Further observations on the nature of radiation-induced chromosomal interchanges recovered from Drosophila sperm

The induction and analysis of numerous translocations (identified genetically and characterized cytologically) between chromosomes 2 and 3 of Drosophila melanogaster have allowed us to reexamine three issues concerning the nature of radiation-induced interchanges in spermatozoa. First, our results support the idea that, relative to their mitotic metaphase length, all major chromosomal regions are similar in their breakability, whether euchromatic (proximal or distal) or heterochromatic. Second, analysis of all our reciprocal exchanges between the two chromosomes shows a statistically significant dependence of the position of the chromosome 2 breakpoint on that of the chromosome 3 breakpoint. Thirdly, our combined cytological and genetic approach strengthens the results of previous analyses, which suggested a strong tendency for chromosomal interchanges to be of the reciprocal type in multiple-break rearrangements. This indicates that if radiation induces chromosome breaks, then the resulting broken ends tend to rejoin in pairs rather than independently.     

Localization of translocation breakpoints in somatic metaphase chromosomes of barley

Karyotype analyses based on staining by acetocarmine followed by Giemsa N-banding of somatic metaphase chromosomes of Hordeum vulgare L. were carried out on 61 reciprocal translocations induced by X-irradiation. By means of computer-based karyotype analyses all of the 122 breakpoints could be localized to defined sites or segments distributed over the seven barley chromosomes. The pre-definition of translocations with respect to their rearranged chromosome arms from other studies rendered it possible to define the break positions even in translocations having exchanged segments equal in size and the breakpoints located distally to any Giemsa band or other cytological marker. The breakpoints were found to be non-randomly spaced along the chromosomes and their arms. All breaks but one occurred in interband regions of the chromosomes, and none of the breaks was located directly within a centromere. However, short and long chromosome arms recombined at random. An improved tester set of translocations depicting the known break positions of most distal location is presented.     

Molecular phylogenetic analysis of the evolution of complex hybridity in Isotoma petraea

Complex hybridity is a rare diploid genetic system of plants, extensively characterized in Oenothera, in which heterozygosity for one or more reciprocal translocations is maintained by means of autogamy and a balanced lethal system. It is visible at metaphase I of meiosis as rings or chains of chromosomes held together by terminalized chiasmata. Phylogenetic analysis based on 274 random amplified polymorphic DNA markers showed that in the Australian endemic Isotoma petraea (Lobeliaceae) the genetic system had a single origin, as a ring-of-six, in the Pigeon Rock population. It subsequently spread to other populations to produce hybrids incorporating additional chromosomes into the rings. Our research supports the suggestion that complex hybridity is an evolutionary response to intense inbreeding and selected because it allows masking of accumulated deleterious alleles.     

Chiasma distributions and chromosome segregation in male and female translocation heterozygous mice analysed using FISH

Two factors postulated to influence the meiotic behaviour of reciprocal translocations were investigated. Firstly, variation in the length of translocated and non-translocated segments was studied in male mice using four different rearrangements involving chromosomes 2 and 4. Secondly, sex-related effects were analysed through comparison of the meiotic behaviour of two translocations in male and female germ cells. In the first part of the study, primary and secondary spermatocytes of male mice carrying a translocation [T(2;4)1Ca, T(2;4)13H, T(2;4)1Sn, or T(2;4)1Go] were screened. Each rearrangement had different proportions of cells with ring and chain quadrivalents at metaphase I; the T(2;4)1Sn heterozygote also had a high rate (45%) of translocation bivalents. In general, the translocations had elevated chiasma frequencies in the rearranged chromosomes compared with structurally normal chromosomes 2 and 4, although the extent of the effect varied. Each rearrangement produced a different array of segregation products at metaphase II, reflecting their contrasting frequencies of multivalent configurations at metaphase I. Comparison of chromosome behaviour at metaphase I and II suggested that certain configurations tended to adopt particular orientations. However, it was also clear that such correlations were imprecise and that other factors, possibly the exact positions of chiasmata, also played a role in multivalent orientation. Two rearrangements, T(2;4)1Go and T(7;16)67H, were analysed in female mice. The frequencies of the various multivalent types at metaphase I differed from those in male carriers of these rearrangements owing to an increased chiasma frequency in oocytes in some of the pairing segments. Not surprisingly, the segregation products seen in metaphase II cells showed some differences from the pattern recorded in male germ cells. For T(2;4)1Go, the sex-related difference in segregation patterns resulted in a diminished expectation of genetically imbalanced gametes, although this was not the case for T(7;16)67H. Received: 6 June 1998 / Accepted: 9 October 1998     

