title: Multiple mutations in the para-sodium channel gene are associated with pyrethroid resistance in Rhipicephalus microplus from the United States and Mexico.
abstract: BACKGROUND
  Acaricide resistant Rhipicephalus microplus populations have become a major problem for many cattle producing areas of the world. Pyrethroid resistance in arthropods is typically associated with mutations in domains I, II, III, and IV of voltage-gated sodium channel genes. In R. microplus, known resistance mutations include a domain II change (C190A) in populations from Australia, Africa, and South America and a domain III mutation (T2134A) that only occurs in Mexico and the U.S.
  
  
  METHODS
  We investigated pyrethroid resistance in cattle fever ticks from Texas and Mexico by estimating resistance levels in field-collected ticks using larval packet discriminating dose (DD) assays and identifying single nucleotide polymorphisms (SNPs) in the para-sodium channel gene that associated with resistance. We then developed qPCR assays for three SNPs and screened a larger set of 1,488 R. microplus ticks, representing 77 field collections and four laboratory strains, for SNP frequency.
  
  
  RESULTS
  We detected resistance SNPs in 21 of 68 U.S. field collections and six of nine Mexico field collections. We expected to identify the domain III SNP (T2134A) at a high frequency; however, we only found it in three U.S. collections. A much more common SNP in the U.S. (detected in 19 of 21 field collections) was the C190A domain II mutation, which has never before been reported from North America. We also discovered a novel domain II SNP (T170C) in ten U.S. and two Mexico field collections. The T170C transition mutation has previously been associated with extreme levels of resistance (super-knockdown resistance) in insects. We found a significant correlation (r = 0.81) between the proportion of individuals in field collections that carried any two resistance SNPs and the percent survivorship of F1 larvae from these collections in DD assays. This relationship is accurately predicted by a simple linear regression model (R2 = 0.6635).
  
  
  CONCLUSIONS
  These findings demonstrate that multiple mutations in the para-sodium channel gene independently associate with pyrethroid resistance in R. microplus ticks, which is likely a consequence of human-induced selection.
authors.forename: Nathan E
authors.initials: NE
authors.lastname: Stone
authors.forename: Pia U
authors.initials: PU
authors.lastname: Olafson
authors.forename: Ronald B
authors.initials: RB
authors.lastname: Davey
authors.forename: Greta
authors.initials: G
authors.lastname: Buckmeier
authors.forename: Deanna
authors.initials: D
authors.lastname: Bodine
authors.forename: Lindsay C
authors.initials: LC
authors.lastname: Sidak-Loftis
authors.forename: John R
authors.initials: JR
authors.lastname: Giles
authors.forename: Roberta
authors.initials: R
authors.lastname: Duhaime
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Miller
authors.forename: Juan
authors.initials: J
authors.lastname: Mosqueda
authors.forename: Glen A
authors.initials: GA
authors.lastname: Scoles
authors.forename: David M
authors.initials: DM
authors.lastname: Wagner
authors.affiliation: Center for Microbial Genetics and Genomics, Northern Arizona University, 1298 S Knoles Drive, Flagstaff 86011-4073, AZ, USA. Joseph.Busch@nau.edu.
authors.forename: Joseph D
authors.initials: JD
authors.lastname: Busch
mesh_terms: D000818:Animals; D001483:Base Sequence; D002417:Cattle; D002418:Cattle Diseases / Q000469:parasitology* / Q000517:prevention & control; D005260:Female; D005838:Genotype; D007305:Insecticide Resistance / Q000235:genetics; D007306:Insecticides; D007814:Larva; D016014:Linear Models; D008800:Mexico; D008969:Molecular Sequence Data; D009154:Mutation; D010641:Phenotype; D020641:Polymorphism, Single Nucleotide; D011722:Pyrethrins; D048494:Rhipicephalus / Q000235:genetics*; D017422:Sequence Analysis, DNA; D015222:Sodium Channels / Q000235:genetics*; D013984:Tick Infestations / Q000517:prevention & control / Q000662:veterinary*; D014481:United States
chemical_list: D007306:Insecticides; D011722:Pyrethrins; D015222:Sodium Channels