Persistent virus integration into the genome of its algal host, Ectocarpus siliculosus (Phaeophyceae)

This study investigated whether Ectocarpus siliculosus virus-1 (EsV-1), a large double-stranded DNA virus infecting brown algae, persists as free episomal DNA or becomes integrated into the host genome. Researchers examined three Ectocarpus siliculosus strains: a healthy control, an infected strain producing virions, and a phenotypically normal latently infected strain. Using pulsed-field gel electrophoresis combined with PCR analysis, they extracted viral DNA sequences from high molecular mass host DNA complexes in infected algal cells. Viral DNA co-migrated with chromosomal DNA during electrophoresis, and PCR detected viral sequences at positions corresponding to host genomic DNA, not just at positions where free viral DNA would appear. The researchers ruled out mechanical trapping or concatenation of viral genomes. Results demonstrate that EsV-1 DNA is covalently integrated into the host genome as a provirus in latently infected cells. The integrated virus is transmitted vertically through meiosis as a Mendelian trait and does not impair host growth, suggesting potential applications as a transformation vector for commercially important brown algal species.

Key findings

• EsV-1 viral DNA is covalently integrated into the Ectocarpus siliculosus host genome, not present as free episomal DNA
• Integrated viral DNA co-migrates with high molecular mass chromosomal DNA during pulsed-field gel electrophoresis
• Viral sequences were detected at positions corresponding to host genomic DNA by PCR analysis in latently infected cells
• The latent integrated virus is inherited as a Mendelian trait through meiosis without reducing host growth
• The integrated provirus could potentially serve as a transformation vector for kelps and other commercially important brown algae