scholarly journals Position-Effect Variegation, Heterochromatin Formation, and Gene Silencing in Drosophila

2013 ◽  
Vol 5 (8) ◽  
pp. a017780-a017780 ◽  
Author(s):  
S. C. R. Elgin ◽  
G. Reuter
Keyword(s):  
Gene Silencing ◽  
Position Effect ◽  
Position Effect Variegation ◽  
Heterochromatin Formation ◽  
Effect Variegation

scholarly journals The impact of genetic background and cell lineage on the level and pattern of gene expression in position effect variegation

2019 ◽  
Vol 12 (1) ◽  
Author(s):  
Sidney H. Wang ◽  
Sarah C. R. Elgin
Keyword(s):  
Genetic Variants ◽  
Genetic Background ◽  
Position Effect ◽  
Cell Lineage ◽  
Inbred Lines ◽  
Position Effect Variegation ◽  
Fourth Chromosome ◽  
Heterochromatin Formation ◽  
Effect Variegation ◽  
The Impact

Abstract Background Chromatin-based transcriptional silencing is often described as a stochastic process, largely because of the mosaic expression observed in position effect variegation (PEV), where a euchromatic reporter gene is silenced in some cells as a consequence of juxtaposition with heterochromatin. High levels of variation in PEV phenotypes are commonly observed in reporter stocks. To ascertain whether background mutations are the major contributors to this variation, we asked how much of the variation is determined by genetic variants segregating in the population, examining both the level and pattern of expression using the fruit fly, Drosophila melanogaster, as the model. Results Using selective breeding of a fourth chromosome PEV reporter line, 39C-12, we isolated two inbred lines exhibiting contrasting degrees of variegation (A1: low expression, D1: high expression). Within each inbred population, remarkable similarity is observed in the degree of variegation: 90% of the variation between the two inbred lines in the degree of silencing can be explained by genotype. Further analyses suggest that this result reflects the combined effect of multiple independent trans-acting loci. While the initial observations are based on a PEV phenotype scored in the fly eye (hsp70-white reporter), similar degrees of silencing were observed using a beta-gal reporter scored across the whole fly. Further, the pattern of variegation becomes almost identical within each inbred line; significant pigment enrichment in the same quadrant of the eye was found for both A1 and D1 lines despite different degrees of expression. Conclusions The results indicate that background genetic variants play the major role in determining the variable degrees of PEV commonly observed in laboratory stocks. Interestingly, not only does the degree of variegation become consistent in inbred lines, the patterns of variegation also appear similar. Combining these observations with the spreading model for local heterochromatin formation, we propose an augmented stochastic model to describe PEV in which the genetic background drives the overall level of silencing, working with the cell lineage-specific regulatory environment to determine the on/off probability at the reporter locus in each cell. This model acknowledges cell type-specific events in the context of broader genetic impacts on heterochromatin formation.

scholarly journals Live Analysis of Position Effect Variegation Reveals Different Modes of Action for HP1 and Su(var)3-9

2021 ◽  
Author(s):  
Keith Andrew Maggert ◽  
Farah J Bughio
Keyword(s):  
Gene Silencing ◽  
Developmental Trajectories ◽  
Position Effect ◽  
Epigenetic Inheritance ◽  
S Phase ◽  
Position Effect Variegation ◽  
Cell Clones ◽  
Effect Variegation ◽  
Random Patterns ◽  
Eukaryotic Genomes

Position Effect Variegation (PEV) results from the juxtaposition of euchromatic and heterochromatic components of eukaryotic genomes, silencing genes near the new euchromatin/heterochromatin junctions. The degree of silencing is itself heritable through S-phase, giving rise to distinctive random patterns of cell clones expressing the genes intermixed with clones in which the genes are silenced. Much of what we know about epigenetic inheritance in the soma stems from work on PEV aimed at identifying the components of the silencing machinery and its mechanism of inheritance. Despite identifying two central gene activities - the Su(var)3-9 histone H3-Lysine-9 methyltransferase and the Su(var)205/HP1 methyl-H3-Lysine-9 binding protein - their role in PEV has been inferred from terminal phenotypes, leaving considerable gaps in understanding how PEV behaves through development. Here, we investigate the phenotypes of Su(var)3-9 and Su(var)205/HP1 mutations in live developing tissues. We discovered that mutations in Su(var)205/HP1 compromise the initial establishment of PEV in early embryogenesis. Later gains of heterochromatin-induced gene silencing are possible, but are unstable and lost rapidly. In contrast, mutations in Su(var)3-9 exhibit robust silencing early in development, but fail to maintain it through subsequent cell divisions. Our analyses show that while the terminal phenotypes of these mutations may appear identical, they have arrived at them through different developmental trajectories. We discuss how our findings further challenge existing models for epigenetic inheritance of heterochromatin-induced gene silencing.

