Viruses are obligate intracellular parasites of diverse hosts ranging from prokaryotes to eukaryotes belonging to all phyla. Several host species have gone extinct at different times, so must have the species-specific viruses. I was interested to know whether any estimates of the number of extinct virus species/genera have been made and whether these have been catalogued. I do not know whether ICTV gives any place to the extinct viruses in its classification scheme.
This would be extremely difficult, as viruses don't leave fossils, in the classic sense. I think the closest you could come would be to look in host DNA for signatured of integrated viruses. However, this would only work for retroviruses; you would miss out on all other types of viruses.
Hi Dr Emily Messina,
I agree it is difficult to gather evidence of extinction of viruses from paleontological studies. But recent paleoproteomics and -genomics approaches may provide important pieces of information about virus-specific proteins within the host proteome samples from pre-historic/ancient evolutionary times. It would be exciting to see developments on these lines in future.
Another point is that certain viruses (viral diseases of humans and animals) have been/are being targeted to be eradicated by human effort (vaccination and other approaches). For example, variola virus is declared eradicated from the world. But for its preservation in certain laboratories, it is going to be extinct. Some other viruses causing disease in humans and animals (e.g. rinderpest virus). And these would not be examples of host-parasite species co-extinction!
Certainly, in the course of evolution, some species disappeared, others changed under the influence of both biotic and abiotic factors. Some died out with their biological masters, others changed their owners and changed themselves. The whole picture is traced hardly ever will. According to individual molecular epidemiological studies, the time of appearance of individual species, types and subtypes can be assumed. For specific facts of the present time, for example, we know when the last strains of wild poliovirus types 2 and 3 were isolated.
Hi Dr Zadorozhna,
So, particularly RNA viruses- with high mutation rates, evolving as a 'quasi-species' have potential to diversify to the extent of becoming a 'new virus species' parasitizing new hosts. And in that case the meaning of virus extinction is blurred.
But, mutagenesis may also be made to push virus to extinction/lethal defection. An interesting article (doi:10.1371/journal.pone.0032550) describes this strategy.
Can we really have a starting point in evolutionary time for virus extinction determinable by our existing tools of investigation?
An interesting question that can (currently) only be answered by estimates. It's incorrect to say that only retroviruses leave genomic traces though. Even if you ignore bacteriophages, it's not true for eukaryotic viruses. This article is worth a read: 10.1126/sciadv.1500527 . It approaches the problem from the other perspective (where did viruses come from, not which have gone) but it illustrates how the ever-increasing sequence and structure databases will shed light on this sort of thing.
Paul
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