The distribution of mitomycin C-induced sister chromatid exchanges in the euchromatin and heterochromatin of the Indian Muntjac

The frequency of sister chromatid exchanges (SCEs) induced by mitomycin C (MMC) in Indian Muntjac chromosomes was determined by the fluorescence plus Giemsa (FPG) technique. Using scanning cytophotometry the relative DNA content of each chromosome was measured with and without acid or alkali pretreatments for C-banding. During acid and alkali treatments, euchromatin lost 20 to 30% of its DNA, while heterochromatin lost less than 5%; an intermediate DNA loss was observed for the short arm of the X chromosome. After growth of cells in the presence of MMC during the first cycle and in the presence of bromodeoxyuridine (BrdU) during the first and second cycles of DNA replication, SCEs in the euchromatin were proportional to DNA content. SCEs at the junctions between the neck of the X chromosome and the long and short arms occurred more frequently than expected. A threshold effect for the induction of SCEs was observed in regions resistant to DNA extraction by acid and alkali treatments (i.e., the neck and short arm of the X chromosome). At high concentrations of MMC, the frequency of SCE at each junction appears to plateau at 0.5.     

Distribution of sister chromatid exchanges in the euchromatin and heterochromatin of the Indian muntjac

The frequency of sister chromatid exchanges (SCEs) was determined for the chromosomes (except Y2) of the Indian muntjac stained by the fluorescence plus Giemsa (FPG) or harlequin chromosome technique. The relative DNA content of each of the chromosomes was also measured by scanning cytophotometry. After growth in bromodeoxyuridine (BrdU) for two DNA replication cycles. SCEs were distributed according to the Poisson formula in each of the chromosomes. The frequency of SCE in each of the chromosomes was directly proportional to DNA content. A more detailed analysis of SCEs was performed for the three morphologically distinguishable regions of the X-autosome composite chromosome. The SCE frequency in the euchromatic long arm and short arm were proportional to the amount of DNA. In contrast, the constitutive heterochromatin in the neck of this chromosome contained far fewer SCEs than expected on the basis of the amount of DNA in this region. A high frequency of SCE, however, was observed at the point junctions between the euchromatin and heterochromatin.     

Effects of supernumerary heterochromatin on chiasma condition in two species of Acrididae (Orthoptera)

Amblytropidia australis and Dichroplus elongatus were found to be polymorphic for supernumerary heterochromatin. In both, basic karyotypes are 2n=22+XO in males.Mitotically unstable extra chromosomes were detected in a population of A. australis. The Bs are telocentric and their number varies from O to 2 within individuals. Mean frequencies of interstitial and total chiasmata at diplotene were compared between individuals with and without Bs. The mean frequency of interstitial chiasmata increases with the number of Bs per cell.A supernumerary terminal segment in S₁₀ pair was observed in a heterozygous condition in several individuals of D. elongatus. The localization and frequency of chiasmata at diplotene were studied. The segment has an intrachromosomal effect since it modifies the location of chiasmata in the bivalent involved.Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET).     

Partial tetrasomy 9(9pter to 9q2101) due to an extra iso-dicentric chromosome.

A 3-year-old boy with partial No. 9 tetrasomy is described. The patient showed markedly retarded physical and mental development as well as multiple congenital anomalies. Routine chromosome analysis revealed an extra C-group chromosome. It had a pronounced secondary constriction at the proximal part of its long arm. Based on studies by a variety of banding techniques, the extra chromosome was identified to be an iso-dicentric No. 9 chromosome with inactivation of one of the two centromeres, the karyotype being 47,XY, + DIC (9)(Q2101). The value of BrdUrd treatment was emphasized in the detection of a very small piece of euchromatin within a long stretch of constitutive heterochromatin.     

