piRNA-Guided CRISPR-like Immunity in Eukaryotes

Eukaryotic genomes contain virus-derived sequences called endogenous virus elements (EVEs). The majority of EVEs are related to retroviruses, which integrate into the host genome in order to replicate. Some retroviral EVEs encode a function; for example, some produce proteins that block infection by related viruses. EVEs derived from nonretroviral viruses - also recently found in many eukaryotic genomes - are more enigmatic. Here, we summarize the evidence that EVEs can act as templates to generate Piwi-interacting RNAs (piRNAs), whose canonical function is sequence-specific silencing of transposable elements (TEs) to maintain genomic integrity. We argue that EVEs may thus enable heritable, sequence-specific antiviral immune memory in eukaryotes-analogous to CRISPR-Cas immunity in prokaryotes.