Showing papers on "Chromosome published in 1982"

A cellular oncogene is translocated to the Philadelphia chromosome in chronic myelocytic leukaemia

Abstract: The transforming genes of oncogenic retroviruses are homologous to a group of evolutionary conserved cellular onc genes. The human cellular homologue (c-abl) of the transforming sequence of Abelson murine leukaemia virus (A-MuL V) was recently shown to be located on chromosome 9. The long arm of this chromosome is involved in a specific translocation with chromosome 22, the Philadelphia translocation (Ph1), t(9; 22) (q34, q11), which occurs in patients with chronic myelocytic leukaemia (CML)3-5. Here we investigate whether the c-abl gene is included in this translocation. Using c-abl and v-abl hybridization probes on blots of somatic cell hybrids, positive hybridization is found when the 22q- (the Philadelphia chromosome), and not the 9q+ derivative of the translocation, is present in the cell hybrids. From this we conclude that in CML, c-abl sequences are translocated from chromosome 9 to chromosome 22q-. This finding is a direct demonstration of a reciprocal exchange between the two chromosomes and suggests a role for the c-abl gene in the generation of CML.

Rabl's Model of the Interphase Chromosome Arrangement Tested in Chinese Hamster Cells by Premature Chromosome Condensation and Laser-UV-Microbeam Experiments

Abstract: In 1885 Carl Rabl published his theory on the internal structure of the interphase nucleus We have tested two predictions of this theory in fibroblasts grown in vitro from a female Chinese hamster, namely (1) the Rabl-orientation of interphase chromosomes and (2) the stability of the chromosome arrangement established in telophase throughout the subsequent interphase Tests were carried out by premature chromosome condensation (PCC) and laser-UV-microirradiation of the interphase nucleus Rabl-orientation of chromosomes was observed in G1 PCCs and G2 PCCs The cell nucleus was microirradiated in G1 at one or two sites and pulse-labelled with 3H-thymidine for 2h Cells were processed for autoradiography either immediately thereafter or after an additional growth period of 10 to 60h Autoradiographs show unscheduled DNA synthesis (UDS) in the microirradiated nuclear part(s) The distribution of labelled chromatin was evaluated in autoradiographs from 1035 cells after microirradiation of a single nuclear site and from 253 cells after microirradiation of two sites After 30 to 60h postincubation the labelled regions still appeared coherent although the average size of the labelled nuclear area fr increased from 142% (0h) to 265% (60h) The relative distance dr, ie the distance between two microirradiated sites divided by the diameter of the whole nucleus, showed a slight decrease with increasing incubation time Nine metaphase figures were evaluated for UDS-label after microirradiation of the nuclear edge in G1 An average of 43 chromosomes per cell were labelled Several chromosomes showed joint labelling of both distal chromosome arms including the telomeres, while the centromeric region was free from label This label pattern is interpreted as the result of a V-shaped orientation of these particular chromosomes in the interphase nucleus with their telomeric regions close to each other at the nuclear edge Our data support the tested predictions of the Rabl-model Small time-dependent changes of the nuclear space occupied by single chromosomes and of their relative positions in the interphase nucleus seem possible, while the territorial organization of interphase chromosomes and their arrangement in general is maintained during interphase The present limitations of the methods used for this study are discussed

Chromosomal location of human kappa and lambda immunoglobulin light chain constant region genes

Abstract: The chromosomal location of human constant region light chain immunoglobulin (Ig) genes has been determined by analyzing a group of human fibroblast/rodent somatic cell hybrids with nucleic acid probes prepared from cloned human kappa and lambda constant region genes. Human chromosomes in each cell line were identified by isoenzyme analysis. The DNA from hybrid cells was digested with restriction endonucleases, size fractionated by gel electrophoresis, transferred to nitrocellulose or DBM paper, and hybridized with (32)P-labeled nucleic acid probes. The C(κ) gene was assigned to human chromosome 2 and the C(λ) genes to chromosome 22, based upon analysis of these hybrid cell lines, and these assignments were confirmed by analysis of subclones. A group of previously unassigned loci can be mapped to chromosome 2 by virtue of their close linkage to C(κ). The λ and κ light chain and heavy chain Ig genes have now been assigned to all three human chromosomes that are involved in translocations with chromosome 8 in human B cell neoplasms. These techniques and probes provide a means to study the detailed arrangement of human Ig genes and their pseudogenes.

