All have both common and unique features of genomic DNA and virion structure. It remains uncertain whether the similarities of different families of this grouping of NCLDV are due to independent recruitment of the same or related host genes during viral replication or whether they simply share a common viral ancestor, although the latter theory is controversial and under heavy scrutiny.
It is probable that these viruses evolved before the separation of eukaryotes into the extant crown groups. The ancestral genome was complex with at least 41 genes, including: (1) the replication machinery; (2) up to four RNA polymerase subunits; (3) at least three transcription factors; (4) capping and polyadenylation enzymes; (5) the DNA packaging apparatus; (6) structural components of an icosahedral capsid and the viral membrane.
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^Colson P, De Lamballerie X, Yutin N, Asgari S, Bigot Y, Bideshi DK, Cheng XW, Federici BA, Van Etten JL, Koonin EV, La Scola B, Raoult D. (2013) "Megavirales", a proposed new order for eukaryotic nucleocytoplasmic large DNA viruses. Arch Virol. 158:2517-2521. doi: 10.1007/s00705-013-1768-6. PubMed PMID 23812617.