Evidence for genetic hybridization between Ixodes scapularis and Ixodes cookei

Ixodes scapularis Say, 1821, the black-legged tick, is becoming established in Canada. The northwards expansion of I. scapularis leads to contact between I. scapularis and I. cookei Packard, 1869, a well-established tick species in Eastern Canada. Examination of I. cookei and I. scapularis collected from New Brunswick revealed ticks with ambiguous morphologies, with either a mixture or intermediate traits typical of I. scapularis and I. cookei, including in characteristics typically used as species identifiers. Genetic analysis to determine if these ticks represent hybrids revealed that 4 had I. cookei-derived mitochondrial DNA but I. scapularis nuclear DNA. In one case the nuclear sequence showed evidence of heterozygosity for I. scapularis and I. cookei sequences, while in the others, the nuclear DNA appeared to be entirely I. scapularis-derived. These data strongly suggest genetic hybridization between these two species. I. cookei and hybrid ticks were readily collected from humans and companion animals and specimens infected with Borrelia burgdorferi, the causative agent of Lyme disease, were identified. These findings raise the issue of genetic introgression of I. scapularis genes into I. cookei and warrant re-assessment of the capacity of I. cookei and I. cookei x I. scapularis hybrids to vector Borrelia infection.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author