The influence of 5-azacytidine on the condensation of the short arm of rye chromosome 1R in Triticum aestivum L. root tip meristematic nuclei

This paper describes the effects of 5-azacytidine on the condensation state of rye (Secale cereale L.) chromatin introduced into the wheat genome (Triticum aestivum L. cv. Beaver). The wheat cultivar Beaver carries a translocation between the short arm of rye chromosome 1R (1RS) and the long arm of wheat chromosome 1B (1BL/1RS). 1RS can be detected using genomic in situ hybridisation and carries a ribosomal DNA (rDNA) locus that can be simultaneously detected using multiple labelling strategies. The rDNA locus divides 1RS into a distal region that is gene rich and a proximal region that is gene poor and highly methylated. 1RS also carries a large block of subtelomeric heterochromatin. The drug, which acts to inhibit DNA methylation in plants, has three pronounced effects on interphase nuclei. (1) It induces aberrant condensation of the rye subtelomeric heterochromatin and in many cases induces sister chromatid separation in the subtelomeric heterochromatin of G2 nuclei. (2) Nuclei trisomic for 1RS are observed at low frequency in treated material and are probably a consequence of aberrant sister chromatid separation or condensation. (3) The drug alters normal condensation of 1RS euchromatin. However, contrary to expectation the effect is not simply to induce decondensation. The proximal region of the arm actually condenses at low levels of drug administration while the distal region remains unaltered or increases its decondensation state. Increasing the concentration of 5-azacytidine induces a biphasic response and at the highest concentration used all regions of the arm show signs of decondensation. Thus the influence of the drug on chromatin condensation depends on the genomic structure. Received: 14 July 1997; in revised form: 26 August 1997 / Accepted: 27 August 1997     

Studies on the linkage map of chromosome 4 of the tomato and on the transmission of induced deficiencies

Various genetic and cytogenetic techniques were applied to an analysis of the linkage map of chromosome 4-a chromosome that is considered to be representative of the tomato complement. Loci have been approximated by standard F₂ linkage tests for 18 genes, including six on the short arm and 12 on the long arm, covering a map distance of 132 units (c.m.). The loci of four key markers were approximated on pachytene chromosomes by a study of radiation-induced deficiencies:clau near the end of the short arm,ful near the euchromatic-heterochromatic boundary of the short arm,ra near the same region on the long arm, ande in the middle of the long arm. Normal transmission for a presumedra deficiency suggests that this gene lies in the heterochromatin of 4L. According to tertiary trisomic segregation,w-4, known by linkage test to be proximal tora, resides on 4L, therefore probably also in the heterochromatic region. The centromere is consequently delimited to a region of 4 c.m. betweenful andw-4. The resultant maps reveal a very much lower crossover rate within heterochromatin—estimated at 0.8 c.m./μ—than for euchromatin—estimated at 4.8 c.m./μ for the short arm and 5.7 for the long arm. Also apparent is a strong tendency of the genes to concentrate toward the centromere of the genetic map and in the proximal sections of the euchromatin of the cytological map. Studies were made of the genetic transmission of various small deficiencies on chromosome 4 as well as a newly discovered deficiency fornv on chromosome 9, supporting the following conclusions. Regardless of their size, deficiendies of euchromatin are not transmitted. Deficiencies of heterochromatin are transmitted to a varying extent depending on their size. A presumed deficiency forra that is too small to be detected cytologically was transmitted without adverse effect on gametes. Somewhat larger deficiencies may be transmitted at reduced rates by female gametes and the largest at extremely low rates, even on the female side.     

Studying Trans-Effects of Modifiers on the Position-Effect Variegation in a Set of Euchromatin–Heterochromatin Rearrangements in Drosophila melanogaster

The effects of suppressors of position-effect variegation were studied in a set of euchromatin–heterochromatin rearrangements of the X chromosome accompanied by inactivation of the gene wapl.The rearrangements differed from one another in the size of the heterochromatic block adjacent to euchromatin, with the euchromatin–heterochromatin border remaining unchanged. In one rearrangement (r20), the position effect caused by a small block of adjacent heterochromatin may be determined by its interaction with the neighboring main heterochromatic region of the X chromosome. Chromosome 3 (the RT chromosome) was found to have a strong suppressing effect on all rearrangements, irrespective of the amount of heterochromatin adjacent to euchromatin. Su-var(3)9, a known suppressor of the position-effect variegation, had a considerably weaker suppressing effect. The RT chromosome had the strongest suppressing effect on the rearrangement r20.     

