Talmudic Question #90

by Welkin Johnson

Is it conceivable that, in evolution, a virus could switch from one genome type and replication style to another?

Comments

[barry]

are there viruses that have alternations of generations? alternate hosts like invertebrate parasites? i don't believe i am so ignorant about viruses. argghh!

[Siobain]

Eugene Koonin put forth the idea that the dsRNA viruses evolved from +ssRNA viruses twenty years ago.
Koonin EV: Evolution of double-stranded RNA viruses: a case for polyphyletic origin from different groups of positive-stranded RNA viruses. Semin Virol 1992, 3:327-339.

Gibbs and Weiler argued that recombination between a ssDNA circovirus and a ssRNA calicivirus led to the common ancestor of ssDNA nanoviruses.
Gibbs, M. (1999). Evidence that a plant virus switched hosts to infect a vertebrate and then recombined with a vertebrate-infecting virus Proceedings of the National Academy of Sciences, 96 (14), 8022-8027 DOI: 10.1073/pnas.96.14.8022

But I think the question as posed is a little too vague. Are "genome types" strictly the Baltimore types? What about the evolution of segmented genomes from monopartite ancestors?

And what did you intend by "replication style?" It can't mean strictly vertical vs strictly horizontal replication (because of the clear ties between viruses and capsid-free transposons). So is this meant molecularly -- the enzyme that catalyzes the reaction changes, from an RNA polymerase to a Reverse transcriptase, for instance? Or that the replication switches from stamping machine to exponential replication?

Without implying there is a common ancestor for large swaths of viruses, I do think that not all distinct groups of viruses necessarily had independent origins. Some viruses have likely evolved from viral ancestors with different genomic architectures. Their evolutionary potential (mutation, recombination rates and often high population sizes) have allowed them to explore far more possibilities than other replicating biological entities.

[Johan Laserna]

(What if all these wonderful SMT talmudic questions could be turned into an equal number of podcasts, TWIM/P/V style? With special guests for each question. Open ended, deep drilling ruminations. I think someone has to clone Elio to bring that about. The world urgently needs more copies of him!)

Elio (blushingly) replies:
There are those who think that one is aplenty. But thanks for the kind and soft words.

Elio

[Christoph Weigel]

Yes, I could imagine a lysogenic phage like Mu integrating close to or within a resident Lambda prophage. Induction/Excision of either prophage would certainly create a mess in most cases but could eventually result in a viable (?) hybrid.

[Nathan Myers]

I assume this means switching between RNA and DNA propagation?

It seems inevitable. I'm guessing it's been much more common to go from RNA to DNA than back, because DNA is more tolerant of mutations that might (to use the technical term) snarl a continuous, connected RNA expression. (Do any RNA viruses travel as fragments?) It seems much more likely in phages because bacterial RNA and DNA seem to slosh around together more freely.

If it does happen, then I would expect to see some lineages that switch routinely, using both modes simultaneously but favoring one over the other according to circumstance. The DNA form might correspond to a more spore-like existence, supporting longer periods between infection, while the RNA form accelerates local epidemics.