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West Nile Virus Experimental Evolution in vivo and the Trade ..

T1 - Culex flavivirus and West Nile virus mosquito coinfection and positive ecological association in Chicago, United States

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Transmission of West Nile Virus through Blood Transfusion in …

When competitions were conducted in chickens, serially and alternately passed WNV demonstrated clear fitness changes. Serial passage in chicks resulted in fitness increases compared to unpassed virus in the homologous host (, , unpaired t-test, P=0.0102). Conversely, after serial passage in mosquitoes WNV displayed significantly decreased replicative fitness in chicks (P=0.0056). WNV from the alternating passage series also experienced significant fitness changes. Alternating passage that concluded in chickens demonstrated fitness gains (P=0.0061) but alternating passage that concluded in mosquitoes did not (P>0.05). Unpassed WNV-WT (wild-type, unmarked WNV derived from the same clone used to generate the marked reference virus) had fitness similar to WNV-REF (). Sera and brain tissue from all chick cohorts were also collected on day 5 post-inoculation and showed no significant difference from day 2 sera (data not shown).

T1 - Test of recrudescence hypothesis for overwintering of west nile virus in gray catbirds

When competitions were conducted in Cx. pipiens, fitness changes also were observed (). Chick-passed WNV displayed significant replicative fitness increases over the unpassed marked reference virus (P=0.0056). Additionally, after 20 serial passes in mosquitoes replicative fitness increases were observed (P=0.0291). Neither the viruses from the alternate passage series nor the control WNV-WT exhibited significant changes in replicative fitness (). In contrast to the results from the competition experiments in chicks and Cx. pipiens there were no significant replicative fitness changes for any of the serially or alternately passed viruses when competed against the unpassed marked reference virus in Cx. quinquefasciatus mosquitoes (, ).

Microbiology Society Journals | A West Nile virus …

Because arboviruses replicate in both arthropod and vertebrate hosts and seem to evolve more slowly than single-host RNA viruses, it is often proposed that they “trade” optimal fitness in either host in exchange for adequate fitness in both. Tests of this “trade-off” hypothesis most often consist of releasing a virus from host alternation and allowing it to specialize on one host or the other, then comparing fitness or genetic sequence data to the unpassed or alternately passed virus , , , , . Due to the complexity of arbovirus transmission cycles, and in many cases the lack of appropriate in vivo models (for dengue virus, for example) these studies have largely been conducted in vitro in tissue culture, with inconsistent results. This lack of consistency appears to be related to differences in virus families, host species, passage regimes and approaches to measuring virus fitness. In most cases, the mechanistic basis for observed trade-offs have not been identified. Moreover, the diversity of experimental systems has made it difficult to identify the merits and defects of the trade-off hypothesis. Here, we used a completely in vivo approach to test whether or not WNV host alternation supports the trade-off hypothesis. By conducting the passage series and the competitions in relevant hosts in vivo, we sought to circumvent several of the caveats required in interpreting many previous studies. In sum, our data support the growing body of evidence that the trade-off hypothesis does not accurately predict WNV population dynamics , . Interestingly, our findings are somewhat at odds with a similar literature developing in the field of alphavirus-host interactions, which tend to support the trade-off hypothesis , , , .The reasons for this are not entirely clear, but may be related to differences in virus replication in vivo (i.e. differences in host factors required for replication or host-cell receptor utilization). Differences in replication and/or mutation rates could also impact genetic diversity or population composition resulting in fitness changes. Additional comparative studies are required to develop a complete understanding of the underlying differences between fitness trade-offs in flaviviruses compared to alphaviruses.

In light of this, we re-examined the trade-off hypothesis by determining the impact of host specialization on WNV fitness, here defined as the capacity for successful genome replication. In particular, (a) bird-specialized WNV, (b) mosquito-specialized WNV, (c) alternately passed WNV and (d) unpassed WNV, were competed against genetically marked WNV in vivo in mosquitoes and chickens. In previous studies we passed WNV exclusively in chicks or mosquitoes 20 times, or passed the virus alternately between mosquitoes and chicks a total of 20 times () , . In this study, the resulting WNV was competed against unpassed marked reference virus (WNV-REF), derived from the same clone used for passage initiation in order to determine whether host specialization leads to fitness gains and/or losses in the WNV system. Our studies, through the use of in vivo model animals for both passage and fitness determination, as well as triplicate performance of each treatment and the use of a higher passage number than typically used, provide a more representative model of the effect of host specialization on WNV fitness and the trade-off hypothesis, than has been possible with cell-culture models.

West Nile Virus Doctoral Dissertation Topics - …

Serial passage in chickens resulted in fitness gains in both chickens and Cx. pipiens mosquitoes, but fitness in a related mosquito species (Cx. quinquefasciatus) was unchanged. These results are at odds with the trade-off hypothesis because although the observed fitness increases in chickens would have been predicted, expected losses in the bypassed host (mosquitoes) were not observed. Notably, the WNV strains that had undergone sequential passage exclusively in chickens had patterns of nucleotide substitution suggesting that they were subject to strong purifying selection during replication in chickens . In addition, intrahost genetic diversity in general was very low after passage in chickens. Collectively these observations suggest that during WNV replication in chickens, high overall fitness is maintained because deleterious mutations are rapidly removed by selection.