Trans-effect of modifiers on position-effect variegation in a set of euchromatin-heterochromatin rearrangements in Drosophila melanogaster]

The effects of suppressors of position-effect variegation were studied in a set of euchromatin-heterochromatin rearrangements of the X chromosome accompanied by inactivation of the gene wapl. The rearrangements differed from one another in the size of the heterochromatic block adjacent to euchromatin, with the euchromatin-heterochromatin border remaining unchanged. In one rearrangement (r20), the position effect caused by a small block of adjacent heterochromatin may be determined by its interaction with the neighboring main heterochromatic region of the X chromosome. Chromosome 3 (the RT chromosome) was found to have a strong suppressing effect on all rearrangements, irrespective of the amount of heterochromatin adjacent to euchromatin. Su-var(3)9, a known suppressor of the position-effect variegation, had a considerably weaker suppressing effect. The RT chromosome had the strongest suppressing effect on the rearrangement r20.     

Preferential derivation of abnormal human G-group-like chromosomes from chromosome 15

Summary The marked binding of antibodies specific for 5-methylcytidine to the short arm of chromosome 15 distinguishes this chromosome from the other human acrocentrics. This method has been used to study over 60 individuals including 12 who did not have Down's syndrome, but who did have an extra G-group sized acrocentric chromosome. In six cases the extra chromosome did not show intensive binding of anti-5-methylcytidine. In the other six cases, the extra chromosome contained a 5-methylcytidine rich band at each end indicating that both ends were derived from chromosome 15 and contained centromeric heterochromatin normally present on the short arm of chromosome 15. The duplication of short arm material in the abnormal chromosomes was confirmed in all cases by quinacrine staining, nucleolar organizer (Ag-AS) staining or C-banding. In three cases, the abnormal chromosome appeared to arise from two different chromosomes 15. Several possible mechanisms for the production of the abnormal chromosome are discussed. The individuals with this abnormal chromosome all showed some degree of mental retardation, but few common physical findings.     

Satellited Y chromosomes: Structure,origin, and clinical significance

Summary Three cases of inherited satellited Y chromosomes (Yqs) were analysed using several cytogenetic techniques. The cytogenetic data of the 14 cases of Yqs chromosomes described to date were reviewed. All Yqs chromosomes carry an active nucleolus organizer region (NOR) in their long arm and must have developed from translocations involving the short arms of the acrocentric autosomes. The structure of the heterochromatic satellite region in the Yqs chromosomes shows conspicuous inter-familial differences; this permits the reconstruction of the translocations from which the various Yqs were derived. Some causal factors leading to the development of Yqs chromosomes are considered: the specific localization of the four satellite DNAs and highly methylated DNA sequences in the karyotype, and some new experimental data on the spatial arrangement of heterochromatic regions in interphase nuclei. These provide distinct evidence for a preferential involvement of the autosomes 15 and 22 in the translocations with the Y heterochromatin. All clinical reports documenting Yqs males born with malformations were reviewed. It appears that the presence of an extra NOR and NOR-associated heterochromatin in the Yqs chromosomes does not cause any phenotypic abnormalities (as long as the Y euchromatin is intact). The possibility that a Yqs chromosome predisposes to non-disjunction and/or to translocations of other chromosomes is discussed.     