Chromosomal localization of the human homolog (c-sis) of the simian sarcoma virus onc gene

Abstract: Nonrandom chromosome rearrangements of chromosome 22 have been identified in different human malignancies. As a result of Southern blot hybridization of a c-sis probe to DNA's from mouse-human somatic cell hybrids, the human homolog (c-sis) of the transforming gene of simian sarcoma virus was assigned to chromosome 22. Hybrids between thymidine kinase-deficient mouse cells and human fibroblasts carrying a translocation of the region q11-qter of chromosome 22 to chromosome 17 were also analyzed. These studies demonstrate that the human c-sis gene is on region 22q11 greater than qter.

Localization of human immunoglobulin kappa light chain variable region genes to the short arm of chromosome 2 by in situ hybridization

Abstract: The genes for human immunoglobulin kappa light chains have been localized in normal lymphocyte and fibroblast chromosomes by in situ hybridization of probes from cloned DNA fragments of the kappa variable region locus. The localization was achieved by counting grains (after autoradiography) over chromosomes in a number of karyotypes. The variable region gene probes hybridized in a cluster on a region of the chromosome 2 short arm close to the centromere (2cen leads to p12). This location was confirmed in lymphocytes from a balanced translocation carrier 46XXt (2; 16) (q13; q22). Our results show that human kappa light chain genes are located in the region of the break point observed in specific chromosomal translocations associated with Burkitt lymphoma.

Assignment of the structural gene for the third component of human complement to chromosome 19

Abstract: The third component of complement (C3) is synthesized and secreted by cultured human primary fibroblasts. A monoclonal antibody having specificity for an antigenic determinant carried by human but not mouse C3 was used to study the continued expression of human C3 in three panels of independently derived human-mouse somatic cell hybrids. Expression of the human product was shown to segregate with human chromosome 19 and with no other chromosome or group of chromosomes. A unique-sequence human genomic C3 DNA clone was isolated and used as a probe in DNA hybridization experiments with DNA prepared from appropriate human-mouse somatic cell hybrids to confirm assignment of the human C3 gene to chromosome 19.

Mapping the Human Genome, Cloned Genes, DNA Polymorphisms, and Inherited Disease

Abstract: It is estimated that the human haploid genome is composed of 3 × 109 nucleotides and that only 10% of it consists of coding and regulatory sequences.14 If a gene is approximately 104 nucleotides in length, which includes the coding region and the intervening and flanking sequences, this estimate would predict that there are about 3–10 × 104 human genes coding for different protein products. Since gene clustering in humans has become evident (for example, the hemoglobin, immunoglobin, and HLA clusters), these estimated gene products may be grouped in from 3000 to 15,000 clusters.15 Further, based upon genetic and molecular studies of microorganisms, Drosophila, and the mouse, there are perhaps 5 × 104 structural genes estimated in humans,14,106,183 which is in agreement with the number of estimated protein products in the human genome. Mapping the human genome involves partitioning the total number of genes into individual maps representing the 24 different human nuclear chromosomes and linearly ordering them on each chromosome. A similar exercise has mapped the 37 genes encoded in the DNA of the mitochondrial genome.2

