Embryonic piRNAs target horizontally transferred vertebrate transposons in assassin bugs

Piwi proteins and the associated Piwi-interacting RNAs (piRNAs) coordinate a surveillance system that protects the animal genome from DNA damage induced by transposable element (TE) mobilization. While the pathway has been described in detail in the fruit fly Drosophila melanogaster, much less is known in more basal insects. Here, we investigated the adaptation of the piRNA pathway to horizontally transferred transposons (HTTs) in the assassin bug Rhodnius prolixus, a primary vector of Chagas disease. Rhodnius acquired specific classes of HTTs by feeding on bats, opossums and squirrel monkeys. By analyzing the temporal dynamics of piRNA cluster expression and piRNA production during critical stages of Rhodnius development, we show that peak levels of [~]28 nt long piRNAs correlate with reduced HTT and resident TE expression primarily during embryogenesis. Strikingly, while resident TEs piRNAs seem to engage in a typical ping-pong amplification mechanism, sense and antisense HTT piRNAs instead overlap by [~]20 nt or do not display ping-pong signatures. These features are explained at least in part by the low number of HTT copies inserted into the piRNA clusters and might point to a non-canonical mechanism of biogenesis. Our data reveal that the piRNA, but not the siRNA pathway, responded to HTTs that were recently transferred from vertebrate tetrapods to a hematophagous insect of medical relevance.