The pattern of R2 retrotransposon activity in natural populations of Drosophila simulans reflects the dynamic nature of the rDNA locus.
The pattern of R2 retrotransposon activity in natural populations of Drosophila simulans reflects the dynamic nature of the rDNA locus.
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DOI：
10.1371/journal.pgen.1000386
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发表时间：
2009-02
期刊：
影响因子：
4.5
通讯作者：
Eickbush TH
中科院分区：
文献类型：
--
作者：
Zhou J;Eickbush TH
人们对使转座元件在种群中保持活性的插入模式和频率知之甚少。反转录转座元件R2专门插入到28S rRNA基因中，在那里它与动物宿主建立了长期、稳定的关系。之前对模拟果蝇实验室种群的研究表明，R2逆转录转座的控制存在于rDNA基因座内。在这份报告中，我们从两个自然种群采集了180个动物的rDNA基因座。研究发现，这两个种群的R2活性模式相似。大约一半的rDNA基因座支持没有或非常低水平的R2转录本，没有证据表明R2逆转录转座。其余一半的rDNA基因座的R2转录本水平几乎在100倍的范围内连续变化，并支持新的逆转录转座事件。对这些群体中18个R2转录水平范围内的品系的rDNA基因座的结构分析表明，R2数量和rDNA基因座大小变化了2倍；然而，R2活性与这些参数中的任何一个都不容易相关。相反，R2活性与rDNA基因座内元素的分布最相关。没有活性的基因座有更大的rDNA单元的连续块，没有R2-插入。这些数据表明了一种模型，在该模型中，rDNA基因座内的频繁重组不断地重新分配R2插入的单位，导致单个基因座内R2活性水平的变化和群体内持久的R2活性。转座子是真核生物基因组中丰富的自私成分。尽管真核生物进化出了控制这些元素的复杂机制，但它们仍在继续繁殖。在这里，我们研究R2反转录转座子，这是一种非常成功的元件，只插入动物的28S rRNA基因内的一个位置。在果蝇的两个自然种群中，我们证明R2的活性与每只果蝇体内R2转录物的水平直接相关。R2转录本的水平反过来又与线状排列的rRNA基因的性质有关，也被称为核仁组织者或rDNA基因座。R2转录水平似乎取决于rDNA基因座内R2插入单位的分布，而不是R2插入的数量或基因座的总大小。我们的发现表明，只有当苍蝇能够识别rDNA基因的一个或多个没有R2插入单位的大区域时，R2转录本和因此的活动才能被阻止。由于重组不断地改变R2插入的rDNA单元的数量和位置，单个基因座在R2不活跃和活跃之间切换，从而维持R2元件的长期生存。
The pattern and frequency of insertions that enable transposable elements to remain active in a population are poorly understood. The retrotransposable element R2 exclusively inserts into the 28S rRNA genes where it establishes long-term, stable relationships with its animal hosts. Previous studies with laboratory stocks of Drosophila simulans have suggested that control over R2 retrotransposition resides within the rDNA loci. In this report, we sampled 180 rDNA loci of animals collected from two natural populations of D. simulans. The two populations were found to have similar patterns of R2 activity. About half of the rDNA loci supported no or very low levels of R2 transcripts with no evidence of R2 retrotransposition. The remaining half of the rDNA loci had levels of R2 transcripts that varied in a continuous manner over almost a 100-fold range and did support new retrotransposition events. Structural analysis of the rDNA loci in 18 lines that spanned the range of R2 transcript levels in these populations revealed that R2 number and rDNA locus size varied 2-fold; however, R2 activity was not readily correlated with either of these parameters. Instead R2 activity was best correlated with the distribution of elements within the rDNA locus. Loci with no activity had larger contiguous blocks of rDNA units free of R2-insertions. These data suggest a model in which frequent recombination within the rDNA locus continually redistributes R2-inserted units resulting in changing levels of R2 activity within individual loci and persistent R2 activity within the population. Transposable elements are abundant selfish components of all eukaryotic genomes. Despite the elaborate mechanisms eukaryotes have evolved to control these elements, they continue to proliferate. Here, we study R2 retrotransposons, highly successful elements that only insert into a site within the 28S rRNA genes of animals. In two natural populations of Drosophila, we show that R2 activity is directly linked to the level of R2 transcripts within each fly. The level of R2 transcripts is in turn linked to properties of the tandemly arranged rRNA genes, also known as the nucleolar organizer or rDNA locus. R2 transcript levels appear to depend upon the distribution of R2-inserted units within the rDNA locus, rather than the number of R2 insertions or the total size of the locus. Our findings suggest that R2 transcripts and hence activity can be prevented only when a fly can identify one or more large regions of the rDNA locus free of R2-inserted units. Because recombinations continually change the number and position of rDNA units with R2 insertions, individual loci switch between R2 inactivity and activity, thus perpetuating the long-term survival of R2 elements.
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