A hypothetical gene flow framework representing the observed distribution of hallmark small DNA tumor virus-related genes observed in opisthokont datasets. Colored lines indicate at least one inferred transfer event for the indicated gene involving at least one member of each virus group. The line does not imply that all members of the group received or maintained the indicated gene. In some cases, the line may represent multiple inferred horizontal gene transfer events occurring at different times. Italics indicate viruses predicted to have circular genomes. Rep: superfamily 3 ATP-dependent replicative DNA helicase with N-terminal nicking endonuclease-like domain. Familiar parvovirus, polyomavirus, and papillomavirus Rep genes are known as NS1, LT, and E1, respectively. Penton: pentameric single β- jellyroll capsid protein. Familiar parvovirus, polyomavirus, and papillomavirus genes of this class are known as VP, VP1, and L1, respectively. We speculate that a subset of parvovirus and circular Rep-encoding single-stranded DNA virus (cressvirus) capsid genes may represent captured midsize eukaryotic linear DNA virus (meldvirus) Penton homologs. Macc: hypothetical membrane-active capsid core protein. Some Macc classes have predicted holin-like hydrophobic domains. Others are predicted to have lipase activity. Adenovirus proteins VI and X are canonical examples of this category. Cah: hypothetical capsid-surface protein with predicted alpha helical (particularly coiled-coil) character. Adenovirus protein IX is a canonical example of this class. A typical hit for candidate Cah proteins in HHpred searches is the reovirus outer capsid protein sigma-1. The adomavirus gene initially designated LO4 encodes a protein that qualifies as a Cah. Hexon: double β-jellyroll major capsid protein that trimerizes to form virion facets. Adenain: papain-like cysteine protease involved in virion maturation. PacK: FtsK/HerA-class P-loop ATPase proteins thought to facilitate viral genome packaging. PolB: viral type B DNA polymerase. Integrase: retrovirus-like (rve) integrase. Prim: homolog of archaeal-eukaryotic primase small catalytic subunits. RT: reverse transcriptase.

Candidate cressviruses encoding papillomavirus-like replicase proteins (RepE1) paired with different classes of inferred capsid proteins. Putatively circular genomes are linearized for display purposes. Hatch marks indicate one kilobase.

A parvovirus-like sequence encoding an exotic HelPol DNA polymerase.

Examples of adomaviruses with three different classes of replicase and three different classes of inferred virion proteins. The circular genome is linearized for display purposes. Hatch marks indicate one kilobase.

Tailtomavirus detected in a sample of seawater from off the coast of Tierra del Fuego. Dali analysis of AlphaFold-predicted structures suggests tailspike-like and single-jellyroll-like virion proteins unified with a polyomavirus-like replicase protein. The inferred circular genome is linearized for display purposes. Hatch marks indicate one kilobase.

An adabscovirus detected in datasets representing Betta fighting fish. The inferred circular genome is linearized for display purposes. Hatch marks indicate one kilobase.

Meldvirus genomes with mixed similarities to the group traditionally referred to as polinton-like viruses. Meld19810 unites a DNA primase-superfamily 2 helicase (Prim-S2H) and tyrosine recombinase (Yrec) with Alpha-class adintovirus virion proteins. Meld23486 unites Beta-class adintovirus-like type B DNA polymerase (PolBb) and retrovirus-like Integrase sequences with virion protein genes that appear to be distantly related to poxvirus virion protein genes. For example, AlphaFold-predicted structures for Meld23486 Hexonrif show a Dali Z-score of 7 for 6BEI (D13 rifampicin resistance major capsid protein, a poxvirus Hexon homolog).

A Cah-class polyomavirus detected in a NASA clean room dataset. The hatched teal box indicates a predicted alpha helix-rich (Cah) domain.