Abstract
Arthropod-borne viruses (arboviruses) of vertebrates including dengue, zika, chikungunya, Rift Valley fever, and blue tongue viruses cause extensive morbidity and mortality in humans, agricultural animals, and wildlife across the globe. As obligate intercellular pathogens, arboviruses must be well adapted to the cellular and molecular environment of both their arthropod (invertebrate) and vertebrate hosts, which are vastly different due to hundreds of millions of years of separate evolution. Here we discuss the comparative pressures on arbovirus RNA genomes as a result of a dual host life cycle, focusing on pressures that do not alter amino acids. We summarize what is currently known about arboviral genetic composition, such as dinucleotide and codon usage, and how cyclical infection of vertebrate and invertebrate hosts results in different genetic profiles compared with single-host viruses. To serve as a comparison, we compile what is known about arthropod tRNA, dinucleotide, and codon usages and compare this with vertebrates. Additionally, we discuss the potential roles of genetic robustness in arboviral evolution and how it may vary from other viruses. Overall, both arthropod and vertebrate hosts influence the resulting genetic composition of arboviruses, but a great deal remains to be investigated.
Highlights
Viruses utilize host machinery in lieu of encoding all requirements for propagation
Many arboviruses such as chikungunya (CHIKV), West Nile (WNV), dengue (DENV), zika (ZIKV), Japanese encephalitis (JEV), Mayaro (MAYV), and Rift Valley fever (RVFV) viruses have emerged in new host species or in new geographical locations demonstrating their ability to adapt to new environments and hosts [3,7,8]
These data suggest that synonymous codon usage is important to replication of arbovirus in both vertebrate and invertebrate systems, but that the pressures placed on replication due to codon reassignments differ between these two hosts
Summary
Viruses utilize host machinery in lieu of encoding all requirements for propagation. viral propagation necessitates close interactions with host cells and fine-tuned co-evolution. As a result of the unique evolutionary pressures imposed by alternating replication in vertebrates and arthropods, trade-offs could be expected to occur to maintain arbovirus viability in both hosts, with adaptation to one host resulting in decreased fitness in the other. Results indicated that host alternation does not automatically limit the adaptive potential of arboviruses, yet consensus sequences remain strikingly consistent in nature over time Combined, these studies largely support that purifying selection is the dominant evolutionary pressure for arboviruses and suggest the mutant swarm may play an important role in their adaptive potential [49,50]. Arboviruses cyclically replicate in highly diverse environments with different evolutionary pressures present; their genomes largely remain constant yet are positioned to evolve rapidly when novel conditions arise. This suggests that known arboviruses are highly adapted to their dual host use and have evolved mechanisms to overcome the challenges associated with replication in diverse host environments
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