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UBC Theses and Dissertations

Characterization of viruses causing lysis of a toxic bloom-forming alga, Heterosigma akashiwo (Raphidophyseae) Tai, Vera


Two viruses, HaRNAV and HaNIV, infecting the toxic bloom-forming alga Heterosigma akashiwo (Hada) Hada ex Hada et Chihara were characterized. HaRNAV is a 25 nm, single-stranded RNA virus with a genome size of approximately 9100 nucleotides. The virus particle has a lipid component as indicated by its sensitivity to chloroform and contains at least 5 structural proteins ranging in apparent size from 24 to 34 kDa. This is the first report of a single-stranded RNA virus infecting a phytoplankton. HaNIV is a 50 nm, double-stranded DNA virus with a 38.5 kb genome. HaNIV particles are also sensitive to chloroform and the capsid contains a major 38 kDa protein and a less abundant 20.5 kDa protein. HaRNAV and HaNIV caused lysis of a different range of strains, but both predominantly caused lysis of strains from the Northeast Pacific. Because different viruses infected different host strains, the complexity of virus-host interactions in the environment is demonstrated. Over 90 % of the HaNIV genome was sequenced and is represented as two contigs, 17 765 and 15 858 base pairs in length. Open reading frames identified from the sequences had similarities with terminase, lysozyme, recombination, and structural proteins of bacteriophages from the Myoviridae, Podoviridae, and Siphoviridae. The homology of HaNIV ORFs with proteins from diverse bacteriophages suggests that the evolution of HaNIV involved numerous horizontal gene transfer events. HaNIV was also shown to be latent. Latency may be an important survival strategy for phytoplankton viruses. Irrefutable evidence proving that the host of HaNIV particles is Heterosigma akashiwo and not a bacterium, however, is imperative before the ecological and evolutionary implications can be determined.

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