Translocation of immunoglobulin VH genes in Burkitt lymphoma

Abstract: We have produced cell hybrids between mouse myeloma cells, which do not produce immunoglobulin chains, and Burkitt lymphoma cells (Daudi), which express surface IgM. Daudi Cells carry a reciprocal chromosome translocation between chromosomes 8 and 14, described as (8;14)(q24;q32). The hybrids were studied for the expression of human immunoglobulin chains and human isozyme markers, for the presence of human chromosomes, and for the presence of the human genes for heavy chain variable regions (VH) and mu and gamma chain constant (C) regions. The results indicate that the expressed mu chain gene is on normal chromosome 14 in Daudi cells. We have also determined that the chromosome 14 involved in the translocation (14q+) carries the gene for C mu and C gamma 1-4 and probably several genes for the variable region (V). Certain hybrids had lost both the chromosomes 14 but had retained the abnormal chromosome 8 (8q-) that carries the terminal end of the long arm of chromosome 14. These hybrids were studied for the presence of human VH, C mu,, and C gamma DNA sequences, and the results indicated that the hybrid cells with the 8q- chromosome contained VH genes that not C genes. Therefore, we conclude that, in the Daudi Burkitt lymphoma, the break in chromosome 14 occurred within the chromosome segment containing V region genes. As a result of the translocation some of these VH genes became associated with chromosome 8. It is possible that the expression of malignancy in Burkitt lymphoma is caused by immunoglobulin V region gene translocation resulting in activation of a gene on the long arm of human chromosome 8.

Chromosome Abnormalities in Malignant Hematologic Diseases

Abstract: Publisher Summary The relatively consistent chromosome changes, especially specific translocations that are closely associated with particular neoplasms, provide convincing evidence for the fundamental role of these changes in the transformation of a normal cell to a malignant cell. In some tumors, these changes are too small to be detected, and the cells appear to have a normal karyotype. However, new chromosome banding techniques have helped in the understanding of the specificity of some of the abnormalities. These techniques allow the identification of each human chromosome and of parts of chromosomes as well. When one considers the number of nonrandom changes that are seen in a cancer such as acute nonlymphocytic leukemia (ANLL), it is clear that not just one gene but rather a class of genes is involved. Moreover, knowledge of the human gene map has developed concurrently with the understanding of the consistent chromosome changes in neoplasia. It is now possible to correlate the chromosomes that are affected with the genes that they carry.

Analysis of chromosome positions in the interphase nucleus of Chinese hamster cells by laser-UV-microirradiation experiments.

Abstract: Unsynchronized cells of an essentially diploid strain of female Chinese hamster cells derived from lung tissue (CHL) were laser-UV-microirradiated (λ=257 nm) in the nucleus either at its central part or at its periphery. After 7–9 h postincubation with 0.5 mM caffeine, chromosome preparations were made in situ. Twenty-one and 29 metaphase spreads, respectively, with partial chromosome shattering (PCS) obtained after micro-irradiation at these two nuclear sites, were Q-banded and analyzed in detail. A positive correlation was observed between the frequency of damage of chromosomes and both their DNA content and length at metaphase. No significant difference was observed between the frequencies of damage obtained for individual chromosomes at either site of microirradiation. The frequency of joint damage of homologous chromosomes was low as compared to nonhomologous ones. Considerable variation was noted in different cells in the combinations of jointly shattered chromosomes. Evidence which justifies an interpretation of these data in terms of an interphase arrangement of chromosome territories is discussed. Our data strongly argue against somatic pairing as a regular event, and suggest a considerable variability of chromosome positions in different nuclei. However, present data do not exclude the possibility of certain non-random chromosomal arrangements in CHL-nuclei. The interphase chromosome distribution revealed by these experiments is compared with centromere-centromere, centromere-center and angle analyses of metaphase spreads and the relationship between interphase and metaphase arrangements of chromosomes is discussed.

Sex chromosome pairing during male meiosis in marsupials.

Abstract: The pairing of the sex chromosomes at pachytene has been examined in twenty-two species of Australian marsupials, including four with complex sex chromosome systems. The axial elements of the sex chromosomes associate in all but one species. However, no synaptonemal complex has been observed between the axes of the X and Y chromosome in any of the examined species. Both the type of association between the sex chromosome axes, and the structural modifications of these axes are conserved within taxonomic groupings. In three species with complex sex chromosome systems, the t(XA), Y, A trivalents do not have a favoured relative orientation of the axes of the Y and A chromosomes, whereas in a fourth species with a t(XA1), t(A2YA1), A2 system the t(XA1) and A2 axes are in a cis arrangement with each other.

