Endogenized polinton-like viruses in the dinoflagellate Oxyrrhis marina uncover novel PolB fusion

Marine viruses are ubiquitous entities that impact the biology of a large fraction of prokaryotic and eukaryotic diversity. Dinoflagellates are heterotrophic, mixotrophic and photosynthetic eukaryotes known for their large and complex nuclear genomes, permanently condensed chromosomes and ability to form toxic algal blooms. We used long-read sequencing to explore the genome of the dinoflagellate <i>Oxyrrhis marina</i> and discovered a novel, endogenized ~25 kbp polinton-like virus (PLV). <i>O. marina</i> PLV (OmPLV) resides primarily in low GC content regions and exhibits a distinctive codon usage pattern, suggesting recent endogenization. OmPLV fragmentation was also observed, suggesting that this element is evolving towards a transposable element-like lifestyle. Notably, OmPLV encodes a unique ~1,500 amino acid fused DNA Helicase SF1-pPolB, representing the largest replication proteins reported so far in Preplasmiviricota. Phylogenetic analysis of the major capsid protein positions OmPLV as a novel PLV lineage related to giant viruses. The discovery of OmPLV provides crucial insight into the evolution and diversification of dsDNA viruses and serves as an important reference point for elucidating the role of endogenized viruses in expansion of dinoflagellate genomes.