Its a common misconception among my students that simpler hosts like bacteria or single celled eukaryotes would host simpler (ie smaller) viruses, but that is certainly not the case.

Another giant virus that infects a protist has been identified and sequenced. Like its close relative Mimivirus, this new virus called Cafeteria roenbergensis virus (CroV) has a very large genome and has many genes not typically found in viruses. Before the discovery of Mimivirus, viruses were not known to encode proteins involved in protein translation. That was a function on which viruses were totally dependent on the host. However, these giant viruses seem to have their fingers in protein translation too, showing us yet another strategy in manipulating host processes. There is also block of genes that appear to be derived from bacteria. The host species, C. roebergensis, eats bacteria, so it would be interesting to know if the bacterial genes were the result of a horizontal gene transfer event from a preferred host food.

Before CroV, all giant viruses identified infect amoebas. CroV infects C. roenbergensis, a marine protist. So what is it about protists that makes them good hosts for such big viruses? Why haven’t we found giant viruses infecting other eukaryotes?
Could the explanation lie in the still murky evolutionary origin of viruses? Another recent paper attempts to put some viruses (nucleocytoplasmic large DNA viruses, including giant viruses, poxes and herpesviruses) into the tree of life along with bacteria, archaea and eukaryotes. Using genes common to all, they showed that these viruses have a very ancient evolutionary origin, probably right around the time of the appearance of eukaryotes. Were the ancestral viruses more cell-like and over time progressively lost genes?