The mapping of highly-repeated DNA families and their relationship to C-bands in chromosomes of Secale cereale

Abstract: The relationship between the chromosomal location of heterochromatin C-bands and of four non-homologous repeated sequence families constituting 8 to 12% of total rye DNA has been investigated in chromosomes of rye (Secale cereale) by in situ hybridisation. Three rye varieties, a set of rye disomic additions to wheat and a triticale were studied. Only centromeric and nucleolar organizer region (NOR) associated C-bands failed to display hybridisation to at least one of the sequences and many telomeric blocks of heterochromatin contained all four repeated sequence families. Both between-variety differences in the chromosomal distribution of repeated sequences, and intravarietal heterozygosities were frequently noted and are probably widespread. — Previously reported deletions of heterochromatin from King II rye chromosomes added to the Holdfast wheat complement were correlated with deletions of some, but not all, of the highly repeated sequence families. A previously unreported loss of some families from King II rye chromosome 4R/7R in a Holdfast wheat genetic background was detected. This loss was not associated with complete deletion of a C-band. A deletion has also probably occurred from the short arm telomere of 4R/7R in the triticale variety Rosner. It is suggested that the families of repeats in rye telomeric heterochromatin which are absent from wheat are selected against in the wheat genetic background.

Conserved Regions of Homologous G-Banded Chromosomes Between Orders in Mammalian Evolution: Carnivores and Primates

Abstract: The recent derivation of a biochemical map of 33 loci of the domestic cat (Felis catus) revealed a striking conservation of chromosomal linkage associations between the cat and humans. A comparison of homologous (by linkage criteria) chromosomes by using conventionally extended and high-resolution G-banding of human and feline chromosomes is presented. Four criteria for establishing probable cytogenetic homologies of chromosomal regions were invoked: (i) map placement of homologous genes to the same chromosomes; (ii) cytological correlation of G-banding pattern; (iii) placement of homologous genes, by regional gene mapping, in the region of cytological homology; and (iv) a requirement that the putative region of homology be ancestral and evolutionarily conserved within their respective orders. Five subchromosomal regions (homologous to human chromosome 1p, 2p, 2q, 12, and X) were found to be conserved and homologous by all the stated criteria. The conserved regions constitute nearly 20% by length of the human chromosomal genome. The implications of conservation of chromosome homologies between mammalian orders whose last common ancestor became extinct more than 60 million years ago is discussed.

The chromosome complement of single-pronuclear haploid mouse embryos following activation by ethanol treatment

Abstract: A technique in which mouse eggs are stimulated to develop parthenogenetically following their brief incubation in 7% ethanol in PBS is described. Very high rates of activation were achieved, and a detailed analysis presented of the class of parthenogenone which develops a single haploid pronucleus following second polar body extrusion. As preliminary studies on presumptive haploid morulae indicated that a proportion of the metaphase spreads examined had an aneuploid chromosome constitution, the incidence of aneuploidy at the first cleavage mitosis was investigated. In the control groups the level of aneuploidy was about 1%, whereas in the ethanol-treated series the incidence ranged from 13 . 6-18 . 8%. Additional pre-treatment of ethanol-activated oocytes in low osmolar medium raised the incidence of aneuploidy to 28 . 3%. Metaphase groups with 18, 19, 21 and 22 chromosomes present were observed in addition to groups with a normal complement of 20 chromosomes. The possible mode of action of ethanol in inducing parthenogenetic activation of mouse oocytes, and a high incidence of aneuploidy, is discussed in relation to previous knowledge of the action of this agent. Preliminary studies using G-banding indicate that the aneuploidy observed appears to arise as a result of non-disjunction which may involve any of the chromosomes of the complement.

Genetic effects of methyl benzimidazole-2-yl-carbamate on Saccharomyces cerevisiae.

