Hilliker A J, Appels R
Chromosoma. 1982;86(4):469-90. doi: 10.1007/BF00330122.
In Drosophila melanogaster X chromosome heterochromatin (Xh) constitutes the proximal 40% of the X chromosome DNA and contains a number of genetic elements with homologous sites on the Y chromosome, one of which is well defined, namely, the bobbed locus, the repetitive structural locus for the 18S and 28S rRNAs. This report presents the localisation of specific repeated DNA sequences within Xh and the employment of this sequence map in constructing new chromosomes to analyse the nature of the heterochromatin surrounding the rDNA region. Repeated sequences were located relative to inversion breakpoints which differentiate Xh cytogenetically. When the rDNA region was manipulated to be in a position in the chromosome so that it was without the Xh which normally surrounds it, the following observations were made. (i) The rDNA region of Xh is intrinsically heterochromatic, remaining genetically active and yet possessing major heterochromatic properties even in the absence of the flanking heterochromatin regions. (ii) The size of the deletion removing the portion of Xh normally located distal to the rDNA region affected the dominance relationship between the X and Y nucleolar organizers (activity/endoreduplication assayed in male salivary glands). The X rDNA without any flanking heterochromatin was dominant over Y rDNA while the presence of some Xh allowed both the X and Y rDNA to be utilized. (iii) Enhancement of the position effect variegation on the white locus was demonstrated to occur as a result of the Xh deletions generated. EMS mutagenesis studies argue that the regions of Xh flanking the rDNA region contain no vital loci despite the fact that they strongly effect gene expression in some genotypes. This is consistent with early studies using X-ray mutagenesis (Lindsley et al., 1960). The pleiotropic effects of deleting specific regions of Xh is discussed in relation to the possible influence of heterochromatin on the organisation of the functional interphase nucleus.
在黑腹果蝇中，X染色体异染色质（Xh）构成了X染色体DNA近端的40%，并包含许多在Y染色体上具有同源位点的遗传元件，其中一个已被明确界定，即截毛位点，它是18S和28S核糖体RNA的重复结构位点。本报告展示了Xh内特定重复DNA序列的定位，以及利用该序列图谱构建新染色体以分析rDNA区域周围异染色质的性质。相对于在细胞遗传学上区分Xh的倒位断点，重复序列得以定位。当rDNA区域在染色体中的位置被操控，使其处于正常情况下围绕它的Xh缺失的位置时，得到了以下观察结果。（i）Xh的rDNA区域本质上是异染色质的，即使在没有侧翼异染色质区域的情况下，仍保持遗传活性并具有主要的异染色质特性。（ii）去除通常位于rDNA区域远端的Xh部分的缺失大小，影响了X和Y核仁组织者之间的显性关系（在雄性唾液腺中测定活性/核内复制）。没有任何侧翼异染色质的X rDNA对Y rDNA具有显性，而一些Xh的存在则允许X和Y rDNA都被利用。（iii）结果表明，由于产生的Xh缺失，导致了白色位点位置效应斑驳的增强。EMS诱变研究表明，尽管rDNA区域侧翼的Xh区域在某些基因型中强烈影响基因表达，但并不包含重要位点。这与早期使用X射线诱变的研究结果一致（Lindsley等人，1960年）。结合异染色质对功能间期核组织的可能影响，讨论了删除Xh特定区域的多效性效应。
