The Effects Of The Silkworm Bm1 And BmSE Retrotransposons On Expression Of Genes

Retrotransposons are important elements in regulating gene expression and shaping genome evolution.Bm1 and BmSE are the two most important SINE retrotransposons in the silkworm genome,however,their functional pattern and mechanism in regulating gene expression have not been demonstrated clearly yet.In order to answer these questions,this study has used EGFP as a reporter gene to clarify the regulatory pattern of how Bm1 and BmSE influence gene expression and explore the mechamism behind this special regulation.The main results are as followings:Bm1 and BmSE retrotransposons have similar regulation patterns in gene expression: When Bm1 and BmSE retrotransposons are inserted in the 5' flanking of gene,they functioned as repressors or silencers,but lost their silence function as they are inserted in the 3' flanking of gene.At the same time,this kind of regulation pattern is also worked in the BmN cell,which means that the mechanism behind this regulation pattern is conserved.Bm1 and BmSE retrotransposons can silence the expression of gene,as long as all three following conditions are available: 1)Locate in specific position: Only when Bm1 and BmSE retrotransposons are inserted in the 5' flanking sequence of gene;2)Inverse sequence: reverse sequence insertion lose the silence function;3)Full and complete sequence: changes like truncated,reversed and mutated in Bm1 and BmSE will lose the silence function.The silence of gene expression of Bm1 and BmSE retrotransposons is not at the transcription level,because no new promoter and new TSS site were observed.Meanwhile,the RNA second structures of those two retrotransposons are not related to gene silence as well.The distributions of Bm1 and BmSE retrotransposons in the genome were investigated,the numbers of predicted genes whose 5'UTR flanking regions with inserted complete and inverse Bm1 and BmSE are 125 and 58,respectively.The frequencies of Bm1 and BmSE insertions at individual loci in domestic and wild silkworm genomes were investigated by using sequencing data of eight silkworm genomes and seven wild silkworm genomes.In total,54 loci in which transposon insertion frequencies are significantly different between the domestic and wild silkworms were found.This provides valuable information for understanding the possible roles of transposons during silkworm domestication.In summary,Bm1 and BmSE retrotransposons have the same regulation pattern in gene expression.Only when the complete and inverse insertion of Bm1 and BmSE in the 5' UTR region can silence gene expression,and this kind of silence effect probabaly occurred in post transcriptional level.In addition,several candidate genes with significantly different frequencies of retrotransposon insertions between the domestic and wild silkworm genomes were found.The results presented in this study help us understanding the role of retrotransposons in both gene regulatory network and silkworm domestication.