Three esterase genes from the tick Boophilus microplus: Comparison between organophosphate susceptible and resistant strains

Different approaches were used to find differences among susceptible and acaricide resistant strains of the southern cattle tick Boophilus microplus. The use of Random-amplified polymorphic DNA showed different patterns of DNA fragments on agarose gel electrophoresis. The unique DNA fragments may be useful for developing probes that can detect acaricide resistance in field populations. A second approach using a strategy based on degenerate primers derived from acetylcholinesterase (AChE) from other species was used to obtain three different esterase nucleotide sequences of cDNA preparations from the southern cattle tick B. microplus. One deduced amino acid sequence had a high homology with conserved amino acids present in the AChE family. The other two cDNA sequences were each identified as carboxyl esterase B. No differences were found in the putative AChE and one of the carboxyl esterase genes when nucleotide sequences were compared between susceptible and organophosphorus resistant strains. Two point mutations were detected in the second carboxyl esterase sequence. One of the mutations leads to an amino acid substitution. The three genes showed different patterns in Southern blot analysis when specific probes derived from each sequence were hybridized with digested genomic DNA. The results indicated polymorphism between susceptible and resistant populations. The point mutation present in the esterase gene was used to design a PCR-based assay to test different populations of the southern cattle tick. The wild type and the mutant alleles were present in all of the six strains tested, except that a pyrethroid resistant strain showed only mutant genotype. The test was also used on single larvae ticks to determine the allelic frequency of mutants within a population. The frequency of mutant allele ranged from 12% in a susceptible strain to 100% in a pyrethroid resistant strain. The results suggest that the mutation may be associated with resistance of B. microplus ticks to pyrethroid compounds.