Abstract: The genetic effects of the mitotic inhibitor methyl benzimidazole-2-yl-carbamate (MBC) have been studied in Saccharomyces cerevisiae. MBC had little or no effect on the frequency of mutation. In some experiments MBC caused an increase in the frequency of mitotic recombination; however, this effect was small and not reproducible. The primary genetic effect of MBC was to induce mitotic chromosome loss at a high frequency. Chromosome loss occurred at equal frequencies for all chromosomes tested (13 of 16). Cells which had lost multiple chromosomes were found more frequently than predicted if individual chromosome loss events were independent. The probability of loss for a particular chromosome increased with length of time cells were incubated with MBC. MBC treatment also increased the frequency at which polyploid cells were found. These results suggested that MBC acted to disrupt the structure or function of the mitotic spindle and cause chromosome nondisjunction.

Association of endosperm reduction with parental imprinting in maize.

Abstract: Among 38 reciprocal translocations between the maize B chromosome and the proximal region of the long arm of chromosome 10 were six interchanges associated with reduced endosperm development. These six have breakpoints that are the most proximal of the set and constitute a graded series with those broken nearer the centromere which have the most abnormal phenotypes. The group of six defines three major regions that produce the endosperm effects. The remaining 32 translocations reduce kernel size very slightly, suggesting the presence of a fourth region distal to all break-points.—The affected class of kernels lacks a paternally derived representative of that segment of 10L translocated to the B centromeric element (B10 chromosome; 10 10 B10). An accompanying class of kernel in which the paternal B10 chromosome is duplicated in the endosperm (10 10 10B B10 B10) is normal. Kernels of the same endosperm constitution synthesized by introducing both 10 and B10 maternally, however, are defective, resembling 10 10 10B. Maternal B109s are therefore unable to compensate for the absence of a paternal B10. Clearly expression of the 10L genes involved supports normal endosperm growth only following pollen transmission.

Localization of the normal allele of T24 human bladder carcinoma oncogene to chromosome 11

Abstract: The development of DNA-mediated gene transfer techniques1,2 has made it possible to identify transforming genes present in certain human tumour cells. Such genes have been shown to induce morphological transformation when used to transfect suitable assay cells3–10. Recently a transforming gene has been isolated by molecular cloning techniques from the T24 (ref. 11) and EJ (ref. 12) human bladder carcinoma cell lines10,13,14. This bladder carcinoma oncogene has been shown to be of human origin, less than six kilobase pairs (kbp) in size, and closely related to the onc genes (v-bas and v-ras) of BALB and Harvey murine sarcoma viruses15–17. These transforming retroviruses arose in nature by transduction of cellular genes from mouse and rat cells, respectively18. To understand better the relationship of the T24 oncogene with other human cellular genes, we have determined the chromosomal location of its normal allele within the human genome. We show here that it is carried on chromosome 11 in normal cells.

Synapsis-dependent allelic complementation at the decapentaplegic gene complex in Drosophila melanogaster.

Abstract: Allelic complementation at the decapentaplegic gene complex (dpp: 2-4-0, cytogenetic location: polytene chromosome bands 22F1-3) of Drosophila melanogaster frequently occurs between site mutations. Two specific instances of allelic complementation are shown to be dependent upon normal somatic chromosome synapsis of homologous dpp genes. Numerous strains have been identified that bear lesions that disrupt allelic complementation when heterozygous with structurally normal chromosomes; each of these 57 strains contains a gross chromosomal rearrangement with a break on chromosome 2. The properties of the rearrangements carried by 50 of these strains are consonant with the idea that their effects are due to a disruption of somatic chromosome synapsis in the dpp region of chromosome arm 2L. In double heterozygotes of simple two-break rearrangements, allelic complementation is restored (presumably through the restoration of structural homozygosity). The types of rearrangements that disrupt complementation have properties very similar to those of rearrangements that disrupt the transvection effect at bithorax [Lewis, E. B. (1954) Am. Nat. 88, 225-239]. The existence of synapsis-dependent allelic complementation is a demonstration of the physiological importance of nuclear organization in gene expression.

