Mecheva I S, Semionov E P
Institute of Genetics, Bulgarian Academy of Sciences, Sofia.
Genetica. 1992;85(3):223-9. doi: 10.1007/BF00132274.
The locations of the ribosomal DNA (rDNA) insertion elements type I and type II along the polytene chromosomes of three Drosophila species of the melanogaster subgroup--D. simulans, D. mauritiana and D. melanogaster--have been compared. In situ hybridization has shown that the intragenomic distribution of type I as well as of type II insertions is different for these related species. In particular, we have revealed rDNA-free autosomal sites, containing type II element sequences within the D. simulans and D. mauritiana chromosomes. This finding confirms the ability of this type of insertion to transpose, as was demonstrated earlier for Bombyx mori. The appearance of the rDNA not associated with the nucleolar organizers, evident by additional nucleoli, occurred with species-specific frequency. At the same time, for all three species the pattern of such changes (an attachment of the nucleoli to varying sites of the chromosomes and the presence of ectopic contacts between them, a composition of the rDNA repeats in the nucleolar material not integrated at the nucleolar organizer) was similar. The number of additional nucleoli in the hybrid polytene nuclei corresponded to the value of the parental species exhibiting nucleolar replicative dominance.
对黑腹果蝇亚组的三种果蝇——拟果蝇（D. simulans）、毛里求斯果蝇（D. mauritiana）和黑腹果蝇（D. melanogaster）——的多线染色体上I型和II型核糖体DNA（rDNA）插入元件的位置进行了比较。原位杂交表明，对于这些近缘物种，I型和II型插入的基因组内分布是不同的。特别是，我们在拟果蝇和毛里求斯果蝇的染色体中发现了不含rDNA的常染色体位点，其中含有II型元件序列。这一发现证实了这种插入类型具有转座能力，正如之前对家蚕所证明的那样。与核仁组织区不相关的rDNA的出现，通过额外的核仁得以体现，其出现频率具有物种特异性。同时，对于所有这三个物种，这种变化的模式（核仁附着于染色体的不同位点以及它们之间存在异位接触，核仁物质中rDNA重复序列的组成未整合到核仁组织区）是相似的。杂种多线核中额外核仁的数量与表现出核仁复制优势的亲本物种的值相对应。