Chromosome distribution in neuroblast metaphase cells of Locusta migratoria L.

Chromosome distribution has been investigated at metaphase and C-metaphase in neuroblast cells of Locusta migratoria embryos. There is no evidence for generalised somatic association of homologues or nucleolus organising chromosomes at either stage. Homologous chromosomes 9 are found closer together than expected in C-metaphase but this seems to be due to their tendency to lie in the centre of the C-metaphase squash.     

Transmission of chromosomal abnormalities: participation of chromosomally unbalanced gametes in fertilization and early development of unbalanced embryos in the Chinese hamster

Using 14 Chinese hamster stocks with various reciprocal translocations, chromosomally unbalanced gametes were produced and used to investigate the participation of the unbalanced gametes in fertilization and the development of unbalanced embryos. The selection of chromosomally abnormal gametes during fertilization was investigated by the chromosomal analysis of meiotic cells in heterozygotes for the 14 reciprocal translocations and pronuclei of fertilized ova obtained from crossing these heterozygotes. Compared with the expected frequencies from meiotic metaphase II (MII) scoring, the frequencies of male pronuclei having commonly a deficiency of chromosome 1 (q14-->q42) or chromosome 3 (p23-->q31) in one-cell embryos decreased significantly. However, the frequencies of male pronuclei with other abnormalities were all consistent with those expected from MII scoring. In contrast, the frequencies of female pronuclei with any karyotype including the same ones, as those decreased in male pronuclei from the translocation heterozygotes were all consistent with those estimated from MII scoring. These results suggest that gametes with nullisomies as well as disomies for any chromosomal segments may mostly participate in fertilization, whereas some sperm nullisomic for the specific segments of chromosomes 1 and 3 may fail to fertilize. On the other hand, the zygotic selection of chromosomal imbalance was investigated by direct analyses of pre-implantation embryos from crosses between chromosomally normal females and male heterozygotes from the 14 stocks with various reciprocal translocations. The chromosomal and morphological analysis revealed that some embryos were arrested in development at the two-cell stage and their common abnormality was partial monosomy for chromosome 1 or 2. Embryos with partial monosomy including chromosomes 1, 3 and 4 showed arrested development at four-eight-cell stages. Among day 4 embryos, some chromosomally unbalanced embryos, mainly with a deficiency of other segments, such as chromosomes 1p, 2q, 5q and 8, had fewer blastomeres than karyotypically normal and balanced embryos. The homology between the mouse and the Chinese hamster chromosomes relating to the developmental abnormalities at early stages was partially confirmed.     

Homoeology of rye chromosome arms to wheat

Summary Cytological markers such as diagnostic C-bands, telocentrics, and translocations were used to identify the arms of rye chromosomes associated with wheat chromosomes at metaphase I in ph1b mutant wheat × rye hybrids. Arm homoeologies of rye chromosomes to wheat were established from the results of metaphase I pairing combined with available data on the chromosomal location of homoeoloci series in wheat and rye. Only arms 1RS, 1RL, 2RL, 3RS, and 5RS showed normal homoeologous relationships to wheat. The remaining arms of rye appeared to be involved in chromosome rearrangements that occurred during the evolution of the genus Secale. We conclude that a pericentric inversion in chromosome 4R, a reciprocal translocation between 3RL and 6RL, and a multiple translocation involving 4RL, 5RL, 6RS, and 7RS are present in rye relative to wheat.     