Mechanisms for the construction and developmental control of heterochromatin formation and imprinted chromosome domains

Development ◽  
1990 ◽  
Vol 108 (Supplement) ◽  
pp. 35-45 ◽  
Author(s):  
Kenneth D. Tartof ◽  
Marilyn Bremer
Keyword(s):  
Position Effect ◽  
Model System ◽  
Chromosome Inactivation ◽  
Position Effect Variegation ◽  
Position Effects ◽  
Developmental Control ◽  
Heterochromatin Formation ◽  
Material Basis ◽  
Chromosome Domains ◽  
Effect Variegation

The study of variegating position effects in Drosophila provides a model system to explore the mechanism and material basis for the construction and developmental control of heterochromatin domains and the imprinted genomic structures that they may create. The results of our experiments in this regard have implications for a diverse assortment of long-range chromosome phenomena related to gene and chromosome inactivation. Specifically, as a consequence of our studies on position effect variegation, we propose a simple mechanism of X-chromosome inactivation, suggest a purpose for genomic imprinting, and postulate a general means for regulating the time in development at which certain genes become heterochromatically repressed.

scholarly journals A Drosophila Homologue of Sir2 Modifies Position-Effect Variegation but Does Not Affect Life Span

Genetics ◽  
2002 ◽  
Vol 162 (4) ◽  
pp. 1675-1685
Author(s):  
Brenda L Newman ◽  
James R Lundblad ◽  
Yang Chen ◽  
Sarah M Smolik
Keyword(s):  
Life Span ◽  
Histone Deacetylase ◽  
Position Effect ◽  
Regulation Of Gene Expression ◽  
Position Effect Variegation ◽  
Creb Binding Protein ◽  
Developmentally Regulated ◽  
Heterochromatin Formation ◽  
Chromatin Regulator ◽  
Effect Variegation

Abstract Control of chromosome structure is important in the regulation of gene expression, recombination, DNA repair, and chromosome stability. In a two-hybrid screen for proteins that interact with the Drosophila CREB-binding protein (dCBP), a known histone acetyltransferase and transcriptional coactivator, we identified the Drosophila homolog of a yeast chromatin regulator, Sir2. In yeast, Sir2 silences genes via an intrinsic NAD+-dependent histone deacetylase activity. In addition, Sir2 promotes longevity in yeast and in Caenorhabditis elegans. In this report, we characterize the Drosophila Sir2 (dSir2) gene and its product and describe the generation of dSir2 amorphic alleles. We found that dSir2 expression is developmentally regulated and that dSir2 has an intrinsic NAD+-dependent histone deacetylase activity. The dSir2 mutants are viable, fertile, and recessive suppressors of position-effect variegation (PEV), indicating that, as in yeast, dSir2 is not an essential function for viability and is a regulator of heterochromatin formation and/or function. However, mutations in dSir2 do not shorten life span as predicted from studies in yeast and worms.

The enhancer of polycomb gene of Drosophila encodes a chromatin protein conserved in yeast and mammals

Development ◽  
1998 ◽  
Vol 125 (20) ◽  
pp. 4055-4066 ◽  
Author(s):  
K. Stankunas ◽  
J. Berger ◽  
C. Ruse ◽  
D.A. Sinclair ◽  
F. Randazzo ◽  
...  
Keyword(s):  
Position Effect ◽  
Polytene Chromosomes ◽  
Polycomb Group ◽  
Position Effect Variegation ◽  
Chromatin Protein ◽  
Heterochromatin Formation ◽  
C Elegans ◽  
Polycomb Group Genes ◽  
Effect Variegation ◽  
Or Gene

