Certain Aquatic Eukaryotes Harbor an OLD-Like Immune Defense System

Key elements of eukaryotic antiviral immunity are evolutionarily conserved with prokaryotic anti-phage defense systems. The Overcoming Lysogenization Defect (OLD) anti-phage defense system is well characterized in prokaryotes but has remained unknown in eukaryotes. Here, the old-like genes are identified in certain aquatic eukaryotes, including the SAR supergroup, Filasterea, Chytridiomycota, and some metazoan lineages (Placozoa, Cnidaria, Spiralia, Hemichordata, and Cephalochordata). Within molluscs, old-like genes are present in bivalves but absent in gastropods and cephalopods. Interestingly, the filasterean Capsaspora owczarzaki, an endosymbiont of the gastropod Biomphalaria glabrata, encodes putative secreted OLD-like proteins, suggesting it may provide its host with symbiont-mediated antiviral protection. Genomic and transcriptomic analyses revealed that old-like genes vary in copy and intron number and are expressed in some eukaryotic lineages. OLD-like proteins retain key structural features but exhibit a different topology from that of prokaryotes. Structural predictions reveal homodimeric dsDNA-bound architectures analogous to that of the Bacillus cereus VD045 GajA OLD homodimer. Furthermore, phylogenetic analysis divided OLD-like proteins into three distinct clusters, one of which likely represents an ancient inheritance from the ancestral Promethearchaeota (formerly Asgard archaea) host. Together, these findings highlight an unexpected presence of old-like genes in certain aquatic eukaryotes, offering new insights into their evolutionary history and potential role in antiviral defense.