Developmental retrotransposon activation primes host immunity for future viral-clearance

Transposons are thought to be largely suppressed under physiological conditions, ensuring that their mobilization is a rare event. By tracking mobilization, we show that during metamorphosis at the Drosophila pupal stage, the Gypsy retrotransposon selectively mobilizes in regenerating tissues. In the newly formed tissues, this wave of Gypsy activation primes the hosts innate immune system by inducing the production of antimicrobial peptides (AMPs). Moreover, early immune-priming functions of Gypsy are essential for combating viral invasion in adult flies: flies with Gypsy being silenced at the pupal stage are unable to clear viruses and succumb to viral infection. Our data reveal that regulated activation of transposons during animal developmental endows a long-term benefit in pathogen warfare.