Transposable elements (TE), defined as mobile genomic DNA sequences, are ubiquitously present in all living organisms studied to date. In these recipient organisms, throughout extensive evolutionary periods, hosts and TEs have rather developed specific interactions to facilitate their coexistence and thereby prevent their mutual extinction. In the germline, specific defense pathways are activated by the host to counteract TE expression, ensuring that TE transposition remains low to preserve the host genome integrity and its proper transmission to the next generation. Nevertheless, TEs abundance suggests that they have certainly adapt their modes of expression/integration upon genome colonization, to insure their proper maintenance and propagation.

To explore further these mechanisms, we constructed a Drosophila melanogaster line in which the mobility of different TE families can be induced due to the ovarian somatic relief of the piRNA pathway (Barckmann et al. 2018, https://doi.org/10.1093/nar/gky761). Thanks to long-read DNA sequencing and a bio-informatic pipeline established in the laboratory (TrEMOLO, https://github.com/DrosophilaGenomeEvolution/TrEMOLO), we precisely determined novel integration sites for five TE families belonging to the endogenous retrovirus group. Interestingly, two of them (ZAM and gtwin) exhibit a dynamic choice of their landing sites. Indeed, we showed that these TEs, to limit competition, have specific expression patterns and integration sites within the host genome. Additionally, we established that their timing of integration during embryogenesis varies.

Unraveling the diverse host-TE interactions that enable this coexistence is a pivotal area of research, that provides valuable insights into the biology of both the host and the genomic parasite like TE.