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PubMed1980

Sequence and heterogeneity in the 5S RNA gene cluster of Drosophila melanogaster.

The sequence of the entire 5S RNA gene of Drosophila melanogaster was determined by sequencing collectively 23 copies contained in a cloned fragment of Drosophila DNA and by sequencing individually four subcloned gene copies. A repetitive heptamer (GCTG CCT) present in variable numbers immediately following the coding sequence, is responsible for the length heterogeneity in the spacer region. Some of the gene copies contain a nucleotide change in the coding region which results in a new site for the restriction enzyme Mn1 I. The variant 5S RNA produced by these gene copies has not been detected in vivo. Two other single nucleotide variations were identified in the spacer region.

Animals↗

PubMed2006

Cytological visualization of DNA transposons and their transposition pattern in somatic cells of maize.

Global genomic analysis of transposable element distributions of both natural (En/Spm, Ac-Ds, and MuDR/Mu) and modified (RescueMu) types was performed by fluorescence in situ hybridization (FISH) on somatic chromosomes coupled with karyotyping of each chromosome. In lines without an active transposable element, the locations of silent En/Spm, Ac-Ds, and MuDR/Mu elements were visualized, revealing variation in copy number and position among lines but no apparent locational bias. The ability to detect single elements was validated by using previously mapped active Ac elements. Somatic transpositions were documented in plants containing an engineered Mutator element, RescueMu, via use of the karyotyping system. By analyzing the RescueMu lines, we found that transposition of RescueMu in root-tip cells follows the cut-and-paste type of transposition. This work demonstrates the utility of FISH and karyotyping in the study of transposon activity and its consequences.

DNA Transposable Elements↗

PubMed2001

Quantification of mitochondrial DNA in peripheral blood mononuclear cells and subcutaneous fat using real-time polymerase chain reaction.

BACKGROUND: With decreased rates of HIV mortality and disease progression attributable to treatment with nucleoside analogue reverse transcriptase inhibitors (NRTIs), attention has now become focused on the toxicities of these forms of treatment. It is believed NRTIs cause a decrease in mitochondrial DNA (mtDNA) synthesis due to their inhibition of DNA polymerase gamma. This hypothesis is supported by in vitro data from muscle biopsies and human lymphoblastic cell lines. The resulting mitochondrial toxicity is thought to manifest itself in a variety of clinical symptoms including fatigue, fat wasting and peripheral neuropathy. A non-invasive test of mitochondrial toxicity is needed to assess toxicity and optimise HIV treatment strategies. Peripheral blood mononuclear cells (PBMC) and subcutaneous fat could be ideal and accessible sources of mtDNA for examining toxicity. OBJECTIVES: The objectives of this study were (a) to develop an assay to quantify the mtDNA copy number of PBMC and obtain reproducible results and (b) to establish the utility of subcutaneous fat as a source of mtDNA for quantification. STUDY DESIGN: PBMC were isolated from blood by centrifugation over Ficoll-Paque and subcutaneous fat was obtained from two 3 mm punch skin biopsies. Following DNA extraction, the mtDNA copy number in each sample was quantified by real-time polymerase chain reaction (PCR). RESULTS: The real-time PCR assay was found to generate consistent and reproducible results with replicates of samples undertaken within the same run, and in two or more different runs, having a mean coefficient of variation of 11.3 and 17.2%, respectively. PBMC and subcutaneous fat contained 409+/-148 and 2042+/-391 copies of mtDNA per cell, respectively. CONCLUSIONS: From the work carried out it can be concluded that firstly, the real-time PCR assay generates consistent and reproducible results, and secondly that mtDNA can be extracted and quantified from PBMC and subcutaneous fat.

Adipose Tissue↗

PubMed1993

Chromatin structures and transcription of rDNA in yeast Saccharomyces cerevisiae.

The chromatin structure of yeast ribosomal DNA was analyzed in vivo by crosslinking intact cells with psoralen. We found that in exponentially growing cultures the regions coding for the 35S rRNA precursor fall into two distinct classes. One class was highly accessible to psoralen and associated with nascent RNAs, characteristic for transcriptionally active rRNA genes devoid of nucleosomes, whereas the other class showed a crosslinking pattern indistinguishable from that of bulk chromatin and was interpreted to represent the inactive rRNA gene copies. By crosslinking the same strain growing in complex or minimal medium, we have shown that yeast cells can modulate the proportion of active (non-nucleosomal) and inactive (nucleosomal) rRNA gene copies in response to variations in environmental conditions which suggests that yeast can regulate rRNA synthesis by varying the number of active gene copies, in contrast to the vertebrate cells studied so far. Whereas intergenic spacers flanking inactive rRNA gene copies are packaged in a regular nucleosomal array, spacers flanking active genes show an unusual crosslinking pattern suggesting a complex interaction of regulatory factors and histones with DNA.

