Abstract
Transposition of two retroelements (Ulysses and Penelope) mobilized in the course of hybrid dysgenesis in Drosophila virilis has been investigated by in situ hybridization on polytene chromosomes in two D. virilis strains of different cytotypes routinely used to get dysgenic progeny. The analysis has been repeatedly performed over the last two decades, and has revealed transpositions of Penelope in one of the strains, while, in the other strain, the LTR-containing element Ulysses was found to be transpositionally active. The gypsy retroelement, which has been previously shown to be transpositionally inactive in D. virilis strains, was also included in the analysis. Whole mount is situ hybridization with the ovaries revealed different subcellular distribution of the transposable elements transcripts in the strains studied. Ulysses transpositions occur only in the strain where antisense piRNAs homologous to this TE are virtually absent and the ping-pong amplification loop apparently does not take place. On the other hand small RNAs homologous to Penelope found in the other strain, belong predominantly to the siRNA category (21nt), and consist of sense and antisense species observed in approximately equal proportion. The number of Penelope copies in the latter strain has significantly increased during the last decades, probably because Penelope-derived siRNAs are not maternally inherited, while the low level of Penelope-piRNAs, which are faithfully transmitted from mother to the embryo, is not sufficient to silence this element completely. Therefore, we speculate that intrastrain transposition of the three retroelements studied is controlled predominantly at the post-transcriptional level.
Highlights
Transposable elements (TEs) are repetitive sequences capable of moving in genomes under certain conditions, and they are widely observed in practically all organisms studied so far
It is noteworthy that we failed to detect Penelope hybridization to chromosome 6, in strain 160, and in all other D. virilis strains studied in our laboratory [18]
We showed that Penelope and Ulysses are able to asymmetrically transpose in D. virilis parental strains even without performing dysgenic crosses that drastically increase the frequency of unrelated TE transpositions in this species
Summary
Transposable elements (TEs) are repetitive sequences capable of moving in genomes under certain conditions, and they are widely observed in practically all organisms studied so far. Host organisms employ multiple strategies to silence TEs and viruses to prevent them from amplifying in the genome, because the vast majority of parasite insertions are likely to be deleterious and impose a fitness cost on the rest of the genome [3,4]. Recent data accumulated from Ceanorharbditis elegans and Drosophila, strongly suggest that RNA interference represents one of the most efficient host processes for silencing transcription and uncontrolled movement of parasite DNA [5,6,7]. Even though eukaryotic genomes have developed multiple systems for silencing TEs, certain families of TEs sometimes go out of control and are able to amplify and jump throughout the chromosomes [8]. The hybrid dysgenesis (HD) syndrome, described in Drosophila melanogaster and Drosophila virilis, represents such a case, where multiple transpositions of TEs lead to harmful consequences [9,10]
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