Site-specific chromosomal rearrangements induced in human diploid cells by x-irradiation

A total of 130 stable, two-break reciprocal translocations were scored in G-banded karyotypes prepared from 375 metaphase spreads from a strain of human diploid fibroblasts irradiated with 400 or 600 rads and analyzed 1-20 mean population doublings later. The chromosomal location of each of the 260 breakpoints was mapped. The sites of 121 chromosomal breaks and deletions in the first postirradiation mitosis were also scored. Unlike the random distribution of these latter events, the translocation breakpoints showed not only a nonrandom distribution among chromosomes but also the existence of specific sites within chromosomes that were more frequently involved in translocations. The most notable finding was a marked excess of translocations involving the short arm of chromosome 1, in particular, band 1p22. The specific types of translocations were random, although the breakpoints were not. Eight of the 12 most frequently involved chromosomal sites were regions in which fragile sites have been mapped in human lymphocytes.     

Chromosomal location of powdery mildew resistance genes and cytogenetic analysis of meiosis in common wheat cultivar Meri

Common wheat cv. Meri was crossed to a set of 21 Chinese Spring monosomic lines to characterize resistance to powdery mildew and to determine the chromosomal location of the gene(s). Monosomic F1 plants were allowed to self-pollinate and to produce F2 seeds. Seedlings of F2 and F3 plants and their parents were inoculated with isolates Ns 2 and 9 of Erysiphe graminis f. sp. tritici. Analysis of obtained data revealed that one major dominant gene conferring resistance is located on chromosome 1B of cv. Meri. The new gene is designated by symbol Pm28. On the basis of the trivalent configuration frequency (without univalent) at the 1st metaphase of meiosis it was found that two reciprocal translocations involving chromosomes 2A/5A and 5B/5D differentiate cv. Meri from cv. Chinese Spring. In the F1 monosomic hybrids, genes causing a decrease in pairing are found on chromosomes 4D and 6D, and genes enhancing pairing--on chromosomes 3A and 7B.     

Topology of constitutional reciprocal translocations in metaphase

We studied in 39 carriers of 26 reciprocal translocations (including five de novo and seven of indeterminate occurrence) the metaphase localization of the derivative chromosomes, their normal non-homologous counterparts (here called A and B), and two control pairs (C and D). In eight familial translocations, we analysed two to five carriers. We digitally captured 10 G-banded lymphocyte metaphases per individual and measured in microns the largest diameter (d) of the metaphase and six intercentromeric distances: (1) der A<-->der B (problem distance 1, pd1), (2) der A<-->B (pd2), (3) der B<-->A (pd3), (4) A<-->B (control distance 1, cd1), (5) the smaller distance between C and D (cd2) and (6) the largest distance between C and D (cd3); in addition, the average between C and D (cd4) was calculated. We used the formula Delta = 100(cd - pd)/d 12 times per metaphase, compared each pd vs. each cd, and tested the differences by the Wilcoxon matched-pair test. Although, in the whole sample there were not significant differences respect to cd1, this distance emerged as the proper control. In the eight familial translocations, the three pd vs. cd1 comparisons revealed that in 19/24 times the pd was smaller but only once reached significance (cd1 vs. pd2 in t[3;4]). In the analysis per individual the pd was smaller than cd1 in 19 (pd1), 22 (pd2) and 22 (pd3) cases although only twice reached significance. We conclude that in some translocations, the derivative chromosomes actually lie close from each other or from a normal non-homologous counterpart.     