The Polycomb group of genes in Drosophila are homeotic switch gene regulators that maintain homeotic gene repression through a possible chromatin regulatory mechanism. The Enhancer of Polycomb (E(Pc)) gene of Drosophila is an unusual member of the Polycomb group. Most PcG genes have homeotic phenotypes and are required for repression of homeotic loci, but mutations in E(Pc) exhibit no homeotic transformations and have only a very weak effect on expression of Abd-B. However, mutations in E(Pc) are strong enhancers of mutations in many Polycomb group genes and are also strong suppressors of position-effect variegation, suggesting that E(Pc) may have a wider role in chromatin formation or gene regulation than other Polycomb group genes. E(Pc) was cloned by transposon tagging, and encodes a novel 2023 amino acid protein with regions enriched in glutamine, alanine and asparagine. E(Pc) is expressed ubiquitously in Drosophila embryogenesis. E(Pc) is a chromatin protein, binding to polytene chromosomes at about 100 sites, including the Antennapedia but not the Bithorax complex, 29% of which are shared with Polycomb-binding sites. Surprisingly, E(Pc) was not detected in the heterochromatic chromocenter. This result suggests that E(Pc) has a functional rather than structural role in heterochromatin formation and argues against the heterochromatin model for PcG function. Using homology cloning techniques, we identified a mouse homologue of E(Pc), termed Epc1, a yeast protein that we name EPL1, and as well as additional ESTs from Caenorhabditis elegans, mice and humans. Epc1 shares a long, highly conserved domain in its amino terminus with E(Pc) that is also conserved in yeast, C. elegans and humans. The occurrence of E(Pc) across such divergent species is unusual for both PcG proteins and for suppressors of position-effect variegation, and suggests that E(Pc) has an important role in the regulation of chromatin structure in eukaryotes.

Different patterns of gene silencing in position-effect variegation

Genome ◽  
2003 ◽  
Vol 46 (6) ◽  
pp. 1104-1117 ◽  
Author(s):  
Vett K Lloyd ◽  
David Dyment ◽  
Donald A.R Sinclair ◽  
Thomas A Grigliatti
Keyword(s):  
Gene Silencing ◽  
Position Effect ◽  
Early Period ◽  
Cellular Level ◽  
Position Effect Variegation ◽  
Small Spot ◽  
Number Of Genes ◽  
Effect Variegation ◽  
Peak Sensitivity ◽  
Phenotypic Instability

Position-effect variegation (PEV) results when a fully functional gene is moved from its normal position to a position near to a broken heterochromatic-euchromatic boundary. In this new position, the gene, while remaining unaltered at the DNA level, is transcriptionally silenced in some cells but active in others, producing a diagnostic mosaic phenotype. Many variegating stocks show phenotypic instability, in that the level of variegation is dramatically different in different isolates or when out crossed. To test if this phenotypic instability was due to segregation of spontaneously accumulated mutations that suppress variegation, four different and well-characterized strains showing PEV for the white+ gene (wm4, wmMc, wm51b, and wmJ) and representing both large and small spot variegators were repeatedly out crossed to a strain free of modifiers, and the phenotypes of these variegators were monitored for 30 generations. Once free of modifiers, these variegating strains were then allowed to reaccumulate modifiers. The spontaneous suppressors of variegation were found to include both dominant and recessive, autosomal and X-linked alleles selected to reduce the detrimental effects of silencing white+ and adjacent genes. The time of peak sensitivity to temperature during development was also determined for these four variegators. Although large and small spot variegators have previously been attributed to early and late silencing events, respectively, the variegators we examined all shared a common early period of peak sensitivity to temperature. Once free of their variegation suppressors, the different variegating strains showed considerable differences in the frequency of inactivation at a cellular level (the number of cells showing silencing of a given gene) and the extent of variegation within the cell (the number of silenced genes). These results suggest that large and small spot variegation may be a superficial consequence of spontaneous variegation suppressors. The nature and number of these spontaneous variegation suppressors depends on the number of genes silenced in a given variegating rearrangement. These results are interpreted in the context of a model that proposes that the different underlying patterns of gene silencing seen in PEV can be attributed directly to the formation of heterochromatin domains possessing different properties of propagation during cell division.Key words: Drosophila melanogaster, position-effect variegation, spontaneous suppressors of variegation.

