Quantitatively Increased Somatic Transposition of Transposable Elements in Drosophila Strains Compromised for RNAi

In Drosophila melanogaster, small RNAs homologous to transposable elements (TEs) are of two types: piRNA (piwi-interacting RNA) with size 23-29nt and siRNA (small interfering RNA) with size 19-22nt. The siRNA pathway is suggested to silence TE activities in somatic tissues based on TE expression profiles, but direct evidence of transposition is lacking. Here we developed an efficient FISH (fluorescence in Situ hybridization) based method for polytene chromosomes from larval salivary glands to reveal new TE insertions. Analysis of the LTR-retrotransposon 297 and the non-LTR retroposon DOC shows that in the argonaut 2 (Ago2) and Dicer 2 (Dcr2) mutant strains, new transposition events are much more frequent than in heterozygous strains or wild type strains. The data demonstrate that the siRNA pathway represses TE transposition in somatic cells. Nevertheless, we found that loss of one functional copy of Ago2 or Dcr2 increases somatic transpositions of the elements at a lower level depending on the genetic background, suggesting a quantitative role for RNAi core components on mutation frequency.在黑腹果蝇（Drosophila melanogaster）中，与转座因子（transposable elements，TEs）同源的小RNA可分为两类：一类是长度为23~29nt的piwi结合RNA（piwi-interacting RNA，简称piRNA），另一类是长度为19~22nt的小干扰RNA（small interfering RNA，简称siRNA）。基于转座因子的表达谱，学界推测siRNA通路可在体细胞组织中抑制转座因子的活性，但目前仍缺乏转座事件的直接证据。本研究开发了一种基于荧光原位杂交（fluorescence in situ hybridization，简称FISH）的高效方法，可用于检测幼虫唾液腺的多线染色体，以揭示转座因子的新插入事件。对长末端重复序列反转录转座子（LTR-retrotransposon）297与非长末端重复序列反转录转座子（non-LTR retroposon）DOC的分析显示，在Argonaute 2（Ago2）与Dicer 2（Dcr2）突变体菌株中，转座因子的新转座事件发生率远高于杂合子菌株或野生型菌株。上述数据证实，siRNA通路可在体细胞中抑制转座因子的转座活动。尽管如此，本研究发现，仅丢失一个功能性Ago2或Dcr2拷贝时，转座因子的体细胞转座率会有所上升，但上升幅度较低，且这一效应依赖于遗传背景，这表明RNA干扰（RNA interference，简称RNAi）核心组分对突变频率具有定量调控作用。