Population Genetics of Ixodes scapularis (Acari: Ixodidae) Based on Mitochondrial 16S and 12S Genes

Blacklegged tick, Ixodes scapularis Say, is the principal vector of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner in the eastern half of the United States. Populations exhibit extreme variation in morphology, host usage, development time, and behavior. We examined sequence variation in the 16S and 12S mitochondrial ribosomal DNA genes to determine genetic relationships among I. scapularis collections from throughout its range. Single strand conformation polymorphism analysis of 300 bp of the 16S molecule was used to identify different haplotypes and estimate their relative frequencies among 198 ticks. Eleven different haplotypes were detected. Haplotype diversity was least in northeastern collections and greatest in the southeast. The 11 haplotypes were sequenced in 24 specimens. In total, 462 bp in the 16S gene and 420 bp in the 12S gene were sequenced to reveal 66 informative sites. Phylogenetic analysis, using I. ricinus L. and I. pacificus Cooley & Kohls as outgroups, revealed 2 clades within I. scapularis. One clade was limited to the South and the other was distributed throughout the range of I. scapularis. Specimens from the Southern United States were basal in the broadly distributed clade. Random amplified polymorphic DNA by polymerase chain reaction patterns examined between members of the 2 clades provided no evidence for reproductive isolation. These patterns suggest that I. scapularis arose in the South but that a large geographic split gave rise to 2 distinct lineages. These lineages now interbreed and are partially sympatric.