Virus-Host Co-speciation: SFV

Nature recently had an interesting article about parasite-host co-speciation. Simian foamy viruses are retroviruses that are prevalent among non-human primates. Although they establish a persistant infection, they have not been associated with any disease and no naturally occuring SFVs have been isolated from humans yet.

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Switzer et al looked at the polymerase gene of 44 different monkey and ape SFVs and a mitochondrial gene from 55 primate species. These genes are highly conserved, so they don't change much over time. Using the sequences of these two genes, the researchers built independent phylogenetic trees. The resulting trees gave similar results both in grouping and in divergence times. In other words, the SFV evolution matched the host evolution in terms of who split off from who, and when. There were some incongruities that suggested cross-species transfer of virus. Overall they had statistical support for 22 co-speciation events.

Here is the abstract:

Although parasite-host co-speciation is a long-held hypothesis, convincing evidence for long-term co-speciation remains elusive, largely because of small numbers of hosts and parasites studied and uncertainty over rates of evolutionary change. Co-speciation is especially rare in RNA viruses, in which cross-species transfer is the dominant mode of evolution. Simian foamy viruses (SFVs) are ubiquitous, non-pathogenic retroviruses that infect all primates. Here we test the co-speciation hypothesis in SFVs and their primate hosts by comparing the phylogenies of SFV polymerase and mitochondrial cytochrome oxidase subunit II from African and Asian monkeys and apes. The phylogenetic trees were remarkably congruent in both branching order and divergence times, strongly supporting co-speciation. Molecular clock calibrations revealed an extremely low rate of SFV evolution, 1.7 x 10(-8) substitutions per site per year, making it the slowest-evolving RNA virus documented so far. These results indicate that SFVs might have co-speciated with Old World primates for at least 30 million years, making them the oldest known vertebrate RNA viruses.

I think that co-speciation is one of the more fascinating and supportive aspects of evolutionary theory. There are also foamy viruses that infect other mammals (eg. cats). I will leave it to the reader to predict what we should find if those viruses and their hosts were included in the above analysis.