POPULATION GENETIC-STRUCTURE OF IXODES PACIFICUS (ACARI, IXODIDAE) USING ALLOZYMES

Genetic analysis of the population structure of the western blacklegge d tick, Ixodes pacificus Cooley & Kohls, was conducted using allozymes . This vector tick transmits the Lyme disease spirochete, Borrelia bur gdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner, in the far-west ern United States. It ranges from British Columbia to Baja California and disjunct populations are present in Oregon, Nevada, Utah, and Ariz ona. Host-seeking adult ticks were collected from vegetation across th e range of the species and were partially fed on rabbits prior to anal ysis. Twelve putative loci were resolved using starch gel electrophore sis. One locus, glucose-6-phosphate isomerase, formed an apparent nort h/south latitudinal dine and showed significant geographic structure. None of the remaining loci exhibited much genetic differentiation. Est imates of gene flow were high relative to other arthropods. Isolation- by-distance analysis suggests a recent and rapid range expansion. We c onclude that the overall lack of differentiation is due high rates of gene flow.