Centromeric DNA from chromosome VI in Saccharomyces cerevisiae strains.

Abstract: The functional sequence from the centromere in chromosome VI ( CEN6 ) of Saccharomyces cerevisiae was narrowed down to a stretch of 500 bp by a Bal31 deletion approach. The DNA sequence in this region shows three long stretches, 40 bp, 96 bp, and 63 bp of 85% and more AT pairs and a pyrimidine purine bias in the individual single strands. We assume that the CEN6 functional sequences encompass these AT-rich stretches because this part shows striking similarities to sequence elements common to CEN3 and CEN11 DNA. A strain comparison revealed that CEN6 DNA sequences are confined to the Saccharomyces genus and probably only to the S. cerevisiae species. CEN6 is not highly conserved within S. cerevisiae strains because EcoRI and HindIII restriction site variants are found with high frequency.

Discontinuous dna variation in the evolution of plant species: the genus lathyrus.

Abstract: The genus Lathyrus is closely related to Vicia and belongs to the family Leguminosae. Most species in this genus are diploids with 2n = 14. The divergence and evolution within this genus is accompanied by a three-fold increase in chromosome size which is directly correlated with a four-fold increase in their nuclear DNA amounts. The fraction of DNA responsible for the variation in total amounts of DNA in this genus consists of repetitive and non-repetitive DNA in a consistent ratio of 4. 1 to 1.0. Quantitative comparisons of the total DNA and of DNA constituents show discontinuity in the pattern of variation among the species of this genus. The constraints upon the composition of DNA among species and the discontinuity in the pattern of variation would suggest genome evolution by a succession of spasmodic changes rather than by a continuous progression. This investigation was carried out with two major objectives in mind, 1) to determine the nature of DNA variation accompanying the evolution of this genus and 2) to compare among species the relative changes in nuclear components which are closely correlated with DNA changes. Multivariate analysis was used to classify the data. MATERIALS AND METHODS

Chromosomal assignment of the human homologues of feline sarcoma virus and avian myeloblastosis virus onc genes.

Abstract: Retroviral transforming genes, v-onc genes, are derived from normal cellular sequences that are called cellular onc (c-onc) genes. DNA from mouse-human somatic cell hybrids that have selectively lost human chromosomes was used in Southern blots to map the chromosomal location of two human onc genes. Cloned human homologues of retroviral onc genes were used as probes. Because the human c-fes gene, which is homologous to feline sarcoma virus, segregates concordantly with human chromosome 15, and the human c-myb gene, which is homologous to avian myeloblastosis virus onc genes, segregates concordantly with human chromosome 6, we have assigned the c-fes and the c-myb genes to human chromosomes 15 and 6, respectively. Nonrandom chromosomal defects involving these human chromosomes have been observed in neoplasms. These studies should be valuable in determining whether specific rearrangements involving these chromosomes result in the abnormal expression of these onc genes in human malignancies.

Chromosomal locations of the genes for rRNA in Escherichia coli K-12.

Abstract: Chromosomal locations of the seven rRNA operons in Escherichia coli K-12 were studied by digesting DNA from various merodiploid strains with SalI restriction enzyme followed by Southern gel analysis with 32P-labeled 23S rRNA as a probe. The seven unique SalI DNA fragments revealed in the autoradiograms were first correlated to the seven rRNA operons previously isolated as hybrid plasmids or transducing phages. The chromosomal locations of six (rrnA, B, C, D, E, and G) of the seven isolated operons were confirmed by increased gene dosage demonstrated in autoradiograms after Southern gel analysis of DNA from relevant merodiploid strains. The gene dosage analysis showed that the location of the remaining operon (now called rrnH) is between metD and proA. No evidence was obtained for the presence of rrnF, which was previously reported to map between aroB and malA. The chromosomal location of rrnH was confirmed by P1 transduction in the following way: a DNA fragment adjacent to rrnH was cloned into pBR322; the resulting hybrid plasmid was integrated at the homologous region of the chromosome of a polA mutant; and the ampicillin resistance marker originally carried by pBR322 was then used for mapping of the nearby rrnH by P1 transduction. A close linkage of rrnH to metD (about 60% cotransduction) was observed, and the data were consistent with the order metD-rrnH-proA. Thus, mapping of all seven rRNA operons has been completed. The present study has also determined the orientation of rrnG and rrnH and demonstrated that the direction of transcription of all the rRNA operons is identical to that of DNA replication.

