Somatic transposition in the Drosophila intestine occurs in active chromatin and is associated with tumor suppressor gene inactivation

Transposable elements (TEs) play a significant role in evolution by contributing to genetic variation through germline insertional activity. However, how TEs act in somatic cells and tissues is not well understood. Here, we address the prevalence of transposition in a somatic tissue, exploiting the Drosophila midgut as a model system. Using whole-genome sequencing of in vivo clonally expanded gut tissue, we map hundreds of high-confidence somatic TE integration sites genome-wide. We show that somatic retrotransposon insertions are associated with inactivation of the tumor suppressor Notch, likely contributing to neoplasia formation. Moreover, by applying Oxford Nanopore long-read sequencing technology, as well as by mapping germline TE activity, we provide evidence suggesting tissue-specific differences in retrotransposition. By comparing somatic TE insertional activity with transcriptomic and small RNA sequencing data, we demonstrate that transposon mobility cannot be simply predicted by whole tissue TE expression levels or by small RNA pathway activity. Finally, we reveal that somatic TE insertions in the adult fly intestine are found preferentially in genic regions and open, transcriptionally active chromatin. Together, our findings provide clear evidence of ongoing somatic transposition in Drosophila and delineate previously unknown underlying features of somatic TE mobility in vivo.