Maternal ovo represses the expression of transposable elements in adult ovaries

In germ cells, repressing transposable elements (TEs) is important to maintain genomic integrity. TE expression and transposition are repressed by PIWI-interacting RNAs (piRNAs). Although many genes for piRNA synthesis have been described, few transcription factors activating their expression have been identified. We previously reported that a transcription factor, maternal Ovo (Ovo-B) protein activates germline-specific gene expression in progenitors of germ cells. In this study, we found that maternal Ovo also activates several genes, including aubergine (aub), for TE silencing. Knocking down maternal Ovo de-repressed TEs in adult ovaries. In addition, embryonic knockdown of aub caused de-repression of TEs in adult Drosophila ovaries. Surprisingly, embryonic knockdown of maternal Ovo affected neither expression of ovo nor its downstream TE-silencing genes in adult ovaries after growth. These results strongly suggest that maternal Ovo is required for TE silencing in ovaries, via transcriptional activation of genes for piRNA synthesis in embryos.