Mitochondrial DNA sequence variation in Ixodes pacificus (Acari : Ixodidae)

The western black-legged tick, Ixodes pacificus, is a primary vector of the spirochaete, Borrelia burgdorferi, that causes Lyme disease. We used varia tion in a 355-bp DNA portion of the mitochondrial cytochrome oxidase III ge ne to assess the population structure of the tick across its range from Bri tish Columbia to southern California and east to Utah. Ixodes pacificus sho wed considerable haplotype diversity despite low nucleotide diversity. Maxi mum parsimony and isolation-by-distance analyses revealed little genetic st ructure except between a geographically isolated Utah locality and all othe r localities. Loss of mtDNA polymorphism in Utah ticks is consistent with a post-Pleistocene founder event. The pattern of genetic differentiation in the continuous part of the range of Ixodes pacificus reinforces recent reco gnition of the difficulties involved in using genetic frequency data to inf er gene flow and migration.