piRNA-Independent Transposon Silencing by The Drosophila THO Complex

piRNAs guide Piwi/Panoramix-dependent H3K9me3 chromatin modification and transposon silencing in the Drosophila germline.  THO is a heteropenameric RNA export complex composed of Hpr1, Tho2, and Thoc5-7.  Null thoc7 mutations displace Thoc5 and 6 from a Tho2-Hpr1 sub-complex, reduce expression of a subset ofgermline piRNAs, and increase steady state expression of a subset of transposon families, suggesting that THO silences transposons by promoting piRNA biogenesis.  Here we show that thoc7 mutations increase transposon transcription, but do not reduce anti-sense piRNAs targeting half of the transcriptionally activated elements, disrupt piRNA-guided H3K9me3 chromatin modification, or block Panoramix-dependent silencing of a reporter transgene.  In addition, unspliced transposon transcripts co-precipitate with THO through a Piwi- and Panoramix-independent mechanism, and piwi mutations dominantly enhance germline defects associated with thoc7 null alleles.  We therefore propose that THO silences transposons through a piRNA-independent mechanism, which functions cooperatively with piRNAs to support germline development.