Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 23, 1999
Publication Date: N/A
Interpretive Summary: The southern cattle tick was eradicated from the U.S. through a campaign that began in 1906 and ended about 50 years later. Unfortunately, the ticks are abundant in Mexico and a quarantine in South Texas along the Mexican border currently prevents their re-entry into this country. However, widespread resistance in Mexico to the pesticides available for controlling the southern cattle tick is threatening the efficacy of procedures currently approved for decontamination of imported cattle. The objective of this research was to use molecular and biochemical techniques to survey several Mexican strains of resistant ticks to identify the physiological mechanisms conferring resistance to pyrethroid and organophosphate pesticides. The results provided strong evidence implicating the existence of specific mechanisms involving increased activity of carboxylesterase proteins that degrade pesticides. Several of these proteins were targeted for further analysis. A fragment of the molecular code for synthesizing an esterase protein was also identified and characterized for possible use in creating a molecular probe to detect resistant ticks. A further accomplishment of this research was the development of a biochemical test to quantitatively measure the ability of different tick strains to degrade the pesticide, permethrin. Future work will identify the specific genes and proteins involved in resistance and lead to the development of rapid, specific diagnostic tests to identify and measure the distribution of resistance mechanisms present in southern cattle tick populations.
We examined several organophosphate and pyrethroid resistant Mexican strains of Boophilus microplus using biochemical and molecular tests to investigate the mechanisms conferring acaricide resistance. Electrophoretic profiles of esterase activity in larval extracts from coumaphos and permethrin resistant strains revealed distinct differences compared to the susceptible strain which inhibitor studies found to be attributable to carboxylesterases. Permethrin hydrolysis assays indicated significantly elevated hydrolytic activity in one pyrethroid resistant strain (Cz), while two other strains (Co and SF) with high resistance to pyrethroids and cross resistance to DDT showed no enhancement of permethrin hydrolysis relative to the susceptible strain. Sequence analysis of the sodium channel mRNA in all strains determined that they did not possess the classic kdr and super-kdr mutations known to confer pyrethroid resistance in several insect species. Using 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 shared significant similarity with 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 the Cz strain.