What flies can and cant teach us about piRNAs
In animals, PIWI-interacting RNAs (piRNAs) serve as guides for the PIWI clade of Argonaute proteins, an evolutionary conserved family of RNA-binding proteins at the heart of eukaryotic gene silencing pathways. Flies teach us that piRNAs protect the germline from transposons and repetitive sequences, ensuring genomic stability during the production of gametes. Transposon silencing appears to be the ancestral function of piRNAs, and transposon-derived piRNAs are found in either the female or male germ cells of arthropods and mammals. While many, perhaps most, steps in the piRNA pathway are conserved among metazoa, the peculiarities of the Dipteran suborder, Brachycera, make flies unrepresentative of the diversity of functions of piRNAs in other arthropods. Moreover, in the mammalian testis, piRNAs play a critical role in coordinating the expression of large numbers of mRNAs essential for the production of functional sperm. This mammalian-specific “pachytene” piRNA pathway plays little if any role in repressing transposons. Thus, just as the ancient RNAi pathway gave rise to the gene-regulating miRNA pathway, so too the ancestral transposon-silencing piRNA pathway spawned a new class of gene-regulating small RNAs, the mammalian pachytene piRNAs.
RNA Therapeutics Institute, University of Massachusetts Medical School, and Howard Hughes Medical Institute