Summary of: West Nile Virus Experimental Evolution in vivo and the Trade-off Hypothesis

Summary of: West Nile Virus Experimental Evolution in vivo and the Trade-off Hypothesis 

By: Elizabeth Mongeon

West Nile virus (WNV) is an RNA arbovirus, transmitted by mosquitoes.  In general, RNA viruses have higher mutation rates than DNA viruses and single-host RNA viruses have higher mutation rates than arboviruses which have multiple hosts.  The trade-off hypothesis suggests the slower mutation rate of arboviruses allows the virus to replicate in both arthropod (mosquitoes) and vertebrate host (birds), rather than specialize for replication in a single host and lack or lose the ability to replicate in other hosts.  There are multiple studies that demonstrate adaptation of arboviruses to a specific host following sustained replication in that host. By specializing to one host, fitness of the virus often increases in that host, but there may be a corresponding decrease in fitness of the virus in the second host (Deardorff, Fitzpatrick, Jerzak, Shi, Kramer, and Ebel ).  

Because of high mutation rates WNV exists as genetically different but related viral particles in a host.  The variation in the mosquito host is greater than in vertebrate hosts.  Brackney correlates certain properties of mosquitoes with higher mutation rates and favor of rare genotypes, increasing the genetic diversity of WNV (Brackney, Beane, and Ebel).  Birds, unlike mosquitos, may serve to purify WNV.  WNV isolated from birds by Jerzak were less genetically diverse than WNV isolated from mosquitos, though the evidence is not conclusive (Jerzak, Bernard, Kramer, and Ebel).

Deardorf, Fitzpatrick, Jerzak, Shi, Kramer, and Ebel hypothesize that trade-offs of WNV may be due to diversifying effects of mosquitoes and restricting effects of birds.  Their study tested the impact of specialization of WNV in mosquitoes and birds on the ability of WNV to replicate in mosquitoes and birds.  The fitness of Bird-specialized WNV, mosquito-specialized WNV, alternately passed WNV (to birds and mosquitoes), and unpassed WNV were compared.  WNV was specialized by twenty consecutive passages in either mosquitoes or chicks.  The study sought to determine whether or not specialization in one host led to fitness gains and losses of WNV in the other host species.  Fitness is the ability of the virus to replicate in a host.  

When the ability of WNV to replicate in chicks was tested, the bird-specialized WNV had increased fitness compared to unpassed WNV.  Mosquito-specialized WNV had decreased fitness in birds compared to unpassed WNV.  Interestingly, alternately passed WNV had increased fitness if the previous pass was into chickens, but decreased fitness if the previous pass was into mosquitoes.  When the ability of WNV to replicate in mosquitoes  (Cx. Pipiens) was tested, replication was increased for both bird-specialized and mosquito-specialized WNV.  Alternately passed WNV did not have an effect on fitness compared to unpassed WNV in mosquitoes.  In another species of mosquito (Cx. Quinquefasciatus) there were no differences in fitness of the specialized and unpassed WNV variants.  In mosquitos, fitness of WNV was not correlated with genetic diversity of WNV.  In birds, the more diverse the WNV, the less fit the WNV to replicate in birds.  In other words, less diverse WNV variants developed in birds were more successful at replicating in birds. 

Alternating passage ending in chicks with subsequent passage into chicks shows the effect of two serial passages.  Fitness gains from two serial passages in chicks were comparable to fitness gains from twenty serial passages (bird-specialized WNC), evidence that the purifying effect of chickens is strong on WNV.  With more passages in mosquitoes (greater than 20) there may be results more consistent with the trade-off hypothesis but most evidence shows mosquitoes to be diversifying. 

The trade-off hypothesis is not supported by these results, at least not completely.  Specialization in birds does increase fitness of WNV in birds and specialization in mosquitoes does decrease fitness in birds.  However, specialization in mosquitoes may or may not increase fitness in mosquitoes and specialization in birds does not decrease fitness in mosquitoes.  Compliance with the trade-off hypothesis differs between host species and the trade-off hypothesis does not accurately predict behavior of West Nile Virus after serial transmission in one host.  

References

Brackney, Doug, Jenniger Beane, and Gregory Ebel. "RNAi Targeting of West Nile Virus in Mosquito Midguts Promotes Virus Diversification." PLoS Pathog. 5.7 (2009): n. page. Web. 27 Apr. 2012. <http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1000502>.

Deardorff, Eleanor, Kelly Fitzpatrick, Greta Jerzak, Pei-Yong Shi, Laura Kramer, and Gregory Ebel. "West Nile Virus Experimental Evolution in vivo and the Trade-off Hypothesis." PLoS Pathog. 7.11 (2011): n. page. Web. April 2012 <http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1002335>.

Jerzak, Greta, Kristen Bernard, Laura Kramer, and Gregory Ebel. "Genetic variation in West Nile virus from naturally infected mosquitoes and birds suggests quasispecies structure and strong purifying selection." J Gen Virol. 86.8 (2005): 2175-2183. Print.