Is the expression of sense and antisense transgenes really sufficient for artificial piRNA production?

Animals have evolved an elegant defense system against a diverse range of selfish elements such as transposons. In animal germ line cells, PIWI proteins and small RNAs associated with PIWI proteins (piRNAs) are at the heart of this defense system. piRNAs are 23-30-nt-long small RNAs that act as sequence-specific guides for PIWI proteins. PIWI proteins possess a slicer activity that is guided by piRNAs; the PIWI-piRNA complex thus silences transposon activity by cleaving transposon RNAs. At present, how de novo piRNA production occurs against a new non-self element is largely unknown. A recent study by Itou et al. using reporter transgenic mice concluded that the concomitant expression of sense and antisense RNA transcripts is sufficient for piRNA production. Our bioinformatic analysis using the same piRNA datasets, however, demonstrates that the introduction of the antisense reporter construct alone produces transgene-derived piRNAs, which is inconsistent with a part of the conclusions of Itou et al.