalies within the Neisseria species trees, strongly supported the view that frequent interspecies recombination has occurred within aroE and glnA . The high levels of sequence variation, and intra- and interspecies recombination, within aroE and glnA did not appear to be due to a 'hitch-hiking' effect caused by positive selection for variation at a neighbouring gene. Our results suggest that interspecies recombinational exchanges with commensal Neisseria occur frequently in some meningococcal 'housekeeping' genes as they can be observed readily even when there appears to be no obvious selection for the recombinant phenotypes.     

From subsistence to market: A case study of the mbuti net hunters

This article describes the recent transition from subsistence to market hunting of some net-hunting Mbuti, a nomadic society of the Ituri Forest of Zaire. The history of this development dates to the late 1950s, as the Mbuti began to have increasing contact with commercial meat traders, called bachuuzi, from outside markets. Before this, the Mbuti had a long history of contact with local swidden agriculturalists, called bakbala, a relationship that continues today. It is sanctioned by religious beliefs, but material exchanges are also important. The Mbuti provide bakbala with meat and other forest resources and receive in return iron implements, tobacco, and cultivated food. The Mbuti view such items now as necessities; however, this does not allow the bakbala to subjugate or control them. Most exchanges take place in the bakbala's village at the Mbuti's discretion, and exchange rates are not fixed. Finally it appears that such trade does not undermine the resource base of either group. The Mbuti's commercial exchanges contrast with this subsistence-oriented system. Unlike the bakbala, the bachuuzi traders establish themselves in the Mbuti's forest camps, where they can promote intensified net hunting and monopolize trade of meat. The Mbuti tolerate the alien traders in camp because they are often a convenient source of food and other desired material goods; however, they do not share the traders' commercial motives. The Mbuti freely manipulate credit to their own advantage and they are often able to evade the traders' efforts at economic control. The future of market hunting is uncertain. Mbuti material needs are changing and the antelope fauna (Cephalophinae) upon which commercial trade depends is being reduced in some areas.A fellowship from the Thomas J. Watson Foundation provided financial support for this research. I am also thankful to Citoyen Bokanga Ekanga Botombele, Commissaire d'Etat du Zaire for his interest in my work and support in obtaining necessary visas, and to Père Thyert of the Catholic Mission of Biambwe for his generous hospitality. Terese Butler Hart, Colin Turnbull, Paul Riesman, Randall Packard, Paul Wholt, and Stuart Marks criticized drafts of this paper. Finally, I am most grateful to the Mbuti with whom I lived for so generously sharing their daily life with me.     

Induction of sister chromatid exchanges by benzidine in rat and human hepatoma cell lines and inhibition by indomethacin

The genotoxic activity of benzidine was studied in two cell lines derived from rat (H4) and human (HepG2) hepatomas which have been shown to be capable of activating certain promutagens. The responses were compared to results in two lung-derived fibroblast lines (IMR-90 and V79) which appear to have little or no metabolizing capability. Benzidine was found to induce sister chromatid exchanges in the two liver-derived cell lines in a dose-dependent fashion but failed to induce sister chromatid exchanges in the fibroblast lines. Since one proposed pathway for benzidine activation involves prostaglandin-mediated metabolism, we tested the effect of pretreatment with indomethacin, an inhibitor of this metabolic pathway. Indomethacin was highly effective in inhibiting benzidine-induced sister chromatid exchanges in both H4 and HepG2 cells. These results suggest that some DNA damage may occur in the livers of fast acetylating species such as the rat without prior N-acetylation and that some amount of DNA damage may occur in the livers of slow acetylating species, even when the liver is not the target organ for carcinogenesis.Abbreviations RI replication index - SCE sister chromatid exchanges     

X-Y exchanges in males of Drosophila hydei carrying the w m Co duplication

Males carrying, inserted on their Y chromosome, a small fragment of X including the w ⁺ (and N ⁺) locus (white-mottled Confluens, w ^m Co), were crossed with the purpose of scoring exceptional progeny. Some of the male and female exceptions were progeny tested and further analysed. Among the various mechanisms which may lead to exceptional offspring, X-Y exchanges proved to occur with a not negligible frequency. The rate was 3%. Nondisjunction accounts for the bulk of the remaining exceptions and appears to be increased considerably in the presence of rearrangements on one or the other of the sex chromosomes.The w ^m Co fragment after having been switched from Y to X by some mechanism other than regular crossing over, may become retransferred to a normal Y chromosome, but at a rate below 3%.     

