A comprehensive RNA virome from molluscan transcriptomes reveals extensive diversity and modular genome evolution

Mollusca, the second-largest animal phylum, includes many aquaculture species used as important food resources for humans. While DNA viruses that threaten molluscan aquaculture have received much attention, molluscan RNA viromes are still understudied. Here, using a multi-stage RdRP discovery pipeline combining six-frame translation, profile-based homology search, and phylogenetic validation, 203 RNA viruses spanning five viral phyla and fifteen viral orders were identified, based on 223 molluscan metatranscriptomes covering eight classes. Phylogenetic analysis, combined with structural modeling, revealed a significant increase in the number of Pisuviricota-related lineages. Extensive modular evolution in viral genomes was observed, including gene rearrangements and co-evolution of capsid and RdRP genes. Host prediction linked 76% of the RNA viruses to a range of eukaryotes. These findings expand the diversity of RNA viruses associated with molluscs and shed light on their phylogenetic relationships, highlighting that molluscs might serve as an important reservoir of diverse RNA viruses infecting a range of eukaryotic hosts.