Distorted heterochromatin replication in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> polytene chromosomes as a result of euchromatin-heterochromatin rearrangements

Studies of the position effect resulting from chromosome rearrangements in Drosophila melanogaster have shown that replication distortions in polytene chromosomes correlate with heritable gene silencing in mitotic cells. Earlier studies mostly focused on the effects of euchromatin-heterochromatin rearrangements on replication and silencing of euchromatic regions adjacent to the heterochromatin breakpoint. This review is based on published original data and considers the effect of rearrangements on heterochromatin: heterochromatin blocks that are normally underrepresented or underreplicated in polytene chromosomes are restored. Euchromatin proved to affect heterochromatin, preventing its underreplication. The effect is opposite to the known inactivation effect, which extends from heterochromatin to euchromatin. The trans-action of heterochromatin blocks on replication of heterochromatin placed within euchromatin is discussed. Distortions of heterochromatin replication in polytene chromosomes are considered to be an important characteristic associated with the functional role of the corresponding genome regions.     

Distorted heterochromatin replication in Drosophila melanogaster polytene chromosomes as a result of euchromatin-heterochromatin rearrangements

Studies of the position effect resulting from chromosome rearrangements in Drosophila melanogaster have shown that replication distortions in polytene chromosomes correlate with heritable gene silencing in mitotic cells. Earlier studies mostly focused on the effects of euchromatin--heterochromatin rearrangements on replication and silencing of euchromatic regions adjacent to the heterochromatin breakpoint. This review is based on published original data and considers the effect of rearrangements on heterochromatin: heterochromatin blocks that are normally underrepresented or underreplicated in polytene chromosomes are restored. Euchromatin proved to affect heterochromatin, preventing its underreplication. The effect is opposite to the known inactivation effect, which extends from heterochromatin to euchromatin. The trans-action of heterochromatin blocks on replication of heterochromatin placed within euchromatin is discussed. Distortions of heterochromatin replication in polytene chromosomes are considered to be an important characteristic associated with the functional role of the corresponding genome regions.     

The distribution of male meiotic pairing sites on chromosome 2 of Drosophila melanogaster: meiotic pairing and segregation of 2-Y transpositions

The distribution of meiotic pairing sites on a Drosophila melanogaster autosome was studied by characterizing patterns of prophase pairing and anaphase segregation in males heterozygous for a number of 2-Y transpositions, collectively coveringall of chromosome arm 2R and one-fourth of chromosome arm 2L. It was found that all transpositions involving euchromatin from chromosome 2, even short stretches, increased the frequency of prophase I quadrivalents involving the sex and second chromosome bivalents above background levels. Quadrivalent frequencies were the same whether the males carried both elements of the transposition or just the Dp (2;Y) element along with two normal chromosome 2s, indicating that pairing is non-competitive. The frequency of quadrivalents was proportional to the size of the transposed region, suggesting that pairing sites are widely distributed on chromosome 2. Moreover, all but the smallest transpositions caused a detectable bias in the segregation ratio, in favor of alternate segregations, indicating that the prophase associations were effective in orienting centromeres to opposite poles. One transposition involving only heterochromatin of chromosome 2 had no effect on quadrivalent frequency, consistent with previous evidence that autosomal heterochromatin lacks meiotic pairing ability in males. One region at the base of chromosome arm 2L proved to be especially effective in stimulating quadrivalent formation and anaphase segregation, indicating the presence of a strong pairing site in this region. It is concluded that autosomal pairing in D. melanogaster males is based on general homology, despite the lack of homologous recombination.by A.C. Spradling     

FISH to meiotic pachytene chromosomes of tomato locates the root-knot nematode resistance gene Mi-1 and the acid phosphatase gene Aps-1 near the junction of euchromatin and pericentromeric heterochromatin of chromosome arms 6S and 6L, respectively

 The root-knot nematode resistance gene Mi-1 in tomato has long been thought to be located in the pericentromeric heterochromatin region of the long arm of chromosome 6 because of its very tight genetic linkage (approx. 1 cM) to the markers Aps-1 (Acid phosphatase 1) and yv (yellow virescent). Using Mi-BAC clones and an Aps-1 YAC clone in fluorescence in situ hybridisation (FISH) to pachytene chromosomes we now provide direct physical evidence showing that Mi-1 is located at the border of the euchromatin and heterochromatin regions in the short arm (6S) and Aps-1 in the pericentromeric heterochromatin of the long arm (6L) close to the euchromatin. Taking into account both the estimated DNA content of hetero- and euchromatin regions and the compactness of the tomato chromosomes at pachytene (2 Mb/μm), our data suggest that Mi-1 and Aps-1 are at least 40 Mb apart, a base pair-to-centiMorgan relationship that is more than 50-fold higher than the average value of 750 kb/cM of the tomato genome. An integrated cytogenetic-molecular map of chromosome 6 is presented that provides a framework for physical mapping. Received: 24 July 1998 / Accepted: 14 August 1998     