Polymorphisms for asymmetric reciprocal translocations in two species of the genus Sideritis L. (Lamiaceae)

Meiotic and karyotypic analyses of six populations of two closely related species belonging to the section Eusideritis of the genus Sideritis (five of S. saetabensis and one of S. tragoriganum) (Lamiaceae), revealed polymorphisms for asymmetric reciprocal translocations. Two populations of S. saetabensis (S-046 and S-089) and the population of S. tragoriganum (S-056) contained heterozygotes and standard homozygotes. No homozygotes for the rearrangement (dicentric) were found. The meiotic behavior of the heterozygotes showed great stability of the dicentric chromosomes in both species due to strict control of meiosis at three levels: (1) chiasma formation, (2) multivalent coorientation at metaphase I, (3) dicentric chromatid separation at anaphase II. During mitosis there may also be strict control of dicentric chromatid separation through 100% parallel separation without interlocking. This stable polymorphism for asymmetric reciprocal translocations is a unique phenomenon that seems to be maintained by over-dominance, thus being a special case of linkage disequilibrium. We give two alternative hypotheses to explain the origin of these polymorphisms and discuss their evolutionary significance.     

A strategy for detecting chromosome-specific rearrangements in rye.

To obtain translocations involving specific chromosomes in rye, a line in which chromosome 1R has large C-bands on its two telomeres but which lacks C-bands (or has very small ones) on the telomeres of the remaining chromosomes was used. About 6% of the plants produced using pollen from irradiated (1.2 krad (1 rad = 10 mGy)) spikes of this line possessed structural changes involving the labeled chromosome. These aberrations included translocations, ring chromosomes, isochromosomes, and telocentrics. It is concluded (i) that all nonlabeled chromosomes have the same probability of participating in reciprocal translocations with the labeled chromosome, 1R, and (ii) that most induced reciprocal translocations involved exchanges of chromosome segments of approximately equal length. The use of lines having the appropriate combination of telomeric C-bands improves the efficiency of obtaining reciprocal translocations involving specific chromosomes that could be used in the construction of detailed physical maps.     

Chromosomal identification and meiotic behavior of reciprocal translocations in a rye polymorphic population. Evolutionary implications

Summary The spontaneous interchange polymorphism of rye cultivar Ailés is composed, as can be deduced from the chromosomal identification of the interchanges analyzed, of several different reciprocal translocations in which the chromosomes of its haploid complement are involved with a similar frequency, except for chromosomes 4R and 6R. Several features of chromosome behavior at metaphase I, such as configuration and orientation of quadrivalents and frequency of chiasmata, were analyzed in structural heterozygotes for different interchanges. The two main factors affecting the orientation of quadrivalents at metaphase I proved to be the morphology of these chromosome associations at metaphase I and, in particular, the frequency of bound chromosome arms that they showed. A genotypic control of alternate orientation of quadrivalents independent of chiasmata frequency was not detected. In addition, the frequency of alternate orientation shows no relation to the fitness. Possible evolutionary implications of the results obtained are discussed.     

Unusual chromosomal mosaicism as a cause of mental retardation and congenital malformations in a familial reciprocal translocation carrier, t(17;22)(q24.2;q11.23)

Familial reciprocal translocations are generally without phenotypic effect, although there is some evidence for a small excess of mental retardation and congenital malformations (MR/CM) in children carrying familial reciprocal translocations. Possible mechanisms whereby such translocations could have a phenotypic effect include cryptic unbalanced rearrangements, uniparental disomy, and disruption of putative genes at the breakpoints, unmasking recessive alleles on the normal homologs. Mosaicism for a supernumerary derivative chromosome in a carrier of a familial reciprocal translocation has not yet been described. We report a boy presenting with MR/CM and a familial reciprocal translocation, t(17;22)(q24.2;q11.23), inherited from the mother. Cytogenetic analysis of peripheral blood lymphocytes showed a balanced karyotype in all 32 analyzed metaphase spreads. Molecular genetic analysis was consistent with biparental origin of the normal homologs. In metaphase spreads from skin fibroblasts a supernumerary chromosome was found in all 24 cells analyzed and could be identified as der(22)t(17;22)(q24.2;q11.23). Several possible segregation modes at meiosis I followed by meiosis II or postzygotic nondisjunction of the der(22) might have led to this unusual chromosomal mosaicism. We propose hidden mosaicism as a possible cause for MR/CM in patients who apparently carry a balanced familial reciprocal translocation.     

