tirant, a Newly Discovered Active Endogenous Retrovirus in Drosophila simulans

Endogenous retroviruses have the ability to become permanently integrated into the genomes of their host, and they are generally transmitted vertically from parent to progeny. With the exception of gypsy, few endogenous retroviruses have been identified in insects. In this study, we describe the tirant endogenous retrovirus in a subset of Drosophila simulans natural populations. By focusing on the envelope gene, we show that the entire retroviral cycle (transcription, translation, and retrotransposition) can be completed for tirant within one population of this species. Endogenous retroviruses are genomic sequences that are widelydispersed throughout the host genome. These sequences con-stitute 8 % of the human genome and represent the remnants of ancient infections by retroviruses (1). Some of these retroviruses were domesticated and generated cellular neogenes, such as the syncytin gene (9, 14). The structure of a canonical endogenous retrovirus consists of three open reading frames (ORFs), which are bordered by long terminal repeats (LTRs). The third ORF encodes the envelope protein (Env), which possesses fusogenic properties and is responsible for the infectious behavior of exog-enous retroviruses. In insects, several endogenous retroviruse