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 tests to investigate the mechanisms conferring acaricide resistance. The electrophoretic profiles of esterase activity in protein extracts from coumaphos and permethrin-resistant strains compared to the susceptible strain revealed distinct differences, which inhibitor studies attributed to carboxylesterases. Esterase hydrolysis assays showed significant enhancement of both total and permethrin hydrolysis in one pyrethroid-resistant strain, with no enhancement in two other strains with very high resistance to pyrethroids. Sequence analysis of sodium channel mRNA fragments in all pyrethroid-resistant strains determined that they did not possess the classic kdr and super-kdr mutations known to confer pyrethroid resistance in several insect species. Using reverse transcriptase-polymerase chain reaction (RT-PCR) with degenerate 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 range 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 very low abundance could provide a marker for the mechanism conferring resistance in this strain.