POPULATION-GENETICS OF IXODES-SCAPULARIS (ACARI, IXODIDAE) BASED ON MITOCHONDRIAL 16S AND 12S GENES

Blacklegged tick, Ixodes scapularis Say, is the principal vector of Bo rrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner in the eastern half of the United States. Populations exhibit extreme variat ion in morphology, host usage, development time, and behavior. We exam ined sequence variation in the 16S and 12S mitochondrial ribosomal DNA genes to determine genetic relationships among I. scapularis collecti ons 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. E leven different haplotypes were detected. Haplotype diversity was leas t in northeastern collections and greatest in the southeast. The 11 ha plotypes were sequenced in 24 specimens. In total, 462 bp in the 16S g ene and 420 bp in the 12S gene were sequenced to reveal 66 informative sites. Phylogenetic analysis, using I. ricinus L. and I. pacificus Co oley & Kohls as outgroups, revealed 2 clades within I. scapularis. One clade was limited to the South and the other was distributed througho ut the range of I. scapularis. Specimens from the Southern United Stat es were basal in the broadly distributed. clade. Random amplified poly morphic DNA by polymerase chain reaction patterns examined between mem bers 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 linea ges now interbreed and are partially sympatric.