First Evidence of Dicistroviruses Infecting Protists

Molecular surveys suggest that RNA viruses are abundant and diverse in the ocean, but the hosts for most of these viruses are unknown because so few have been cultivated. Here, we present the genomes of five positive-sense, single-stranded RNA (+ssRNA) viruses isolated from tropical seawater that infect green algae in the genus Tetraselmis (order Chlorodendrales). Phylogenetic analyses of multiple genes placed these closely related viruses within the family Dicistroviridae (order Picornavirales) making them the first viruses within the bounds of the Dicistroviridae family demonstrated to infect an organism other than arthropods. The RNA-dependent and capsid gene sequences of the Tetraselmis RNA viruses (TetRNAV01-05) cluster with others recovered from diverse environmental water samples or aquatic invertebrate tissues, and together they form a strongly supported sister clade to those of viruses in the genus Triatovirus. The TetRNAVs harbor a unique intergenic internal ribosome entry site (IRES), suggesting a translation strategy distinct from that described in arthropod-infecting dicistroviruses. Our results suggest that many uncultivated viruses presumed to infect invertebrates, because they were detected in invertebrate-derived samples and their genes cluster within the family Dicistroviridae, may instead be protist-infecting viruses.