On the Control of the Distribution of Meiotic Exchange in DROSOPHILA MELANOGASTER

The effects of eight recombination-defective meiotic mutants on crossing over within the X heterochromatin were examined. Since none permit substantial frequencies of exchange within heterochromatin although six lessen or abolish constraints on the location of exchanges within euchromatin, the systems that prohibit exchange within heterochromatin and that govern where exchanges will occur in euchromatin are under separate genetic control.—A minor component of the effects of mei-218 is the production of nonhomologous exchanges; of mei-9 is the recovery of deleted chromatids; and of mei-41 is the recovery of deleted chromatids and/or a low frequency of heterochromatic exchanges.     

Pachytene pairing and metaphase I configurations in a tetraploid somatic Lycopersicon esculentum x L. peruvianum hybrid.

In the tetraploid somatic hybrid between the diploid Lycopersicon species L. esculentum (tomato) and L. peruvianum, synaptonemal complexes formed quadrivalents in 73 of the 120 sets of four chromosomes (60.8%) in 10 cells studied in detail at pachytene. Of these, 43 had one pairing partner exchange, 22 had two, and 8 had three, very close to a Poisson distribution. The points of pairing partner exchange were concentrated at the middle of the two arms. The frequency per arm corresponded with physical arm length. There was a sharp drop around the centromere, and pericentric heterochromatin had a slightly lower probability of being involved in pairing partner exchange than euchromatin. The chromosomes align before pairing and there are several points of pairing initiation, with concentrations at or near the ends and the centromere. From zygotene to late pachytene the quadrivalent frequency decreased considerably. At late pachytene it was lower than expected with the observed high frequency of pairing partner exchange. Pairing affinity between species was only slightly lower than affinity within species, in spite of considerable genetic differentiation. The frequency of recombination nodules increased from early to late zygotene and then decreased strongly to full pachytene. There is a highly significant negative correlation between percent pairing and SC length. At metaphase I the frequency of quadrivalents was 0.444, and branched quadrivalents were rare, probably caused by interference and restriction of chiasma formation to distal euchromatin. Metaphase I quadrivalent frequency is a relatively good indication of pairing affinity in this material.     

The odd-even effect in mitotically unstable B chromosomes in grasshoppers

The odd-even effect, by which B chromosomes are more detrimental in odd numbers, has been reported in plants and animals. In grasshoppers, there are only a few reports of this effect and all were referred to as traits related to the formation of aberrant meiotic products (AMPs). Here we review the existing information about B chromosome effects on AMPs, chiasma frequency and the number of active nucleolus organizer regions (NORs) per cell. Polysomy for A chromosomes and B chromosomes are two kinds of chromosome polymorphism frequently found in grasshoppers. In some aspects, e.g. meiotic behaviour and mitotic instability leading to individual mosaicism (in the case of mitotically unstable Bs), polysomic As show similar characteristics to B chromosomes. In fact, polysomy is regarded as one of the main mechanisms for B chromosome origin. Here we review some features of meiotic behaviour in known cases of polysomy and mitotically unstable Bs in grasshoppers, in looking for possible causes for the odd-even effect. In all these traits, the odd-even effect was apparent, although its appearance was not universal in any case, with variation among species or populations within the same species. The equational division and lagging of the extra chromosomes, when univalents, could favour the appearance of abnormal meiotic products, and the formation of bivalents, when there are two or more extra chromosomes, inhibits this process. Therefore, the odd-even effect might be a consequence of the concomitant operation of both aspects of extra chromosome meiotic behaviour. The possibility that the odd-even effect might result from an increase in cell stress generated by odd numbers is suggested.     

