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. 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. 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.

中文翻译：

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遗传背景和细胞谱系对位置效应变异中基因表达水平和模式的影响。

基于染色质的转录沉默通常被描述为随机过程，这主要是由于在位置效应变异（PEV）中观察到的镶嵌表达，其中常染色质报告基因在某些细胞中由于异染色质的并置而沉默。PEV表型的高水平变异通常在报告者种群中观察到。为了确定背景突变是否是造成这种变异的主要因素，我们询问了多少变异是由群体中分离的遗传变异决定的，使用果蝇Drosophila melanogaster作为模型检查了表达水平和表达模式。使用第四个染色体PEV报告基因系39C-12的选择性育种，我们分离出了两个表现出不同对比程度的自交系（A1：低表达，D1：高表达）。在每个自交群体中，观察到的杂种程度具有显着相似性：两个自交系之间沉默程度的90％变异可以通过基因型来解释。进一步的分析表明，该结果反映了多个独立的反式基因座的综合作用。虽然最初的观察是基于在蝇眼中评分的PEV表型（hsp70-白色报告基因），但使用在整个蝇中评分的β-gal报告基因也观察到了类似程度的沉默。此外，在每个自交系中杂色的模式变得几乎相同。尽管表达程度不同，但在A1和D1品系的同一个眼睛象限中仍发现大量色素富集。结果表明，背景遗传变异在确定实验室动物中通常观察到的PEV的可变程度方面起主要作用。有趣的是，不仅杂种的程度在自交系中变得一致，而且杂种的模式也看起来相似。将这些观察结果与局部异染色质形成的扩散模型相结合，我们提出了一种增强的随机模型来描述PEV，在该模型中，遗传背景驱动了沉默的总体水平，并与细胞谱系特异性的调控环境一起确定了开/关概率。每个细胞中的报告基因座。在对异染色质形成更广泛的遗传影响的背景下，该模型确认了特定于细胞类型的事件。自交系的杂色程度不仅保持一致，而且杂色的模式也看起来相似。将这些观察结果与局部异染色质形成的扩散模型相结合，我们提出了一种增强的随机模型来描述PEV，在该模型中，遗传背景驱动了沉默的总体水平，并与细胞谱系特异性的调控环境一起确定了开/关概率。每个细胞中的报告基因座。在对异染色质形成更广泛的遗传影响的背景下，该模型确认了特定于细胞类型的事件。自交系的杂色程度不仅保持一致，而且杂色的模式也看起来相似。将这些观察结果与局部异染色质形成的扩散模型相结合，我们提出了一种增强的随机模型来描述PEV，在该模型中，遗传背景驱动了沉默的总体水平，并与细胞谱系特异性的调控环境一起确定了开/关概率。每个细胞中的报告基因座。在对异染色质形成更广泛的遗传影响的背景下，该模型确认了特定于细胞类型的事件。我们提出一个增强的随机模型来描述PEV，其中遗传背景驱动沉默的总体水平，并与细胞谱系特定的调节环境一起确定每个细胞中报告基因位点的开/关概率。在对异染色质形成产生更广泛的遗传影响的情况下，该模型承认特定于细胞类型的事件。我们提出一个增强的随机模型来描述PEV，其中遗传背景驱动沉默的总体水平，并与细胞谱系特定的调节环境一起确定每个细胞中报告基因位点的开/关概率。在对异染色质形成更广泛的遗传影响的背景下，该模型确认了特定于细胞类型的事件。