Reciprocal translocations in Saccharomyces cerevisiae formed by nonhomologous end joining

Reciprocal translocations are common in cancer cells, but their creation is poorly understood. We have developed an assay system in Saccharomyces cerevisiae to study reciprocal translocation formation in the absence of homology. We induce two specific double-strand breaks (DSBs) simultaneously on separate chromosomes with HO endonuclease and analyze the subsequent chromosomal rearrangements among surviving cells. Under these conditions, reciprocal translocations via nonhomologous end joining (NHEJ) occur at frequencies of approximately 2-7 x 10(-5)/cell exposed to the DSBs. Yku80p is a component of the cell's NHEJ machinery. In its absence, reciprocal translocations still occur, but the junctions are associated with deletions and extended overlapping sequences. After induction of a single DSB, translocations and inversions are recovered in wild-type and rad52 strains. In these rearrangements, a nonrandom assortment of sites have fused to the DSB, and their junctions show typical signs of NHEJ. The sites tend to be between open reading frames or within Ty1 LTRs. In some cases the translocation partner is formed by a break at a cryptic HO recognition site. Our results demonstrate that NHEJ-mediated reciprocal translocations can form in S. cerevisiae as a consequence of DSB repair.     

Chromosome modifications induced in mouse spermatogonia through exposure to high-energy neutrons]

The incidence of reciprocal translocations induced in mouse spermatogonia has been studied in CBA mice given X-ray or neutron exposure. Analysis of dividing spermatocytes at diakinesis-first metaphase stage of meiosis shows that in X-irradiated mice there is a linear dose-response relationship. After exposure to fast neutrons the yield of translocations follows a humped curve with a maximum of chromosome exchanges after exposure to 100 rad.     

No induction of interchromosomal effect in male meiosis of Chinese hamsters with reciprocal translocations

Chinese hamsters from five strains with reciprocal translocations, T(1;3)7Idr, T(1;3)8Idr, T(1;2)9Idr, T(7;9)16Idr, and T(1;5)17Idr, and a karyotypically normal strain, CHS/Idr, were used to look for an interchromosomal effect by chromosomal analysis of meiotic cells and one-cell embryos. The frequencies of nondisjunction at first meiosis in five normal (+/+) males, calculated by doubling the number of hyperhaploid cells, ranged from 0.43% to 1.33%, and there was no significant difference in frequency among individuals. On the other hand, the frequency of hyperhaploid cells in males heterozygous for each translocation ranged from 3.0% to 11.8%, and the frequency of hyperhaploid cells with an extra translocation-unrelated chromosome ranged from 0.2% to 0.4%, which is no different from that estimated from scoring of +/+ males at the second meiotic metaphase. In one-cell embryos from crosses between karyotypically normal females and male heterozygotes for T(1;2)9Idr and T(7;9)16Idr, 1.1% and 0.5% of embryos had an extra translocation-unrelated chromosome. Compared with the control, the frequency of meiotic nondisjunction showed no increase in male heterozygotes for the reciprocal translocations. Therefore, the results suggest that multivalents and rearranged chromosomes existing at first and second meiosis in male Chinese hamsters exert no influence on segregation of normal bivalents and chromosomes unrelated to the rearrangements.     

