PIWI protein-interacting RNA pathway – an adaptor for the exaptation of transposable elements

Abstract

Transposable elements or transposons (TEs) are abundant genome components in almost all multicellular organisms. Their activity is double-sided: they could be harmful and destructive for the genes and genomes, and yet could carry beneficial information (new regulatory motifs, new lncRNA and protein-coding genes, etc.) The host response to this double-sided nature of TEs is the evolution of various systems for TE silencing and control at DNA, transcriptional and post-transcriptional level. TE control systems neutralize the harmful effects of TEs and help the beneficial traits of TE sequences to be integrated into the host genome and become part of cellular signaling or regulatory systems. So the co-evolution between TEs and TE control systems could be considered the main road to TE exaptation and functionalization. The main focus of this review is the complex interplay between TEs and PIWI/piRNA system, an ancient regulatory system in animals, active mostly in the germline and early embryo. The roles of TEs, PIWI/piRNAs, and their interactions are discussed in more detail in two fields that together are crucial for evolution itself: animal reproduction and stress response.