Chromatin↗

PubMed2003

Stochastic gene expression as a many-body problem.

Gene expression has a stochastic component because of the single-molecule nature of the gene and the small number of copies of individual DNA-binding proteins in the cell. We show how the statistics of such systems can be mapped onto quantum many-body problems. The dynamics of a single gene switch resembles the spin-boson model of a two-site polaron or an electron transfer reaction. Networks of switches can be approximately described as quantum spin systems by using an appropriate variational principle. In this way, the concept of frustration for magnetic systems can be taken over into gene networks. The landscape of stable attractors depends on the degree and style of frustration, much as for neural networks. We show the number of attractors, which may represent cell types, is much smaller for appropriately designed weakly frustrated stochastic networks than for randomly connected networks.

Algorithms↗

PubMed1994

Mitochondrial DNA diversity in the pea aphid Acyrthosiphon pisum.

The pea aphid Acyrthosiphon pisum has been established in North America for at least a century and occurs on a broad range of host plants. Despite its importance as a crop pest, there is little understanding of the genetic structure of its populations or the extent of genetic divergence associated with different host plant utilization. This study examined the extent of mitochondrial DNA (mtDNA) diversity among 35 clones of pea aphids from alfalfa. Polymorphisms were detected at only 2 of 126 restriction sites, suggesting the same impoverished level of genetic diversity found in prior allozyme studies. However, length variation was common and apparently arose as a result of tandem repeats in two regions of the mtDNA molecule. Region 1 occurred in proximity to the control region of the molecule, while region 2 was close to a cluster of tRNA genes flanking the NAD-dehydrogenase subunit 3 gene on the opposite side of the mtDNA molecule. Each of the aphid clones was homoplasmic for a single length variant in region 1, which varied with respect to the number of copies of a 120-bp repeat. By contrast, one-third of the clones were heteroplasmic at region 2, where they possessed a variable number of copies of a 210-bp repeat. Reanalysis of clones after 30 generations of parthenogenetic reproduction established the stability of length variants over this interval, suggesting their value in studies of the genetic structure of aphid populations.

Animals↗

PubMed1999

Human retrovirus-5 in rheumatic disease.

It has long been suggested that retroviral infection may play a role in the pathogenesis of autoimmune rheumatic disease. Particles resembling retroviruses have been reported in tissue from patients with Sjögren's syndrome, lupus and rheumatoid arthritis, and molecular mimicry between retroviral antigens and host proteins has been proposed as a mechanism of induction of autoimmunity. Since 1980, four distinct human infectious retroviruses have been discovered, HTLV-I, HTLV-II, HIV-1 and HIV-2. We recently cloned part of a new human retrovirus genome, designated human retrovirus-5 (HRV-5) and demonstrated that this is not endogenous and is therefore a novel infectious retrovirus. Because symptoms resembling arthritis, polymyositis and Sjögren's syndrome occur in individuals infected with HTLV-I and HIV-1, we investigated the possibility that HRV-5 was associated with idiopathic rheumatic disease. Using nested PCR, HRV-5 we demonstrated that proviral DNA was present in approximately 50% of synovial samples of arthritic joints and was also found in over 10% of blood samples of patients with rheumatoid arthritis and systemic lupus erythematosus. HRV-5 proviral DNA was not detectable in affected tissues of autoimmune diseases and was found in only one of over 200 tissues taken at autopsy from non-rheumatoid patients. Sequence analysis of the amplified viral segment showed genetic variation between samples with maintenance of the open reading frame typical of a replicating infectious retrovirus. Thus HRV-5 appears to be a human retrovirus found with a very low genome copy number in most tissues, but which is increased to detectable levels in inflamed joints and blood from patients with rheumatic disease. Whether HRV-5 is aetiologically important in these diseases remains to be determined.

