Experimental Evolution of West Nile Virus at Higher Temperatures Facilitates Broad Adaptation and Increased Genetic Diversity.

Rachel L Fay,Graham G Willsey,Kiet A Ngo,Lili Kuo,Laura D Kramer,Alexander T Ciota

doi:10.3390/v13101889

Abstract

West Nile virus (WNV, Flaviviridae, Flavivirus) is a mosquito-borne flavivirus introduced to North America in 1999. Since 1999, the Earth’s average temperature has increased by 0.6 °C. Mosquitoes are ectothermic organisms, reliant on environmental heat sources. Temperature impacts vector–virus interactions which directly influence arbovirus transmission. RNA viral replication is highly error-prone and increasing temperature could further increase replication rates, mutation frequencies, and evolutionary rates. The impact of temperature on arbovirus evolutionary trajectories and fitness landscapes has yet to be sufficiently studied. To investigate how temperature impacts the rate and extent of WNV evolution in mosquito cells, WNV was experimentally passaged 12 times in Culex tarsalis cells, at 25 °C and 30 °C. Full-genome deep sequencing was used to compare genetic signatures during passage, and replicative fitness was evaluated before and after passage at each temperature. Our results suggest adaptive potential at both temperatures, with unique temperature-dependent and lineage-specific genetic signatures. Further, higher temperature passage was associated with significantly increased replicative fitness at both temperatures and increases in nonsynonymous mutations. Together, these data indicate that if similar selective pressures exist in natural systems, increases in temperature could accelerate emergence of high-fitness strains with greater phenotypic plasticity.

Highlights

Flavivirus is a genus of single stranded, positive sense, enveloped RNA viruses which result in at least 50–100 million infections annually [1]
A significantly higher viral load was measured for West Nile virus (WNV) passaged at 30 ◦ C compared to 25 ◦ C (* p ≤ 0.05 one-way analysis of variance (ANOVA) w/multiple comparisons, Tukey’s post-test)
Genome to pfu ratios fluctuated during passage, with values ranging from 6.0 × 102 –6.0 × 103 copies/pfu, yet overall trends for infectious particles were similar to those measured for WNV genomes (Figure 2B)

Summary

Introduction

Flavivirus is a genus of single stranded, positive sense, enveloped RNA viruses which result in at least 50–100 million infections annually [1]. Several mosquito-borne flaviviruses, including West Nile virus (WNV), Zika virus (ZIKV), and dengue virus (DENV), cause emerging infectious diseases. WNV emerged in the western hemisphere in 1999 and was responsible for an outbreak of neuroinvasive disease. In the following five years, WNV became endemic throughout the U.S WNV is the most geographically widespread flavivirus and has caused the largest outbreak of neuroinvasive illness in history [2,3]. Since its emergence in 1999, there have been approximately 52,000 WN human cases diagnosed in the U.S, resulting in over 2300 fatalities [4]. WNV lineage 2 has risen in prevalence in Europe, with an increasing frequency of outbreaks in recent years [5]

Methods

Results

Conclusion