piRNA function in germline development

Small non-coding RNAs bound to Argonaute (Ago) proteins have emerged as major players in epigenetic regulation of gene expression in plants and animals. The effector RNA molecules include the 21–25 nucleotide (ntd) long microRNAs (miRNAs) and the small interfering RNAs (siRNAs), which direct sequence specific target mRNA silencing. Based on phylogenetic analysis, Argonaute proteins can be subdivided into two subfamiliesAgo clade and Piwi clade. While, siRNAs and miRNAs associate with the Ago clade Argonautes, recent studies have identified a class of 24–30 ntd long Piwi-interacting RNAs (piRNAs), which bind to Piwi clade Argonautes. The piRNAs have important roles in germline development in flies, fish and mice. Here, we review recent studies bearing on the biological roles of piRNAs in the germline.