Vertical transmission of RNA molecules in Caenorhabditis nematodes

In organisms composed of a single cell, nucleic acids of viral origin may be transmitted directly to the daughter cells at cell division, without ever passing through a virion stage outside of the cell. These nucleic acids usually encode a RNA dependent RNA polymerase (RdRP) that enables their replication. For some of these agents, such as Narnaviruses, no capsid protein is expressed, and thus they are referred to as capsidless viruses. By contrast, other viruses, such as the family Totiviridae, encode a capsid protein in addition to the RdRP. Here we identify putative capsidless viral RNAs in animals for the first time - in nematodes closely related to the model laboratory organism C. elegans. We show that these RNAs are transmitted vertically through the germline of these animals. The hosts appear deficient in some molecular pathways of defense against double-stranded RNAs. Our work provides the first evidence that animal cells harbour capsidless viruses.