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The phylogenomics of evolving virus virulence

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  1. skybrian
    (edited )
    People often talk about viruses evolving to be less virulent but it seems a better answer is “it depends on the situation.” From the article: The article goes on to talk about how genetic...

    People often talk about viruses evolving to be less virulent but it seems a better answer is “it depends on the situation.”

    From the article:

    A commonly stated idea is that there is often an evolutionary trade-off between virulence and transmissibility because intra-host virus replication is necessary to facilitate inter-host transmission but may also lead to disease, and it is impossible for natural selection to optimize all traits simultaneously. In the case of MYXV, this trade-off is thought to lead to ‘intermediate’ virulence grades being selectively advantageous: higher virulence may mean that the rabbit host dies before inter-host transmission, whereas lower virulence is selected against because it does not increase virus transmission rates. A similar trade-off model has been proposed to explain the evolution of HIV virulence. However, many doubts have been raised about the general applicability of the trade-off model, virus fitness will be affected by traits other than virulence and transmissibility, contrary results have been observed in experimental studies and relatively little is known about evolutionary trade-offs in nature. For example, in the case of the second virus released as a biocontrol against European rabbits in Australia — rabbit haemorrhagic disease virus (RHDV) — there is evidence that virulence has increased through time, probably because virus transmission often occurs through blow flies that feed on animal carcasses, making host death selectively favourable. Similarly, experimental studies of plant RNA viruses have shown that high virulence does not necessarily impede host adaptation and, in the case of malaria, higher virulence was shown to provide the Plasmodium parasites with a competitive advantage within hosts.

    Other factors in addition to evolutionary trade-offs can shape the level of virulence in an emerging virus. For example, ‘short-sighted’ virulence evolution within a single host may be detrimental for inter-host transmission, and newly emerged ‘spillover’ infections that have experienced only a limited number of transmission events are likely to have virulence levels that have not yet been optimized for transmissibility by natural selection. Accordingly, for spillover infections, ongoing transmission may be largely at the mercy of random drift effects, including the severe population bottlenecks that routinely accompany such events. Finally, it is possible that virulence may sometimes simply be a coincidental by-product of selection for another trait or selection for transmission in another species.

    Theory therefore tells us that natural selection can increase or decrease pathogen virulence, depending on the particular combination between host, virus and environment. Although providing a useful framework, theory can provide only useful generalities because the relevant factors vary substantially and need to be assessed on a case-by-case basis. Virulence evolution could, however, be better understood if its genomic basis were known.

    The article goes on to talk about how genetic sequencing might be used to figure out what’s really happening and gives examples where that’s been done, including cases where viruses evolved to be more virulent.

    4 votes