Direct cell-to-cell transmission of retrotransposons.

Transposable elements are abundant in host genomes but are generally considered to be confined to the cell in which they are expressed, with the notable exception of endogenous retroviruses. Here, we identify a group of long terminal repeat (LTR) retrotransposons that infect the germline from somatic cells within the Drosophila ovary, despite lacking the fusogenic Envelope protein typically required for retroviral entry. Instead, these elements encode a short transmembrane protein, sORF2, which bears structural features reminiscent of viral cell-cell fusogens. Through genetics, imaging, and electron microscopy, we show that sORF2 localizes to invasive somatic protrusions that contain retroviral capsids and establish physical contact with the oocyte membrane. In the absence of sORF2, protrusion formation and soma-to-germline transmission of capsids are abolished. Remarkably, sORF2-like proteins are widespread among insect retrotransposons and also occur in piscine nackednaviruses and avian picornaviruses. These findings reveal a noncanonical, Envelope-independent transmission mechanism shared by retrotransposons and non-enveloped viruses.