Molecular and biochemical survey of acaricide resistance mechanisms in larvae from Mexican strains of the southern cattle tick, Boophilus microplus

We examined the larvae of several organophosphate and pyrethroid-resistant Mexican strains of Boophilus microplus using biochemical and molecular test s to investigate the mechanisms conferring acaricide resistance. The electr ophoretic profiles of esterase activity in protein extracts from coumaphos and permethrin-resistant strains compared to the susceptible strain reveale d distinct differences, which inhibitor studies attributed to carboxylester ases. Esterase hydrolysis assays showed significant enhancement of both tot al and permethrin hydrolysis in one pyrethroid-resistant strain, with no en hancement in two other strains with very high resistance to pyrethroids. Se quence analysis of sodium channel mRNA fragments in all pyrethroid-resistan t strains determined that they did not possess the classic kdr and super-kd r mutations known to confer pyrethroid resistance in several insect species . Using reverse transcriptase-polymerase chain reaction (RT-PCR) with degen erate primers designed from conserved regions of insect esterase amino acid sequences, a B. microplus larval cDNA fragment was isolated whose deduced amino acid sequence was significantly similar to esterases from a wide rang e of species. In Northern blot RNA analysis the cDNA hybridized to a 2.1 kb mRNA that was abundant in all resistant strains except one, in which a ver y low abundance could provide a marker for the mechanism conferring resista nce in this strain. Published by Elsevier Science Ltd.