title: Host alternation is necessary to maintain the genome stability of rift valley fever virus.
abstract: BACKGROUND
  Most arthropod-borne viruses (arboviruses) are RNA viruses, which are maintained in nature by replication cycles that alternate between arthropod and vertebrate hosts. Arboviruses appear to experience lower rates of evolution than RNA viruses that replicate in a single host. This genetic stability is assumed to result from a fitness trade-off imposed by host alternation, which constrains arbovirus genome evolution. To test this hypothesis, we used Rift Valley fever virus (RVFV), an arbovirus that can be transmitted either directly (between vertebrates during the manipulation of infected tissues, and between mosquitoes by vertical transmission) or indirectly (from one vertebrate to another by mosquito-borne transmission).
  
  
  RESULTS
  RVFV was serially passaged in BHK21 (hamster) or Aag2 (Aedes aegypti) cells, or in alternation between the two cell types. After 30 passages, these single host-passaged viruses lost their virulence and induced protective effects against a challenge with a virulent virus. Large deletions in the NSs gene that encodes the virulence factor were detectable from the 15(th) serial passage onwards in BHK21 cells and from the 10(th) passage in Aag2 cells. The phosphoprotein NSs is not essential to viral replication allowing clones carrying deletions in NSs to predominate as they replicate slightly more rapidly. No genetic changes were found in viruses that were passaged alternately between arthropod and vertebrate cells. Furthermore, alternating passaged viruses presenting complete NSs gene remained virulent after 30 passages.
  
  
  CONCLUSIONS
  Our results strongly support the view that alternating replication is necessary to maintain the virulence factor carried by the NSs phosphoprotein.
authors.affiliation: Molecular Genetics of Bunyavirus, Department of Virology, Institut Pasteur, Paris, France.
authors.forename: Sara
authors.initials: S
authors.lastname: Moutailler
authors.forename: Benjamin
authors.initials: B
authors.lastname: Roche
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Thiberge
authors.forename: Valérie
authors.initials: V
authors.lastname: Caro
authors.forename: François
authors.initials: F
authors.lastname: Rougeon
authors.forename: Anna-Bella
authors.initials: AB
authors.lastname: Failloux
mesh_terms: D000330:Aedes / Q000821:virology; D000818:Animals; D002460:Cell Line; D006224:Cricetinae; D004195:Disease Models, Animal; D019143:Evolution, Molecular; D042822:Genomic Instability; D051379:Mice; D012295:Rift Valley Fever / Q000473:pathology / Q000821:virology; D012296:Rift Valley fever virus / Q000235:genetics* / Q000472:pathogenicity*; D012376:Rodent Diseases / Q000473:pathology / Q000821:virology; D017384:Sequence Deletion; D014774:Virulence; D014776:Virus Cultivation