Two classes of piRNAs separate silencing from diversity. Based on the limited number of piRNAs in a single cell (Fig. 1) and the skewed distribution of their sequence abundance (Fig. 2), only the topmost abundant piRNA sequences can be present in every cell. These commonly detected sequences originate from a few top-ranked piRNA-generating regions and exhibit a strong preference for uridine at their 5′-most position (1U) (Figs. 3, 4). The 1U-preference of the Zuc-processor complex modulates the abundance of individual piRNAs (Fig. 3). Abundance correlates with function (Figs. 3E,F, 5). Based on their sequence abundance, piRNAs can be divided into functional classes. A few topmost abundant sequences dominate piRNA silencing (silencers). A biological threshold (red dotted line) separates these piRNAs from the bulk of low abundant sequences that cannot be present in every cell. Rare piRNA establishes cell-to-cell diversity. On convergent targets, these piRNAs could act as Modifiers and promote reproductive polymorphism. In isolation, extremely low abundant piRNAs might never act. However, over time, these highly diverse sporadic sequences could serve as a resource for evolutionary tinkering and bolster adaptation to novel genomic invaders. The functional sequence space of piRNAs is concise and can be regulated to ensure careful self/nonself discrimination.
