Virus resistance in the toxic bloom-forming dinoflagellate Heterocapsa circularisquama to single-stranded RNA virus infection

HcRNAV is the only known cultured dinoflagellate-infecting RNA virus. Lysis of its host dinoflagellate Heterocapsa circularisquama caused by HcRNAV is followed by apparent cell regrowth. Here we investigate the mechanism supporting the survival phenomenon. The proportion of normal cells with intact nucleus decreased to ～8% by 3 days post infection, and then, increased to > 90% at 15 days post infection. There were abnormal cells lacking an intact nucleus, and this was followed by propagation of virus-resistant survivor cells. The proportion of HcRNAV-resistant cells in three different subcultures and temporal fluctuations were compared: a clonal H. circularisquama culture without virus inoculation (virus-sensitive, VS), a surviving isolate from the HcRNAV-inoculated Culture-VS incubated in autoclaved medium (virus-resistant, VR) and a portion of Culture-VR incubated with HcRNAV (VR incubated with virus, VR + V). The proportion of HcRNAV-resistant cells in Culture-VS was 0% and in Culture-VR + V was > 94% during the experiment; and Culture-VR fluctuated from 4% to 71%. Hence, the virus resistance was assumed to be reversible. Using Northern hybridization, viral genome accumulation was not detected in Culture-VR + V cells either inoculated with HcRNAV or transfected with HcRNAV-genome; thus, intracellular viral RNA replication was assumed to be interrupted in the virus-resistant cells.