Investigation of a preferentially transmitted Aegilops sharonensis chromosome in wheat.

Abstract: An attempt to produce a set of addition lines of Aegilops sharonensis to the wheat variety ‘Chinese Spring’ produced only one addition line. This was due to preferential transmission of one chromosome from Ae. sharonensis. This chromosome was studied in detail by established cytological methods of chromosome observation and by the newer techniques of C-banding and in situ hybridization of a cloned DNA sequence. The chromosome was found to be partially homologous to an Ae. sharonensis chromosome of similar behaviour in another wheat addition line. The incomplete homology of the two Ae. sharonensis chromosomes was due to the presence of a translocated segment of a wheat chromosome. — Substitution lines of the Ae. sharonensis chromosome for wheat homoeologous group 4 were produced and the Ae. sharonensis chromosome thereby designated 4 S l .

Cytogenetic mapping of the duplicated segment of chromosome 12 in lymphoproliferative disorders.

Abstract: Recently, an extra chromosome 12 has been reported to occur with high frequency in peripheral blood lymphocytes from patients with chronic B-cell lymphocytic leukaemia (B-CLL)1,2. Here we report a unique chromosomal abnormality, which suggests that the segment q13 to q22 on chromosome 12 carries the important genes that are duplicated in those lymphoproliferative disorders characterized by trisomy for this chromosome.

Chromosome and nucleotide sequence differentiation in genomes of polyploid Triticum species.

Abstract: The nature of genome change during polyploid evolution was studied by analysing selected species within the tribe Triticeae. The levels of genome changes examined included structural alterations (translocations, inversions), heterochromatinization, and nucleotide sequence change in the rDNA regions. These analyses provided data for evaluating models of genome evolution in polyploids in the genus Triticum, postulated on the basis of chromosome pairing at metaphase I in interspecies hybrids. The significance of structural chromosome alterations with respect to reduced MI chromosome pairing in interspecific hybrids was assayed by determining the incidence of heterozygosity for translocations and paracentric inversions in the A and B genomes of T. timopheevii ssp. araraticum (referred to as T. araraticum) represented by two lines, 1760 and 2541, and T. aestivum cv. Chinese Spring. Line 1760 differed from Chinese Spring by translocations in chromosomes 1A, 3A, 4A, 6A, 7A, 3B, 4B, 7B and possibly 2B. Line 2541 differed from Chinese Spring by translocations in chromosomes 3A, 6A, 6B and possibly 2B. Line 1760 also differed from Chinese Spring by paracentric inversions in arms 1AL and 4AL whereas line 2541 differed by inversions in 1BL and 4AL (not all chromosomes arms were assayed). The incidence of structural changes in the A and B genomes did not coincide with the more extensive differentiation of the B genomes relative to the A genomes as reflected by chromosome pairing studies. To assay changing degrees of heterochromatinization among species of the genus Triticum, all the diploid and polyploid species were C-banded. No general agreement was observed between the amount of heterochromatin and the ability of the respective chromosomes to pair with chromosomes of the ancestral species. Marked changes in the amount of heterochromatin were found to have occurred during the evolution of some of the polyploids. The analysis of the rDNA region provided evidence for rapid “fixation” of new repeated sequences at two levels, namely, among the 130 bp repeated sequences of the spacer and at the level of the repeated arrays of the 9 kb rDNA units. These occurred both within a given rDNA region and between rDNA regions on nonhomologous chromosomes. The levels of change in the rDNA regions provided good precedent for expecting extensive nucleotide sequence changes associated with differentiation of Triticum genomes and these processes are argued to be the principal cause of genome differentiation as revealed by chromosome pairing studies.