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Yes - it would have to be a kind of cross between a giant virus (Mimi- or Pandora- like) and a mycoplasma. In some conditions, it could infect host cells as a virus, with complete dissociation of the infecting particle at some point after entry, then DNA replication and formation of new infectious particles. In other conditions, the infectious particle itself could become capable of growth and division. In this case, the infectious particle would have to be something like a bacterial endospore - metabolically inert itself, but capable of producing a vegetative cell under the right conditions. Of course, you would need a large genome to code for these two different life cycles, but maybe 2Mb would be enough.

While such a beast may be a theoretical possibility, I am not sure it could exist in vivo. Viral lifestyles select for rapid replication, and loss of useless genes, so a few cycles of virus-style replication might select for trimmed down genomes that had lost essential genes for the cellular replication phase. Maybe the only way this could work in nature is if the two replication cycles had to alternate. But that would also require a rather unique set of environmental conditions. (Host cells periodically becoming very rare, thus favoring a cellular lifestyle, then nutrients becoming very sparse, thus favoring viral infection of host cells). The host would have to be some kind of autotroph, I guess, and once its population had been eradicated by the viral recplication cycle, the cellular form of our hypothetical organism could then grow as a heterotroph, feeding ghoulishly on the remains of the dead autotrophic host cells until everything is used up. This kind of thing could perhaps occur in a rather isolated environment, that is only rarely re-seeded by susceptible host autotrophs.

OK, now back to grading undergraduate lab reports.....

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