Mosquitoes Put the Brake on Arbovirus Evolution: Experimental Evolution Reveals Slower Mutation Accumulation in Mosquito Than Vertebrate Cells

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Abstract
Like other arthropod-borne viruses (arboviruses), mosquito-borne dengue virus (DENV) is maintained in an alternating cycle of replication in arthropod and vertebrate hosts. The trade-off hypothesis suggests that this alternation constrains DENV evolution because a fitness increase in one host usually diminishes fitness in the other. Moreover, the hypothesis predicts that releasing DENV from host alternation should facilitate adaptation. To test this prediction, DENV was serially passaged in either a single human cell line (Huh-7), a single mosquito cell line (C6/36), or in alternating passages between Huh-7 and C6/36 cells. After 10 passages, consensus mutations were identified and fitness was assayed by evaluating replication kinetics in both cell types as well as in a novel cell type (Vero) that was not utilized in any of the passage series. Viruses allowed to specialize in single host cell types exhibited fitness gains in the cell type in which they were passaged, but fitness losses in the bypassed cell type, and most alternating passages, exhibited fitness gains in both cell types. Interestingly, fitness gains were observed in the alternately passaged, cloned viruses, an observation that may be attributed to the acquisition of both host cell–specific and amphi-cell-specific adaptations or to recovery from the fitness losses due to the genetic bottleneck of biological cloning. Amino acid changes common to both passage series suggested convergent evolution to replication in cell culture via positive selection. However, intriguingly, mutations accumulated more rapidly in viruses passed in Huh-7 cells than in those passed in C6/36 cells or in alternation. These results support the hypothesis that releasing DENV from host alternation facilitates adaptation, but there is limited support for the hypothesis that such alternation necessitates a fitness trade-off. Moreover, these findings suggest that patterns of genetic evolution may differ between viruses replicating in mammalian and mosquito cells. Dengue virus (DENV) is an arthropod-borne virus of immense public health concern. Recent studies suggest that the rate of DENV evolution has accelerated, as well as the disease severity (virulence), most likely driven by the increased transmission and selection by the vector, Aedes aegypti, over the last 50 years. Despite the nearly global impact of DENV, little is known about the forces that influence its host range evolution, a deficiency confounded by the absence of a practical vertebrate model. We used vertebrate (Huh-7) and invertebrate (C6/36) cells lines as surrogate hosts for studying DENV host range evolution. Using genetically homogeneous or heterogeneous DENV for serial passages (specialization) in either cell line, or alternating passage, we demonstrated that (i) viruses allowed to specialize in single host cells exhibited fitness gains but fitness losses in the bypassed cell line; (ii) most viruses passaged in alternating cycles exhibited detectable fitness gains in both cell lines; (iii) amino acid changes common in both passage series suggested convergent evolution via positive selection; and (iv) there was limited support that DENV evolution is constrained by alternate host transmission and replication. These results provide insights into the likelihood and extent of shifts towards higher DENV virulence.