Environmentally-induced epigenetic conversion of a piRNA cluster

Abstract

Transposable element (TE) activity is repressed in animal gonads by PIWI-interacting RNAs (piRNAs), a class of small RNAs produced by specific loci made of TEs insertions and fragments. Current models propose that these loci are functionally defined by the maternal inheritance of piRNAs produced during the previous generation, raising the question of their first activation in the absence of piRNAs. Taking advantage of an inactive cluster of P-element derived transgene insertions, we show here that raising flies at high temperature (29°C) instead of 25°C results in a rare but invasive epigenetic conversion of this locus into an active piRNAs producing one. The newly acquired epigenetic state is stable over many generations even when flies are switch back to 25°C. The silencing capacities, piRNA production and chromatin modifications of the cluster are all identical whether conversion occurred by maternal piRNA inheritance or by high temperature. We also demonstrate that in addition to high temperature, a single homologous transgene inserted elsewhere in the genome is required to activate the locus.We thus have identified a minimal system of three components to create a stable piRNA producing locus: 1) a locus with multiple TE derived sequences; 2) an euchromatic copy of these sequences and 3) elevated temperature. Altogether, these data report the first case of the establishment of an active piRNA cluster by environmental changes. It highlights how such variations of species natural habitat can become heritable and shape their epigenome. SIGNIFICANCE STATEMENTRecently, we have witnessed great progress in our understanding of the silencing of Transposable Elements (TEs) by piRNAs, a class of small RNAs produced by piRNA clusters. At each generation, piRNA clusters are supposed to be activated by homologous piRNAs inherited from the mother raising the question of the making of the first piRNAs. Here, we report the birth of a stable and functional piRNA cluster induced by high temperature without maternal inheritance of homologous piRNAs. We propose a minimal system to create a piRNA cluster: a sufficient number of repeated sequences, a euchromatic copy of these sequences and an increase in the production of antisense RNA. Casier et al. -Thermic activation of a piRNA cluster 3

Maternally inherited siRNAs initiate piRNA cluster formation

“…Several studies demonstrate the critical role of cytoplasmic piRNA inheritance from the mother to the progeny in initiating piRNA production (Casier et al, 2019;de Vanssay et al, 2012;Le Thomas et al, 2014b). Together these findings suggest that piRNA biogenesis is governed by a trans-generational feed-forward loop in which piRNA biogenesis is promoted by RDC complex, which in turn is deposited on chromatin guided by cytoplasmically inherited piRNAs.…”

“…Consistent with siRNAs being dispensable for maintenance of piRNA biogenesis, a previous study showed that the siRNAs are not required for the activity of an artificial piRNA cluster (T1/BX2) (de Vanssay et al, 2012) . On the other hand, increase in the level of antisense transcripts was proposed to be linked to spontaneous activation of BX2 (Casier et al, 2019), suggesting a general role of antisense RNA (and hence the siRNA pathway) in activation of piRNA clusters. Overall, our results and the previous findings suggest a two-step model of cluster activation (Fig.…”