Fig. 1

Different ways to control TE expression. a piRNAs. piRNAs are produced from piRNA clusters, genomic spots where new TEs can integrate. piRNAs can act through two mechanisms. In the nucleus, piRNAs bind to Piwi proteins. They also bind in anti-sense to TE mRNA being transcribed, triggering histone methylation of TEs and thus inhibiting recruitment of Pol II. This leads to the silencing of TE expression. In the cytoplasm, piRNAs bind to other Argonaute proteins, triggering TE mRNA degradation. b Repressor proteins. A functional P element produces the transposase that triggers its excision and transposition. When repressor proteins are transmitted from the mother through cytoplasm or when the P element is degenerated, it produces an alternatively spliced mRNA. This mRNA encodes a non-functional transposase that will act as a repressor by competing with the functional transposase, and trigger the production of more alternatively spliced mRNA. This positive repression loop, where the repressor protein activates its own production, prevents the transposition of the TE. c DNA methylation. The TE is methylated, preventing its expression