Population structure of Australian isolates of the cattle tick Rhipicephalus (Boophilus) microplus

Since its introduction in Australia, and despite decades of movement control, the cattle tick, Rhipicephalus (Boophilus) microplus, has spread widely in suitable habitat in the Northern Territory and the state of Queensland (QLD). On the east coast, its southerly distribution is limited by a tick quarantine line on the border with the state of New South Wales (NSW). Resistance to the popular acaricide amitraz emerged in the early 1980s and its distribution is rapidly increasing at the present time. This study examines the genetic structure of amitraz-resistant and susceptible populations in Queensland and the relationship of endemic populations in the state of Queensland with outbreak populations in the state of New South Wales. Ticks from paired susceptible and resistant field isolates were collected from five locations, including outbreak populations south of the quarantine line, and, following resistance testing, larvae were genotyped using 13 microsatellites. Three of the four populations located south of the tick quarantine line showed low variability (Ho 0.48; with 2.36–3.55 alleles per locus), presumably as a result of strong founder effects and genetic drift. All Queensland populations showed high variability (Ho 0.67–0.74; with 7.00–9.82 alleles per locus) yet even these populations showed evidence of past bottlenecks, a likely consequence of the use of acaricides. Reduced gene flow at distances as low as 4.2 km was indicated by significant differentiation of most populations. However, local selective effects on resistance alleles in the absence of gene flow cannot be discarded as an explanation. There was no clear pattern of differentiation by region or by resistance status.