Three-way differentiation of sister chromatids in endoreduplicated (M3) chromosomes of Bloom syndrome B-lymphoid cell line

Summary The three-way differentiation of sister chromatids (3-way SCD) in M₃ endoreduplicated chromosomes in a Bloom syndrome (BS) B-lymphoid cell line, suggested that in addition to exchanges between sister chromatids (intra-exchanges), non-sister chromatid exchanges (inter-exchanges) also occur, especially in BS high SCE cells. In BS diploid chromosomes such inter-exchanges probably get confused with intra-exchanges when total SCEs are accounted for. Bloom syndrome high SCE cells probably do not follow the same bromodeoxyuridine (BrdU) uptake pattern over three cell cycles as normal cells. The 3-way SCD in M₃ endoreduplicated chromosomes can be explained on the basis of Schvartzman's second model (1979) as well as Miller's model (1976), depending on the pattern of uptake of BrdU over three cell cycles. An interference in the previous events of exchanges in the following cell cycle (i.e., cancellation of SCEs) in BS chromosomes was observed in some regions, though not in high numbers.     

Y chromosome-specific mutations induced by a giant transposon in Drosophila hydei

Summary The w ^m Co duplication of Drosophila hydei (Dp (1; Y) 16B2-17B1) contains 13–16 bands in salivary gland chromosomes. The duplication resides preferentially in the X heterochromatin or on the Y chromosome. In some stocks frequent (up to 4×10^-3) exchanges of the duplication occur between different Y chromosomes (T(X; Y) and free Y) or between the X and the Y chromosome. About 60% of the T(X; Y)-Y exchanges induce mutations in the Y chromosomal male fertility genes of the recipient Y chromosome. From the mutational spectrum generated by the T(X; Y)-Y transpositions and from the variable efficiency as acceptor of different X-Y translocations it can be concluded that the exchanges show a remarkable site specificity: distal positions in the long arm of the Y chromosome are occupied preferentially. More proximal positions in the long arm of insertions into the short arm of the Y chromosome are found only with a lower frequency. No transpositions to the autosomes have been recovered. Duplications are lost with highly differing frequencies. The losses are not linked with insertions of the w ^m Co element into a new position and are more frequent than transpositions. Therefore, we regard the w ^m Co element as a giant transposon.     

Reciprocal Exchanges Instigated by Large Heterologies in the B2 Gene of Ascobolus Are Not Associated with Long Adjacent Hybrid DNA Stretches

In the gene b2 of Ascobolus immersus, large heterologies increase the frequencies of reciprocal exchanges on their upstream border (corresponding to the high non-Mendelian segregation side). Tests were made to determine whether these reciprocal exchanges, instigated by large heterologies, resulted from the blockage of a Holliday junction bordering a hybrid DNA tract extending from the end of the gene to the heterology. Three types of experiments were performed to answer this question. In all cases, results did not correlate the presence of reciprocal exchanges instigated by large heterologies with the presence of adjacent hybrid DNA tracts. These reciprocal exchanges were rarely associated with postmeiotic segregation at upstream markers, they were not associated with gene conversion of a marker within the interval and their frequency was not decreased by decreasing the frequency of hybrid DNA formation in the gene. These results led to the proposal of the existence of a precursor to reciprocal exchange different from a single branch-migrating Holliday junction. This precursor migrates rightward and its migration is dependent on the DNA sequence homology. The existence of this precursor does not exclude that reciprocal exchanges resulting from the maturation of single Holliday junctions bordering adjacent hybrid DNA tracts could also occur.     