Autoimmune Diseases↗

PubMed1993

DNA addition or deletion is associated with a major karyotype polymorphism in the fungal phytopathogen Colletotrichum gloeosporioides.

A 1.2 Mb minichromosome resolved by pulsed-field electrophoresis was present in two independent race 3 isolates of Colletotrichum gloeosporioides causing Type B anthracnose specifically on Stylosanthes guianensis cv. Graham in Australia. This chromosome was absent in duplicate isolates representing races 1, 2 and 4 which infect other S. guianensis cultivars. A gene library was prepared specifically from the 1.2 Mb minichromosome and ten independent DNA clones unique to this chromosome were identified by differential hybridisation to whole chromosome probes. All of the ten selected probes hybridised only to the 1.2 Mb minichromosome unique to the race 3 isolates but not to any chromosome in isolates of the other races. These ten probes also hybridised only to restriction-digested DNA of race 3 and were thus both chromosome- and strain-specific for Type B C. gloeosporioides. Hybridisation analysis of NotI fragments of the 1.2 Mb minichromosome with these sequences indicated that they were not tightly clustered on the chromosome. These data demonstrate that the variation in the occurrence of the 1.2 Mb minichromosome did not arise by rearrangement of the genome of a progenitor strain but involved either large scale deletion or addition of DNA. The 1.2 Mb minichromosome did not contain a cloned high-copy-number repeat sequence present on all other mini- and maxichromosomes, suggesting addition from a genetically distinct strain. All ten chromosome-specific DNA probes hybridised to a 2.0 Mb chromosome in all races of C. gloeosporioides causing Type A anthracnose on Stylosanthes spp. including S. guianensis.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromosome Aberrations↗

PubMed1989

The ribosomal genes of the mosquito, Aedes aegypti.

The characterisation of the ribosomal genes of the mosquito, Aedes aegypti, is described. Preliminary experiments using a cloned Drosophila ribosomal DNA (rDNA) repeat to probe Southern transfers of Ae. aegypti genomic DNA has indicated that the rDNA repeat of Ae. aegypti is 9.0 kb in length and that individual rDNA repeats exhibit a high degree of homogeneity with respect to length and the position of restriction enzyme recognition sites within the rDNA. The preliminary mapping data together with partial digestion experiments demonstrate that, as in all other higher eukaryotes, the rDNA repeats are arranged in a head-to-tail, tandemly repeating manner. The restriction mapping of cloned rDNA repeats confirmed the largely uniform length of the Ae. aegypti rDNA repeat and provided a more detailed physical map of the DNA. A restriction site polymorphism was detected in one clone (Aar9) which contains an extra HincII site, which is not present in three other clones studied (Aar1, Aar3, or Aar7). Transcription mapping has allowed the allocation of identities to the various restriction fragments and the approximate positioning of the transcription unit. The estimate of rDNA repeat copy number in Ae. aegypti (approximately 500 copies per haploid genome) is similar to the estimate reported for the closely related species, Aedes albopictus, of 430 copies per haploid genome. Ribosomal DNA thus comprises approximately 0.6% of the total Ae. aegypti genome. Analysis of the variation of the rDNA repeat unit both within individual mosquitoes and between strains of Ae. aegypti, has severed to confirm the remarkable homogeneity of the rDNA repeat unit in this insect.

Aedes↗

PubMed2003

Diversity in coding tandem repeats in related Neisseria spp.

BACKGROUND: Tandem repeats contained within coding regions can mediate phase variation when the repeated units change the reading frame of the coding sequence in a copy number dependent manner. Coding tandem repeats are those which do not alter the reading frame with copy number, and the changes in copy number of these repeats may then potentially alter the function or antigenicity of the protein encoded. Three complete neisserial genomes were analyzed and compared to identify coding tandem repeats where the number of copies of the repeat will have some structural consequence for the protein. This is the first study to address coding tandem repeats that may affect protein structures using comparative genomics, combined with a population survey to investigate which show interstrain variability. RESULTS: A total of 28 genes were identified. Of these, 22 contain coding tandem repeats that vary in copy number between the three sequenced strains, three strain specific genes were included for investigation on the basis of having >90% identity between repeated units, and three genes with repeated elements of >250 bp were included although no length variations were seen in the genomes. Amplification, and sequencing of repeats showing altered copy number, of these 28 coding tandem repeat containing regions, from a set of largely unrelated strains, revealed further repeat length variation in several cases. CONCLUSION: Eighteen genes were identified which have variation in repeat copy number between strains of the same species, twelve of which show greater diversity in repeat copy number than is present in the sequenced genomes. In some cases, this may reflect a mechanism for the generation of antigenic variation, as previously described in other species. However, some of the genes identified encode proteins with cytoplasmic functions, including sugar metabolism, DNA repair, and protein production, in which repeat length variation may have other functions. Coding tandem repeats appear to represent a largely unexplored mechanism of generating diversity in the Neisseria spp.

