Dynamics of West Nile virus evolution during infection of wild birds, mosquitoes, and the human brain: Unraveling the complexities of selection, drift, and fitness

RNA virus emergence can in large part be attributed to their rapid rates of evolution. Low fidelity of viral RNA polymerases (10-6 to 10-4 substitutions per nucleotide copied), coupled with rapid replication rates leads to the formation of large and genetically complex intrahost populations. Intrahost diversity provide viruses with the ability to quickly adapt to shifting fitness landscapes, either as a product of infecting new hosts or host environments. These intrahost variants can also collectively contribute to the phenotype of the population, influencing viral fitness and disease. Moreover, the dynamic nature of viral populations provides a "moving target" for antiviral defenses and severely limits our ability to develop new drugs and vaccines.;Several studies using WNV have assessed how different host types impact arbovirus population structure, revealing that viral populations are more diverse in mosquitoes compared to birds. In mosquitoes, purifying selection is weak and virus diversification is driven by the action of RNA interference, which creates an intracellular milieu that favors rare genotypes. In contrast, purifying selection in birds is strong and the innate antiviral response is suspected to be dominated by type I interferon. This cycling of genetic diversification in mosquitoes and selective constraint in birds leads to slower rates of evolution compared to many single-host viruses. The mechanisms for arbovirus adaptation, however, are incompletely understood.;We allowed WNV to replicate in wild-caught American crows, house sparrows and American robins to assess how natural selection shapes RNA virus populations in ecologically relevant hosts that differ in susceptibility to virus-induced mortality. After five sequential passages in each bird species, we examined the phenotype and population diversity of WNV through fitness competition assays and next-generation sequencing (NGS). We demonstrate that fitness gains occur in a species-specific manner, with the greatest replicative fitness gains in robin-passaged WNV and the least in WNV passaged in crows. Sequencing data revealed that intrahost WNV populations were strongly influenced by purifying selection and that the overall complexity of the viral populations was similar among passaged hosts. However, the selective pressures that control WNV populations seem to be dependent on the bird species. Specifically, crow-passaged WNV populations contained the most unique mutations and defective genomes, but the lowest average mutation frequency. Therefore, our data suggest that WNV replication in the most disease-susceptible bird species is positively associated with virus mutational tolerance, likely via complementation, and negatively associated with the strength of selection.;We sought to determine whether important enzootic (Culex tarsalis, Cx. quinquefasciatus, and Cx. pipiens) and bridge vectors (Aedes aegypti) of WNV have differential impacts on viral mutational diversity and relative fitness. Using NGS, we report high genetic diversity during WNV infection of mosquitoes, with species dependent impacts on rates of WNV evolution (~2x greater divergence within Cx. quinquefasciatus). Within mosquitoes, WNV that escaped known anatomical barriers to transmission also underwent stochastic reductions in genetic diversity that was subsequently recovered during intratissue population expansions. Cycles of genetic drift and weak purifying selection within a single mosquito infection resulted in accumulation of deleterious mutations in the virus population (i.e. mutational load). Consequently, the expectorated (i.e. transmitted) WNV had lower relative fitness in avian cells compared to input virus.;We describe a fatal case of WNV encephalitis in which we analyzed tissue obtained from specific brain regions at autopsy using NGS and immunohistochemistry. Despite similar levels of WNV replication between the cortical and subcortical regions, injury was only observed in the subcortical grey matter brain regions. In addition, expression of specific interferon-stimulated genes and WNV amino acid variation was higher in injured tissues. Analysis of WNV populations revealed no evidence for bottlenecks between tissues, indicating that the viral populations could move relatively freely among the regions studied and viral genetic diversity is more likely shaped by natural selection than genetic drift. Therefore, this observational, patient-based data suggests that neuronal injury and the strength of viral selection pressure may be associated with the level of the innate immune response; however, confirmation is needed with additional human samples and in animal models.;Taken together, these results reveal important insights into the deterministic and stochastic forces that shape WNV populations during infection of different hosts and tissues. In general, birds maintain fitness through natural selection and mosquitoes randomly shuffle the variant repertoire, decreasing relative fitness. Therefore, arboviruses cycle between levels of relative fitness as they cycle between hosts. Moreover, we can now make predictions about the evolutionary rates, fitness outcomes, and adaptive potential from specific transmission cycles. For example, WNV may evolve more rapidly while maintaining higher relative fitness from a Cx. quinquefasciatus-robin cycle than a Cx. pipiens-crow cycle, which will increase adaptive potential and facilitate emergence. (Abstract shortened by UMI.).