Rye chromosome translocations in hexaploid wheat: a re-evaluation of the loss of heterochromatin from rye chromosomes

Summary Using in situ hybridization techniques, we have been able to identify the translocated chromosomes resulting from whole arm interchanges between homoeologous chromosomes of wheat and rye. This was possible because radioactive probes are available which recognize specific sites of highly repeated sequence DNA in either rye or wheat chromosomes. The translocated chromosomes analysed in detail were found in plants from a breeding programme designed to substitute chromosome 2R of rye into commercial wheat cultivars. The distribution of rye highly repeated DNA sequences showed modified chromosomes in which (a) most of the telomeric heterochromatin of the short arm and (b) all of the telomeric heterochromatin of the long arm, had disappeared. Subsequent analyses of these chromosomes assaying for wheat highly repeated DNA sequences showed that in type (a), the entire short arm of 2R had been replaced by the short arm of wheat chromosome 2B and in (b), the long arm of 2R had been replaced by the long arm of 2B. The use of these probes has also allowed us to show that rye heterochromatin has little effect on the pairing of the translocated wheat arm to its wheat homologue during meiosis. We have also characterized the chromosomes resulting from a 1B-1R translocation event.From these results, we suggest that the observed loss of telomeric heterochromatin from rye chromosomes in wheat is commonly due to wheat-rye chromosome translocations.     

Influence of a rare unstable B-chromosome on chiasma frequency and nonhaploid sperm production in Dichroplus pratensis (Melanoplinae,Acrididae)

Three B-containing individuals of Dichroplus pratensis were found among 300 males collected in several Argentine localities. The B-chromosome is a partially euchromatic short telocentric and it is mitotically unstable. The B-chromosome number of testis cells ranged from 0 to 4 in two males and from 0 to 3 in the remaining one; this variation was inter- as well as intrafollicular. Meiotic behaviour of the Bs was very regular, association of Bs was usually chiasmate and no lagging univalents were observed during first division. The presence of Bs increased the production rate of abnormal spermatids and this effect was more pronounced in follicles with odd numbers of Bs. Chiasma frequency and between-cell variance were also increased in B-containing cells. A possible interaction between the Bs and the polymorphic centric fusions present in the species is discussed in relation to chiasma frequency determination.     

The B-chromosomes of two species ofCylindrotettix (Leptysminae,Acrididae)

Twenty-four males ofCylindrotettix obscurus and ten ofC. santarosae (Leptysminae, Acrididae) from Rio Claro, Brazil, were cytologically analysed. Both species are 2n = 23 ♂ (22+XO), all chromosomes being telocentric except for 4 acrocentric pairs ofC. obscurus. Two different B-chromosomes were found inC. obscurus: (1) a small, mitotically unstable telocentric (B^T) and (2) a medium-sized, mitotically stable submetacentric (B¹). B¹ has a large pericentromeric heterochromatic portion and two distal euchromatic segments where both terminal associations and interstitial chiasmata are regularly formed producing a ring univalent at prophase I. B¹ is almost certainly an iso-chromosome which has undergone a centromeric shift through pericentric inversion or centric transposition (heterochromatin duplication or deletion could also be the cause of arm inequality). InC. santarosae, a single male carried a small mitotically stable telocentric B-chromosome. Both B types ofC. obscurus significantly increased the production of abnormal sperm (diploid, tetraploid and micro-spermatids) when compared to 0B individuals although their effects were differential: B^T induced higher frequencies of microspermatids especially in those follicles with odd B^T numbers, while in B¹ carriers the increase was chiefly due to polyploid spermatids. None of the Bs ofC. obscurus affected cell mean chiasma frequency nor the between-cell variance. The B ofC. santarosae had no effect on abnormal sperm production but it probably increased cell mean chiasma frequency.