Widespread Expression of piRNA-like Molecules in Somatic Tissues

Piwi-interacting RNA (piRNA) are small RNA abundant in the germline across animal species. In fruit flies and mice, piRNA were implicated in maintenance of genomic integrity by transposable elements silencing. Outside of the germline, piRNA have only been found in Drosophila ovarian follicle cells. Previous studies further reported the presence of multiple piRNA-like small RNA (pilRNA) in fly heads and a small number of pilRNA in mouse tissues and human NK cells. Here, by analyzing high-throughput small RNA sequencing data in more than 130 fruit fly, mouse and rhesus macaque samples, we show widespread presence of pilRNA displaying all known characteristics of piRNA in multiple somatic tissues of these three species. In mouse pancreas and macaque epididymis, pilRNA abundance was compatible with piRNA abundance in the germline. Using in situ hybridizations, we further demonstrate pilRNA co-localization with mRNA expression of Piwi-family genes in all macaque tissues. These findings indicate that piRNA-like molecules might play important roles outside of the germline. Overall design: Small RNA expression profiles were examined in five rhesus macaque tissues (testis, epididymis, prostate, seminal vesicles, cortex) and one mouse tissue (epididymis).