Comparison of two DNA viruses infecting the marine brown algae Ectocarpus siliculosus and E. fasciculatus

The marine brown algal genus Ectocarpus contains two species, E. siliculosus and E. fasciculatus. Field populations of both species include plants with infection symptoms caused by DNA viruses. We have established clonal cultures from infected and normal host plants and investigated the properties of the endogenous viruses. Both host species contain virus particles with a hexagonal cross-section and a diameter of ca. 150 nm. The genomes of both virus types consist of double-stranded DNA, approximately 320 kb in size. Restriction digestion with Sfil revealed differences between the two virus genomes. However, PCR experiments suggest that at least one gene, which encodes a major capsid protein, is quite similar in both virus species. In cross-infection experiments the E. siliculosus virus did not initiate an infection cycle in E. fasciculatus. In contrast, the E. fasciculatus virus infected E. siliculosus zoospores. The resulting plants showed aberrant symptoms and produced virus particles which were not infectious. We conclude that the two Ectocarpus species are hosts for different, but closely related viruses.