Adhesins, Bacterial↗

PubMed2002

Similar patterns of genomic alterations characterize primary mediastinal large-B-cell lymphoma and diffuse large-B-cell lymphoma.

To address the possible genetic relationship between primary mediastinal large-B-cell lymphoma (PMLBCL) and diffuse large-B-cell lymphoma (DLBCL), we compared DNA copy number changes identified by comparative genomic hybridization (CGH) analysis of 40 PMLBCL and 91 DLBCL tumors. We assessed their karyotypes by G-banding; amplification of MYC, BCL2, and REL genes by Southern blotting; and incidence of nonpolymorphic BCL6 mutations by single-strand conformation polymorphism analysis (SSCP). Overall, CGH identified overlapping and nonoverlapping patterns of DNA copy number changes in the two groups. Among the latter changes, gains of chromosomes 8, 11, 15, and 16 and losses of chromosomes 5, 10, 15, 16, 17, and 20 were seen only in DLBCL, and gains of chromosomes 10, 21, and 22 and losses of chromosomes 11, 13, and 18 were seen only in PMLBCL. Several overlapping changes were identified in both groups, with variation in incidence. Statistical analysis of these changes showed significant gains of chromosomes 3 (P <or= 0.05) and 7q (P <or= 0.05) in DLBCL and gains of chromosomes 9 (P <or= 0.05) and 19 (P <or= 0.05) and the X chromosome (P <or= 0.05) and loss of chromosome 4 (P <or= 0.05) in PMLBCL. Frequent recurring DNA amplification at 2p13-15 and less frequent amplification at 6p21, 12q13, and 18q21 were noted in both groups. Recurring amplification at 1q21 was seen only in DLBCL, whereas nonrecurring amplification at 10p11.2 and 15q22-24 was seen only in PMLBCL. G-banded karyotype analysis identified t(3;14)(q27;q32) in one and t(14;18)(q32;q21) in two cases of PMLBCL. Seven of 13 cases exhibited SSCP variants in the 5' noncoding region of BCL6. In addition, 19 of 24 PMLBCLs assayed for BCL6 protein expression by immunohistochemistry showed positive results, indicating an origin from a germinal center (GC)-derived B cell. Based on these data, we conclude that PMLBCL is a distinct entity among GC-derived high-grade DLBCLs.

Adolescent↗

PubMed1992

The structure and evolution of subtelomeric Y' repeats in Saccharomyces cerevisiae.

The subtelomeric Y' family of repeated DNA sequences in the yeast Saccharomyces cerevisiae is of unknown origin and function. Y's vary in copy number and location among strains. Eight Y's, from two strains, were cloned and sequenced over the same 3.2-kb interval in order to assess the within- and between-strain variation as well as address their origin and function. One entire Y' sequence was reconstructed from two clones presented here and a previously sequenced 833-bp region. It contains two large overlapping open reading frames (ORFs). The putative protein sequences have no strong homologies to any known proteins except for one region that has 27% identity with RNA helicases. RNA homologous to each ORF was detected. Comparison of the sequences revealed that the known long (Y'-L) and short (Y'-S) size classes, which coexist within cells, differ by several insertions and/or deletions within this region. The Y'-Ls from strain Y55 also differ from those of strain YP1 by several short deletions in the same region. Most of these deletions appear to have occurred between short (2-10 bp) direct repeats. The single base pair polymorphisms and the deletions are clustered in the first half of the interval compared. There is 0.30-1.13% divergence among Y'-Ls within a strain and 1.15-1.75% divergence between strains in the interval. This is similar to known unique sequence variation but contrasts with the 8-18% divergence among the adjacent subtelomeric repeats, X. Subsets of Y's exhibit concerted evolution; however, more than one variant appears to be maintained within strains. The observed sequence variation disrupts the first ORF in many Y's while most of the second ORF including the putative helicase region is unaffected. The structure and distribution of the Y' elements are consistent with having originated as a mobile element. However, they now appear to move via recombination. Recombination can account for the homogenization within subsets of Y's but does not account for the maintenance of different variants.