THE CHROMOSOMES OF PACHYPHYTUM (CRASSULACEAE)

Eleven of the 12 species of Pachyphytum, all that are available, have n = 31–33 standard chromosomes, or a multiple. Accessory chromosomes were found in some or all collections of four species; some cells of one plant have more than 50 of them. Accessory chromosomes often occur in groups at metaphase I, corresponding to their origin from one to several chromocenters of prophase I. Intraspecific polyploidy occurs within five species, with diploids to 12-ploids (n = ca. 186) in P. compactum and diploids to decaploids (n = ca. 160) in P. hookeri. Although the basic chromosome number is high, evidence from meiosis in certain hybrids shows that the basic 31–33 chromosomes are probably all different: they do not pair with each other and they do not duplicate each other. Polyploids, with 62 or more chromosomes, are probably autopolyploids: they form multivalents, and the chromosomes they contribute to hybrids pair with each other. Three different probable hybrids have been found in the wild, and more than 300 hybrids have been produced in cultivation.     

A selective method for studying primary sex chromosome non-disjunction in females and males and X-Y exchange in males of Drosophila melanogaster including a demonstration of euchromatic X-Y exchange

A selective method was developed, based on negative complementation of the Abruptex alleles of the Notch focus, for studying primary sex chromosome non-disjunction in females and males of Drosophila melanogaster and X-Y exchange in males. The results show that the frequency of primary non-disjunction of structurally normal X chromosomes was lower than the frequency of X-derY non-disjunction in males. Double exchange between the X and the derY chromosome in the male occurs with a frequency of at least 0.091%. Single exchanges are naturally expected to occur with even higher frequency. Exchanges were interestingly at least partly of cuchromatic nature. The origin of these exchanges is at least partly of gonial origin.     

A model for the production of chromosome damage by Mitomycin C

A model is presented, which is based on the idea that the chromosome damage induced by Mitomycin C results directly from repair or misrepair of DNA molecules responsible for the linear continuity of the chromosomes. Testing the model with human cells confirms the prediction that exchanges with complete joining occur between chromosome regions containing homologous, repetitive DNA. Most probably incomplete exchanges involve homologous, but unique DNA sequences. — Prerequisites determining the MC-induced aberration patterns are the distribution of the chemical due to compartmentalization, the somatic pairing of chromosomes, and the occurrence of repeated or unique DNA sequences. — The scoring of different classes of MC-induced chromatid aberrations (attenuation, constriction, gap, break) in alcohol/acetic acid-fixed chromosome has a limited value.     

THE VASCULAR SYSTEM OF THE INFLORESCENCE AXIS OF ANDROPOGON GERARDII (GRAMINEAE) AND ITS BEARING ON CONCEPTS OF MONOCOTYLEDON VASCULAR TISSUE

The vascular bundles in the inflorescence axis of Andropogon gerardii occur in inner and outer systems. The inner system is made up of large, early developing strands that, at earliest stages of development, are precocious (= the appendage they are to serve has not yet been initiated). The outer system consists of later developing smaller strands that are open ended in a proximal direction (= strands differentiate basipetally in the cortex below the appendage they serve). Bundles of both the inner and outer systems are not connected to other procambium early in their development but exist as isolated strands. The veins of the inner system of the inflorescence axis occur as sympodia. The presence of inner and outer systems in the vascular tissue is common to most monocotyledons. However, amongst monocotyledons, only certain grasses have been shown to have strands of the inner system that are isolated early in development. Many dicotyledons have large strands which are precocious and some have smaller, later developing strands which are open ended in a proximal direction, hence they occur as isolated strands. These smaller strands in dicotyledons occur between large strands. Certain dicotyledons have an inner and an outer system of veins. Of these, some have veins of the inner system that differ from the inner system bundles of monocotyledons in that they also form part of the outer system of veins, or develop at a different time. One other dicotyledon with an inner and outer system, Bougainvillea, differs from monocotyledons only in that the bundles of the outer system do not seem to be isolated early in their development and anastomoses are seen between the inner and outer systems. Thus, it appears that monocotyledons differ from dicotyledons only in the presence of independent inner and outer systems of vascular bundles in the former. Thus, the hypothesis of Zimmermann and Tomlinson that there are basic differences between monocotyledon and dicotyledon vascular systems is not substantiated. It is even suspected that monocotyledon and dicotyledon vascular systems will be demonstrated to be modifications of a basic plan consisting of large, acropetally differentiating and smaller, basipetally differentiating strands.     