scholarly journals The Impact of Genetic Background and Cell Lineage on the Level and Pattern of Position Effect Variegation

2019 ◽  
Author(s):  
Sidney H. Wang ◽  
Sarah C.R. Elgin
Keyword(s):  
Genetic Background ◽  
Position Effect ◽  
Cell Lineage ◽  
Inbred Lines ◽  
Laboratory Population ◽  
Position Effect Variegation ◽  
Fourth Chromosome ◽  
Heterochromatin Formation ◽  
Effect Variegation ◽  
The Impact

AbstractBackgroundChromatin-based transcriptional silencing is often described as a stochastic process, largely because of the mosaic expression observed in position effect variegation (PEV), where a euchromatic reporter gene is juxtaposed with heterochromatin. Here we closely examine the impact of genetic background on PEV phenotypes in the fruit fly, Drosophila melanogaster.ResultsUsing consecutive generations of selective breeding, we isolated, from a single laboratory population, two inbred lines exhibiting contrasting degrees of variegation (A1: low expression, D1: high expression). Within each inbred population, remarkable similarity is observed in both the degree and the pattern of variegation. 89.63% of the differences between the two inbred lines in the degree of silencing can be explained by genotype, while a modest but significant sex effect is also observed. Further analyses of the PEV phenotype in the progeny of crosses between A1 and D1 suggest that the genotypic effect is the result of the combined effect of multiple independent trans-acting loci. While the initial observations are based on a PEV phenotype scored in the fly eye (hsp70-white reporter), similar degrees of silencing were observed using a beta-gal reporter that can be scored across the whole fly. The pattern of variegating hsp70-white expression among individual flies becomes almost identical after five generations of inbreeding. Using a reporter inserted into the heterochromatic fourth chromosome, image analysis found significant enrichment of pigmentation in the ventral-posterior quadrant in both the A1 and D1 lines, and in the F1 and F2 progeny produced from a cross between A1 and D1, despite different degrees of expression.ConclusionsCombining these results with the spreading model for local heterochromatin formation, we propose an augmented stochastic model to describe PEV. In this model, the genetic background, which determines the overall level of silencing, works with the cell lineage specific regulatory environment to determine the on/ off probability at the reporter locus in each cell. This model acknowledges cell-type specific events, as well as the general impact of heterochromatin formation.

scholarly journals Comparative Analysis of Position–Effect Variegation Mutations in Drosophila melanogaster Delineates the Targets of Modifiers

Genetics ◽  
1998 ◽  
Vol 148 (2) ◽  
pp. 733-741
Author(s):  
Georgette L Sass ◽  
Steven Henikoff
Keyword(s):  
Drosophila Melanogaster ◽  
Chromosomal Rearrangements ◽  
Position Effect ◽  
Reporter Genes ◽  
Class Ii ◽  
Class I ◽  
Position Effect Variegation ◽  
Heterochromatin Formation ◽  
Transposon Insertion ◽  
Effect Variegation

Abstract In Drosophila melanogaster, heterochromatin-induced silencing or position–effect variegation (PEV) of a reporter gene has provided insights into the properties of heterochromatin. Class I modifiers suppress PEV, and class II modifiers enhance PEV when the modifier gene is present in fewer than two doses. We have examined the effects of both class I and class II modifiers on four PEV mutations. These mutations include the inversions In(1)wm4 and In(2R)bwVDe2, which are classical chromosomal rearrangements that typify PEV mutations. The other mutations are a derivative of brownDominant, in which brown+ reporters are inactivated by a large block of heterochromatin, and a P[white+] transposon insertion associated with second chromosome heterochromatin. In general, we find that class I modifiers affect both classical and nonclassical PEV mutations, whereas class II modifiers affect only classical PEV mutations. We suggest that class II modifiers affect chromatin architecture in the vicinity of reporter genes, and only class I modifiers identify proteins that are potentially involved in heterochromatin formation or maintenance. In addition, our observations support a model in which there are different constraints on the process of heterochromatin-induced silencing in classical vs. nonclassical PEV mutations.

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