Primer: Giant viruses of protists.

Giant viruses infecting protists have expanded our understanding of viral diversity. These members of the phylum Nucleocytoviricota possess genomes that can exceed 2.5 Mb and encode hundreds to thousands of proteins, including functions once thought unique to cellular organisms. They infect amoebae, algae, and other unicellular eukaryotes, forming large factories and interacting with virophages and mobile genetic elements. Their virions display diverse architectures, from icosahedral particles to amphora-shaped pandoraviruses and tail-bearing tupanviruses. Comparative genomics highlights evolutionary processes such as horizontal gene transfer, gene duplication, de novo gene birth, and genome rearrangements, which contribute to their genomic diversity. Despite their widespread presence in aquatic and terrestrial ecosystems, many aspects of their biology remain unknown, including the roles of lineage-specific ORFan genes (open reading frames with no detectable homologs in current databases) and the molecular strategies used to manipulate hosts. This Primer provides an accessible overview of protist-infecting giant viruses, summarizing their classification, virion architecture, replication strategies, and evolutionary scenarios, and points to key questions that remain open.