Reidentification of chromosomal CUP1 translocations in the wine yeasts Saccharomyces cerevisiae

Reciprocal translocations between chromosomes XVI and VIII were revealed in eight Saccharomyces cerevisiae strains (mostly wine ones) using pulse-field electrophoresis of native chromosomal DNAs and their hybridizations with the CUP1 and GAL4 probes. New and reciprocal translocations of at least the gene CUP1 occur at the expense of crossing-over in the hybrids of such strains with the genetic lines of normal karyotype during meiosis. Relationship between these reciprocal translocations and the sulfite (Na₂SO₃) resistance gene SSU1-R is discussed.     

The centromere structure in Robertsonian wheat-rye translocation chromosomes indicates that centric breakage-fusion can occur at different positions within the primary constriction

Univalent chromosomes at meiotic metaphase I have a tendency to misdivide at the centromeres. Fusion of the misdivision products may produce Robertsonian translocations. The fine structure of the centromeres in Robertsonian wheat-rye translocation chromosomes was analyzed by fluorescence in situ hybridization (FISH) using two centromere-specific DNA clones: pRCS1, derived from rice, and pAWRC1, derived from rye. Clone pRCS1 hybridizes to the centromeres of all grasses including wheat and rye, whereas clone pAWRC1 is rye specific and hybridizes only to the centromeres of rye. Four of the six wheat-rye translocations derived from a single centric misdivision event (1st generation translocations) had hybrid centromeres, with approximately half of the centromere derived from rye and half from wheat. In the two other 1st generation translocations, the entire centromere was derived from rye. Among eight reconstructed wheat and rye chromosomes that originated from two consecutive centric misdivision-fusion events (2nd generation translocations), T1BS.1BL (derived from T1BS.1RL and T1RS.1BL) and one of three T2BS.2BL (derived from T2RS.2BL and T2BS.2RL) had hybrid centromeres. T1RS.1RL (derived from T1BS.1RL and T1RS.1BL), two of three T2BS.2BL, and all three T2RS.2RL (derived from T2RS.2BL and T2BS.2RL) had rye centromeres. All three 3rd generation translocations had hybrid centromeres with approximately half of the centromere derived from rye. There were no indications that the composite structure of the centromere in these chromosomes affected their behavior in mitosis or meiosis. These observations support the notion of a compound structure of the centromere in higher organisms, and indicate that during the centric breakage-fusion event, centromere breakage may occur in different positions along the segment of the chromosome that interacts with the spindle fibers. Normal behavior of the 1st, 2nd, and 3rd generation centric translocations in mitosis and meiosis indicates that, at least in wheat and rye, centromeres are not chromosome specific.     

Redundant segments inZea mays detected by translocations of monoploid origin

Twenty-two independently occurring spontaneous reciprocal translocations were isolated from monoploid X diploid crosses in maize and their breakpoints were determined. As 12 of the translocations involved the same two chromosomes and had breakpoints at approximately the same positions (6L. 2–3, 7L. 2–3) and two other translocations appeared to be identical with breakpoints at 2L. 9, 6L. 4, 14 of the 22 translocations probably arose by crossing over within duplicate segments of nonhomologous chromosomes. Thus, at least part of the bivalents seen at diakinesis and chromatid bridges seen at anaphase I in monoploid plants appear to be generated by recombination between redundant chromosome segments. The other eight translocations each occurred once. Because our evidence indicates that recombination between nonhomologous illegitimately synapsed chromosome segments does not occur in maize, these were probably also produced by recombination between redundant segments. If one assumes that their breakpoints also mark regions of interchromosomal redundancy, other conclusions can be reached: A) corn does not contain detectable homoeologous chromosomes, thus it is precently a true diploid, and B) as exchanges giving rise to translocations did not occur in the centromeres or proximal heterochromatin, these regions either do not possess redundancy or are rarely involved in chiasma formation. Furthermore, the duplicated segments in the genome giving rise to translocations in haploid microsporocytes probably have the same serial order with respect to the centromere.This work was partially supported by U.S. Atomic Energy Commission Contract AT(11-1)-2121.