The genome of the brown alga Ectocarpus siliculosus contains a series of viral DNA pieces, suggesting an ancient association with large dsDNA viruses

Abstract

BackgroundEctocarpus siliculosus virus-1 (EsV-1) is a lysogenic dsDNA virus belonging to the super family of nucleocytoplasmic large DNA viruses (NCLDV) that infect Ectocarpus siliculosus, a marine filamentous brown alga. Previous studies indicated that the viral genome is integrated into the host DNA. In order to find the integration sites of the viral genome, a genomic library from EsV-1-infected algae was screened using labelled EsV-1 DNA. Several fragments were isolated and some of them were sequenced and analyzed in detail.ResultsAnalysis revealed that the algal genome is split by a copy of viral sequences that have a high identity to EsV-1 DNA sequences. These fragments are interspersed with DNA repeats, pseudogenes and genes coding for products involved in DNA replication, integration and transposition. Some of these gene products are not encoded by EsV-1 but are present in the genome of other members of the NCLDV family. Further analysis suggests that the Ectocarpus algal genome contains traces of the integration of a large dsDNA viral genome; this genome could be the ancestor of the extant NCLDV genomes. Furthermore, several lines of evidence indicate that the EsV-1 genome might have originated in these viral DNA pieces, implying the existence of a complex integration and recombination system. A protein similar to a new class of tyrosine recombinases might be a key enzyme of this system.ConclusionOur results support the hypothesis that some dsDNA viruses are monophyletic and evolved principally through genome reduction. Moreover, we hypothesize that phaeoviruses have probably developed an original replication system.

Provirophages in the Bigelowiella genome bear testimony to past encounters with giant viruses

“…The large majority of predicted genes (63/83 = 76%) are orphans, a characteristic shared with giant virus genomes. Thus, it is likely that this large DNA stretch is the remnant of an integrated NCLDV genome similar to those previously observed in various eukaryotic genomes (19,(24)(25)(26)(27). Inserts of likely NCLDV origin identified in the moss genome were also reported as transcriptionally silent (19).…”

Amoeba/Amoebal Symbiont Genetic Transfers: Lessons from Giant Virus Neighbours

“…These genes are thought to play critical roles in the timing and physical processes of virion release from the host cells, and further research demonstrated a shift from E. huxleyi to EhV sphingolipid gene expression during the infection cycle, thus allowing viruses to regulate eukaryotic pathways necessary for their life cycle [110] . Of note, DNA fragments of another phycodnavirus, Ectocarpus siliculosus virus-I, were also described to be interspersed in the host's genome in regions containing DNA repeats, pseudogenes and genes involved in DNA replication, integration and transposition [111] .…”

Persistent virus and addiction modules: an engine of symbiosis

“…For example, Phycodnaviruses that infect multicellular brown algae do integrate their DNA. Ectocarpus siliculosus shows the world wide occurrence of the endogenized virus; EsV-1 [72]. Feldmania brown algal species, such as Emiliania Huxley, are similar.…”

“…With EsV-1, the DNA integrates as one copy and establishes a lifelong persistent infection in its host. In addition, there are distinct subgenomic loci (fragments) of EsV-1 related viral DNA integrated at various sites in the host [72]. These loci appear able to express viral encoded endonucleases and transposase.…”