Amino Acid Sequence↗

PubMed1990

The effect of T-DNA copy number, position and methylation on reporter gene expression in tobacco transformants.

Inter-transformant variability in the expression of introduced genes was studied in the R1 and R2 generations of 10 tobacco transformants, produced by Agrobacterium-mediated transformation. In replicated and physiologically equivalent material, tranformants showed considerable variability in the expression of the reporter gene uidA as shown by transcript levels and beta-glucuronidase (GUS) activity. However, homozygous R2 material could be investigated for seven of the transformants and among these, and in one line in which two inserts could segregate independently, this inter-transformant variability was reduced to simple bimodal expression. The two levels of expression for GUS activity in leaves were high or low (approximately 2.5 or 0.3 nmol cm-2 min-1 respectively), with no continuous variation. Transformants in the high group had single T-DNA insertions, while those in the low group had multiple T-DNA insertions, at the same or different loci. Within each group, although T-DNA was apparently integrated at different sites in the plant genome, there was no evidence of position effects. GUS activity levels of the transformants were very similar in the field and in environmentally controlled conditions under high or low light. Plants with multiple insertions and low expression also tended to have increased methylation of the integrated T-DNA.

Bacterial Proteins↗

PubMed2003

Quantitative real-time PCR based on single copy gene sequence for detection of Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis.

OBJECTIVE: To establish a method for quantification of Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis from subgingival plaque by real-time polymerase chain reaction (PCR) technique. MATERIAL AND METHODS: Bacterial cells from both species were obtained from type culture and counted microscopically. Cellular suspension in sterile distilled water was used for DNA extraction by boiling for 20 min, with a mineral oil cover. Primers for PCR were selected from sequences of LktC gene (A. actinomycetemcomitans) and Arg-gingipain (P. gingivalis) to yield amplicons below 100 bp. SYBR Green I based real-time PCR was adjusted to quantify separately both species. RESULTS: A good sensitivity and specificity were obtained for both species, although the yield was better for A. actinomycetemcomitans. A good repeatability of cycle threshold (CT) was encountered, so coefficient of variation was below 6% at every initial copy number. CONCLUSION: A new method of quantification of A. actinomycetemcomitans and P. gingivalis based on SYBR Green real-time PCR is presented. Its good sensibility and repeatability will allow its application to analysis of subgingival plaque samples.

Aggregatibacter actinomycetemcomitans↗

PubMed1991

Techniques used to define human MHC antigens: polymerase chain reaction and oligonucleotide probes.

The various HLA class II specificities, identifiable by serological or cellular typing reagents, reflect underlying polymorphisms of the constituent alpha and beta chains, encoded at several loci within the HLA-D region. Variation in these genes is concentrated in allelic hypervariable regions but can also be found elsewhere. Sequence-specific oligonucleotides (SSO) can be used to detect individual alleles with a high degree of accuracy by probing dot-blotted DNA, amplified to high copy number by the polymerase chain reaction (PCR). The classic DR specificities (DR1-DRw14) can be identified with certainty by sequential hybridization to a series of 14 SSO probes. In addition, fine resolution of specificities such as the subtypes of DR4 is possible using a combination of SSO probes and group-specific amplification in which PCR primers designed to amplify only DR4 alleles are employed. Similar methods can be applied to the typing of DQ and DP alleles even on whole blood samples which have been stored at -20 degrees C for up to 8 years. A full HLA class II type may be obtained on a large number of samples simultaneously without the need to separate and store viable lymphocytes. Thus, the sensitivity and robustiness of this technique give it major applications in the analysis of histocompatibility and disease associations, particularly in circumstances where facilities for the initial preparation and storage of samples may be limited.

Alleles↗

PubMed1991

Frequency and organization of papA homologous DNA sequences among uropathogenic digalactoside-binding Escherichia coli strains.