Postreplication repair in E. coli: strand exchange reactions of gapped DNA by RecA protein

Summary We have used a sensitive gel electrophoresis assay to detect the products of Escherichia coli RecA protein catalysed strand exchange reactions between gapped and duplex DNA molecules. We identify structures that correspond to joint molecules formed by homologous pairing, and show that joint molecules are converted by RecA protein into heteroduplex monomers by reciprocal strand exchanges. However, strand exchanges only occur when there is a 3-terminus complementary to the single stranded DNA in the gap. In the absence of a complementary free end, the two DNA molecules pair and short heteroduplex regions are formed by localised interwinding.     

A comparison of the geometric configurations adopted by radiation-induced chromatid interchanges in animals and plants

An analysis of radiation-induced chromatid interchanges in four animal and five plant species indicates that polarised (P) and X-type interchanges occur relatively more frequently in the plants, but non-polarised (N) and U type interchanges occur more frequently in the animals. An explanation of this difference, compatible with the observed geometric shapes of these interchanges and their differing propensity to incompleteness, is that; (a) in all the organisms examined, U- and X-type exchanges are formed during interphase between polarised chromosomes, i.e. they are notional ‘PU’ and ‘PX’ types, (b) the ratio ‘PU’ : ‘PX’ is similar in both animals and plants, (c) a proportion of these exchanges is established during interphase between looped and unlooped (and possibly terminal overlapping) segments, and this proportion is greater in animals; as a result of subsequent chromatin condensation these ‘PU’ and ‘PX’ exchanges may be converted to NU and NX types respectively, (d) ‘PX’ exchanges which are not converted by this mechanism, and NX exchanges which arise in late prophase as a result of this mechanism, are unstable and may be converted during prometaphase to the more stable NU and PU configurations respectively, (e) transformations (d) are relatively more frequent in the animal species because the flexural and tensional regidities of the chromosomes are greater than in the plant chromosomes.     

Unequal crossing over is the principal pathway of homologous recombination in tandem duplications of Escherichia coli

Homologous recombination between direct DNA repeats in tandem duplications usually leads to their dissociation. An even number of crossovers between two copies of a duplication should lead to the formation of diploid segregants, i.e., to the preservation of the duplication. However, in studies of the genotype of diploid segregants in heterozygous tandem duplications of Escherichia coli, it was shown that they arise by unequal exchanges between sister chromosomes rather than by intrachromosomal exchanges. Generally, these exchanges lead to the establishment of the homozygous state of (heterozygous) duplications. Since the available data suggest that the exchange between sister chromosomes may be coupled with DNA replication, it is supposed that unequal exchanges between direct DNA repeats occur in the process of DNA replication.     

Q-bands in Lilium and their relationship to C-banded heterochromatin

In L. pardalinum, narrow bands of quinacrine fluorescence are distributed throughout the chromosomes. These vary in intensity from dull to bright, and their constant pattern allows all chromosomes to be recognized. Bright bands occur at some centromeres, and near all three nucleolar constrictions. In L. longiflorum, similar Q-bands occur along chromosomes, but they are less distinctive and their pattern does not closely match that of L. pardalinum. Also, L. longiflorum does not have bright regions at or near primary and secondary constrictions. Most Q-bands do not coincide with dark Giemsa C-bands, except for the bright nucleolar and centromeric regions in L. pardalinum. All C-banded heterochromatin stains identically after SSC pretreatment, dark with Giemsa and bright with quinacrine.— The many Q-bands of varying intensity, wide distribution and constant pattern, unrelated to C-bands, may be analogous to mammalian Q-bands. Such universality is expected if Q-bands area fundamental component of chromosome architecture.