DsRNA sequencing revealed a previously missed terminal sequence of a +ssRNA virus that infects dinoflagellate Heterocapsa circularisquama.

Heterocapsa circularisquama RNA virus (HcRNAV) is the only dinoflagellate-infecting RNA virus cultured. However, only two strains of HcRNAV have been registered with complete genome sequences (strains 34 and 109 for UA and CY types, respectively). To extend the genomic information of HcRNAV, we performed full-genome sequencing of an unsequenced strain of HcRNAV (strain A8) using the fragmented and primer-ligated double-stranded RNA (dsRNA) sequencing (FLDS) method. The complete genome of HcRNAV A8 with 4457 nucleotides (nt) was successfully determined, and sequence alignment of the major capsid protein gene suggested that A8 was a UA-type strain, consistent with its intraspecific host specificity. The complete sequence was found to be 80 nt longer at the 5' terminus than the registered sequences of HcRNAV strains (34 and 109), suggesting that FLDS is more reliable for determining the terminal sequence than conventional methods (5' Rapid Amplification of cDNA End). Our study contributes to a better understanding of dinoflagellate-infecting viruses with limited sequence data.