West Nile virus (WNV, family Flaviviridae: Flavivirus) is an arthropod-borne virus (arbovirus) that has demonstrated remarkable success since being introduced to North America in 1999. Within three years after its introduction the virus had adapted to local mosquito vectors and within 8 years had become fully endemic , , . Viruses with RNA genomes, like WNV, have higher mutation rates than those of most DNA viruses due to error-prone replication . However, arboviruses seem to evolve more slowly compared to single-host RNA viruses . The trade-off hypothesis is a commonly postulated theory suggesting that this slower rate derives from the biological requirement for alternating replication in two taxonomically divergent hosts (vertebrates and arthropods). Under the trade-off hypothesis, virus fitness in both hosts is reduced in comparison to single host viruses, which can “specialize” on a single host environment [Recently reviewed by Ciota and Kramer ]. Several studies have reported that releasing arboviruses from host alternation and allowing sustained replication in a single host results in rapid adaptation to the specialized host, often with a corresponding fitness loss in the bypassed host, providing support for the trade-off hypothesis , , , .

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  • Culex flavivirus and West Nile virus mosquito …

    KW - West Nile virus

  • Virus Replication/genetics; West Nile virus/physiology* Substances

    KW - West Nile virus

  • Under the trade-off hypothesis, virus fitness ..

    hypothesis - west nile virus survivor -- wnv research -- conservancy of the phoenix inc -- pantheist nature conservancy

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How Many People Here Have Rh Negative Blood? - …

Arthropod-borne viruses, including West Nile virus (WNV) have long been considered to be subject to a fitness “trade-off” because they must replicate in at least two taxonomically divergent hosts in order to perpetuate. Results of studies testing this trade-off hypothesis have been inconsistent and largely dependent on which virus family is studied and which experimental system used. Therefore, considerable ambiguity exists in the literature regarding how host alternation influences virus population biology. Accordingly, we allowed WNV to adapt to each of its main hosts (mosquitoes and birds) for 20 sequential passes in order to determine whether host alternation in nature imparts a fitness compromise, as predicted by the trade-off hypothesis. After host-specialization, passed viruses were competed in vivo in mosquitoes and chicks against an unpassed marked reference virus to determine whether replicative fitness gains or losses had occurred. Our results demonstrate that the trade-off hypothesis accurately predicts WNV adaptation in the avian environment but not in mosquitoes. Overall, our results suggest that WNV adaptation is controlled by alternating cycles of genetic expansion in a permissive mosquito environment and restriction in avians, where purifying selection is dominant.

[Page 3] at the GodlikeProductions Conspiracy Forum

Serial passage of WNV in mosquitoes resulted in slight fitness gains in one species of mosquito (Cx. pipiens, the host in which the virus was sequentially passed), no change in a related species (Cx. quinquefasciatus), and extreme fitness losses in chickens. These findings seem to support the trade-off hypothesis. Purifying selection is relaxed in mosquitoes leading to high genetic diversity , . It therefore seems likely that much of the genetic diversity generated during mosquito infection consists of mutations that are selectively neutral in mosquitoes but are slightly or strongly deleterious in the chick environment, leading to chick-specific fitness declines. This observation is supported by our analysis of the relationship between virus fitness and the genetic diversity within the WNV test population (). Moreover, our results suggest that the mechanistic basis for the observed fitness trade-off following mosquito passage is likely related to intrahost genetic diversity and different selective environments in each host type.

Proceedings of the National Academy of Sciences

In nature, arthropod-borne viruses (arboviruses) perpetuate through alternating replication in vertebrate and invertebrate hosts. The trade-off hypothesis proposes that these viruses maintain adequate replicative fitness in two disparate hosts in exchange for superior fitness in one host. Releasing the virus from the constraints of a two-host cycle should thus facilitate adaptation to a single host. This theory has been addressed in a variety of systems, but remains poorly understood. We sought to determine the fitness implications of alternating host replication for West Nile virus (WNV) using an in vivo model system. Previously, WNV was serially or alternately passed 20 times in vivo in chicks or mosquitoes and resulting viruses were characterized genetically. In this study, these test viruses were competed in vivo in fitness assays against an unpassed marked reference virus. Fitness was assayed in chicks and in two important WNV vectors, Culex pipiens and Culex quinquefasciatus. Chick-specialized virus displayed clear fitness gains in chicks and in Cx. pipiens but not in Cx. quinquefasciatus. Cx. pipiens-specialized virus experienced reduced fitness in chicks and little change in either mosquito species. These data suggest that when fitness is measured in birds the trade-off hypothesis is supported; but in mosquitoes it is not. Overall, these results suggest that WNV evolution is driven by alternate cycles of genetic expansion in mosquitoes, where purifying selection is weak and genetic diversity generated, and restriction in birds, where purifying selection is strong.

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