Comparisons of two large phaeoviral genomes and evolutionary implications

The evolution of viral genomes has recently attracted considerable attention. We compare the sequences of two large viral genomes, EsV-1 and FirrV-1, belonging to the family of phaeoviruses which infect different species of marine brown algae. Although their genomes differ substantially in size, these viruses share similar morphologies and similar latent infection cycles. In fact, sequence comparisons show that the viruses have more than 60% of their genes in common. However, the order of genes is completely different in the two genomes, suggesting that extensive recombinational events in addition to several large deletions had occurred during the separate evolutionary routes from a common ancestor. We investigated genes encoding components of signal transduction pathways and genes encoding replicative functions in more detail. We found that the two genomes possess different, although overlapping, sets of genes in both classes, suggesting that different genes from each class were lost, perhaps randomly, after the separate evolution from an ancestral genome. Random loss would also account for the fact that more than one-third of the genes in one viral genome has no counterparts in the other genome. We speculate that the ancestral genome belonged to a cellular organism that had once invaded a primordial brown algal host.