House dust mites use a plant-like siRNA pathway to silence transposable elements

Evidence from the past decade has established that RNA interference (RNAi) plays versatile and important roles in regulating gene expression in all domains of life [1, 2]. There are three major classes of small RNAs, including microRNAs (miRNAs), short interfering RNAs (siR-NAs), and Piwi-interacting RNAs (piRNAs). miRNAs primarily regulate functional genes and play important roles in development and other processes, while siRNAs and piRNAs primarily regulate nonfunctional transcripts, serving as defense mechanisms against transposable elements (TEs) and viruses [3]. Plants do not have piRNAs and use siRNAs made by Dicer to silence TEs [4]. Animals usually have siRNAs and piRNAs, both of which can be used to silence TEs. E.g., in vertebrates and flies, piRNAs play major roles in silencing TEs [5]. Caenorhabditis elegans uses secondary siRNAs made by RNA-dependent RNA polymerases (RdRP)-not by Dicer-to silence TEs [6]. However, at least in the Tc3 transposon, a conserved family of DNA transposons, secondary siRNAs are initiated by piRNAs in C. elegans [7]. Regardless, piRNAs in animals are either directly or indirectly involved in silencing TEs. In this issue, Mondal and Flynt reported house dust mites do not possess any piRNA pathway [8]. To maintain genome stability, they use Dicer-dependent siRNAs to silence TEs. This finding puts house dust mites in a special position in the evolution of small RNA pathways, bearing a plant-like system for silencing TEs in an animal system. Indeed, the size of siRNAs in house dust mites is similar to that made by plant Dicer-like 3 (DCL3) [9], which is also involved in silencing transposons. This research further diversifies the functions of animal small RNAs and demonstrates a unified TE-regulation mechanism in plants and animals.