Environmentally-induced epigenetic conversion of a piRNA cluster

Transposable element (TE) activity is repressed in animal gonads by PIWI-interacting RNAs (piRNAs) produced by piRNA clusters. Current models propose that piRNA clusters are functionally defined by the maternal inheritance of piRNAs produced during the previous generation. Taking advantage of an inactive cluster of P-element derived transgene insertions in Drosophila melanogaster, we show here that raising flies at high temperature (29°C) instead of 25°C results in stable conversion of this locus into an active piRNAs producing one reporting thus the first case of the establishment of an active piRNA cluster by environmental changes and without maternal inheritance of homologous piRNAs. Overall design: Small RNAs were prepared from ovaries of Drosophila females and high-throughput sequenced using Illumina Hi-seq 2500 by Fasteris. 9 samples were processed All genotypes contain seven repeats of /P(lacW) /inserted in tandem on the second chromosome. This cluster, called BX2, can be in different epigenetic states, i.e. producing or not small RNAs called piRNAs. BX2theta and BX2* genotype piRNAs but not BX2OFF. P(TARGET) is a P(lacZ) transgene inserted in the second chromosome but at another place and used as a readout of the epigenetic state of BX2: if BX2 is ON, P(TARGET) is repressed, if BX2 is OFF, P(TARGET) is expressed.