Genomic analyses of Symbiomonas scintillans show no evidence for endosymbiotic bacteria but does reveal the presence of giant viruses

• DOI: 10.1371/journal.pgen.1011218
• Publication Date: 2024-04-01T17:48:56Z
• Authors: Anna Cho; Gordon Lax; Samuel J. Livingston; Yumiko Masukagami; Mariia Naumova; Olivia Millar; Filip Husnik; Patrick J. Keeling
• Keywords: Eukaryote; Endosymbiosis; Genome size

Symbiomonas scintillans Guillou et Chrétiennot-Dinet, 1999 is a tiny (1.4 μm) heterotrophic microbial eukaryote. The genus was named based on the presence of endosymbiotic bacteria in its endoplasmic reticulum, however, like most such endosymbionts neither identity nor functional association with host were known. We generated both amplification-free shotgun metagenomics and whole genome amplification sequencing data from S . strains RCC257 RCC24, but unable to detect any sequences known lineages bacteria. absence endobacteria further verified FISH analyses. Instead, numerous contigs assemblies RCC24 closely related prasinoviruses infecting green algae Ostreococcus lucimarinus , Bathycoccus prasinos Micromonas pusilla (OlV, BpV, MpV, respectively). Using BpV as reference, we assembled near-complete 190 kbp draft encoding all hallmark prasinovirus genes, well two additional incomplete distinct viruses RCC257, three similar viral genomes which collectively call SsVs. A multi-gene tree showed SsV types branched within highly supported clades each BpV2, OlVs, MpVs, respectively. Interestingly, transmission electron microscopy also revealed nm virus-like particle morphology size endosymbiont originally reported Overall, conclude that currently does not harbour an bacterium, associated giant viruses.