Genetic mechanisms of virus evolution and emergence
Allison, Andrew Brownell
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Although the emergence of a novel pathogen from an existing virus (or viruses) may be a multi-faceted process involving many interdependent viral, host, and/or environmental factors, the fundamental catalyst is genetic variation of the virus. For recombination and reassortment, both mechanisms involve the direct exchange of genetic material from two (or more) parental viruses, such that the novel virus is a genetic chimera of the parental viruses. For the recombinant or reassortant to replicate efficiently, be packaged and assembled correctly, and subsequently be competently transmitted in nature, the genetic elements derived from each parental virus must be structurally and functionally compatible in order for the novel virus to be viable. In the research on recombination in alphaviruses and reassortment in orbiviruses presented here, findings suggest this to be case, as both examples reinforce the notion that the genetic and/or structural compatibility between the parental viruses was likely a prerequisite for the emergence of the novel virus(es). In the two following examples of viral evolution involving overprinting and mutation, the emergence of the novel virus or variant was not dictated by the exchange of genes from different parental viruses, but rather by a change in existing sequence. For overprinting in rhabdoviruses, this involved the de novo synthesis of a novel protein through the use of an overlapping reading frame, thus increasing the coding capacity of the virus without incorporating any additional genetic elements. In the case of mutation in parvoviruses, the change in existing sequence allowed the new virus variant to jump species, thereby expanding the host range of the virus. In the description of the viruses presented within, genetic variability, whether through the process of change (overprinting or mutation) or exchange (recombination or reassortment), resulted in the emergence of a novel virus that was not only altered genetically, but also resulted in changes in the pathogenicity, antigenicity, ecology, and/or epidemiology of the virus relative to the parental virus(es), thereby shaping the evolutionary pathway of each novel virus.
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