The frequency of selected papA DNA sequences among 89 digalactoside-binding, uropathogenic Escherichia coli strains was evaluated with 12 different synthetic 15-base probes corresponding to papA genes from four digalactoside-binding piliated recombinant strains (HU849, 201B, and 200A). The papA probes encode amino acids which are common at the carboxy terminus of all strains, adjacent to the proximal portion of the intramolecular disulfide loop of strain 210B, or predicted to constitute the type-specific epitope for each of the four recombinant strains or other epitopes of strain HU849. The presence among the strains of DNA sequence homology to the papA probes was determined by in situ colony hybridization. Hybridization data suggest that there is a high frequency of homologous papA DNA sequences corresponding to selected regions of the papA gene from strain HU849 among the clinical strains. The following nucleotide locations which encode portions of the mature HU849 PapA are detected in a high percentage (42 to 70%) of clinical isolates: 208 to 222, 310 to 324, 478 to 492, 517 to 531, 553 to 567, and 679 to 693. These sequences encode portions of the predicted protective, immunogenic, and/or antigenic epitopes of this PapA. The data also indicate considerable heterogeneity of papA sequences among the strains, especially in the region of nucleotide bases corresponding to positions 391 to 418. These oligonucleotides encode the predicted PapA type-specific immunogenic dominant epitope. Determination of the extent of genetic variability in the papA gene among digalactoside-binding strains will require more extensive DNA sequencing of prototypic papA genes, additional hybridization studies employing other papA gene oligonucleotide probes, and assessment of the different pap operons and their copy number in each strain.

Amino Acid Sequence↗

PubMed1997

Two variants of quantitative reverse transcriptase PCR used to show differential expression of alpha-, beta- and gamma-fibrinogen genes in rat liver lobes.

Quantitative reverse transcriptase PCR (RT-PCR) is a sensitive method for the measurement of mRNA copy number. However, the methodology has gained a reputation for poor reproducibility, leading to concern over the validity of much of the data generated using this technique. We have developed two variants of quantitative competitive RT-PCR using a synthesized RNA as an internal standard to measure precisely the relative levels of alpha-, beta- and gamma-fibrinogen mRNAs in the four lobes of the rat liver. In the first of these variants we altered only the amount of total RNA in the RT-PCR reaction, keeping the amount of internal standard RNA and the number of PCR cycles constant. In the second variant only the number of PCR cycles was altered, and the amounts of total RNA and standard RNA were kept constant. Both variants of RT-PCR allowed calculation of the number of mRNA copies, which did not differ significantly between the two techniques. Of the two variants, the second gave better reproducibility, and the intra-assay coefficient of variation for this technique was 14% (n = 20). Using these two variants we have shown that there are different numbers of fibrinogen mRNAs in the four liver lobes for each of the three genes (alpha-fibrinogen F = 14.64, P = 0.0003; beta-fibrinogen F = 3.74, P = 0.04; gamma-fibrinogen F = 3.75, P = 0.04). In conclusion, by using two variants of quantitative competitive RT-PCR we have shown that this technique can be used to give reproducible results, and the low intra-assay coefficient of variation suggests that quantitative RT-PCR should be the technique of choice for accurate measurement of mRNA copy number.

Animals↗

PubMed2001

Population analysis of an unusual NOR-site polymorphism in brown trout (Salmo trutta L.).

A population analysis of an unusual NOR-site polymorphism previously detected in brown trout (Salmo trutta L.) from North-western Spain was carried out in 225 individuals from 20 native populations from this area. The analysis performed has permitted us to reveal: (i) the ubiquity of this phenomenon in most river basins from NW Spain, 13 extra-NORs not observed in standard trout being detected; (ii) the rDNA constitution of all extra-NORs, as confirmed by CMA3-staining and rDNA-FISH, and their capability to constitute their own nucleolus; (iii) the constant location of extra-NORs within individuals and their stable transmission across generations; and (iv) the telomeric location of Ag-NORs, which were randomly distributed in the karyotype of S. trutta, mostly in the heterozygous condition. Repetitive sequences in the IGS or scattered along rDNA units, rather than chromosome rearrangements, could play an important role in the dispersion of NORs. The polymorphism described seems to have a single geographical origin, since a positive correlation was demonstrated between the degree of polymorphism and geographical distance to a central point of the river basin where the phenomenon showed a higher intensity. The distribution of NOR-site variation as compared with allozyme variation in the populations studied, showed significant statistical differences. Selection against high copy number, or mutation due to changes in NOR location, could be acting on this polymorphism to explain the observed distribution. The high dispersion of NOR-site variants within and among populations suggests the transposition phenomenon responsible for NOR jumping could be still active.

Animals↗