Abstract
The goal of the present study was to assess the gene expression of xenobiotic metabolizing enzymes (XMEs) Cytochrome P-450 (CYP) and carboxylesterase (CE) related to detoxification of synthetic pyrethroids, plus acetylcholinesterase (AChE), in field isolates of acaricide-resistant Rhipicephalus microplus. The XMEs expression levels were assessed by mRNA measurement using quantitative reverse transcription PCR. The XME expression levels of field-isolated acaricide-resistant ticks were compared against acaricide-susceptible reference ticks used in this study as a gene expression baseline and represented as relative expression units (REU). Field isolates were subjected to toxicological bioassays and determined resistant to all the Pyr acaricides (Pyr), whereas most of them were found susceptible to organophosphorous acaricides (OP), with the exception of three isolates, which exhibited moderate resistance to Diazinon. Significantly higher levels of CYP were detected in pyrethroid-resistance ticks when compared to Su ticks (P<0.01). A linear regression analysis showed that pyrethroid acaricide resistance levels of R. microplus were proportional to the CYP expression levels (correlation coefficient (R):0.85; P<0.05). Analysis on CE expression levels showed only one isolate resistant to Pyr and OP with a statistically significant increase (P<0.01). AChE expression levels showed statistically significant (P<0.01) subexpression in all tick isolates when compared to the susceptible reference. Our results suggest that pyrethroid acaricide resistance in the cattle tick may be diagnosed by measuring the CYP expression levels using quantitative PCR.
Highlights
Pesticide resistance in arthropods is a multifactorial phenomenon involving behavioral, biochemical, and metabolic mechanisms designed to counteract the effect of the pesticide on the target organism [1, 2]
The ability of arthropods to exhibit resistance against pesticides is due to underlying genetic mechanisms that may involve the expression of one or several xenobiotic metabolizing enzymes (XMEs) genes [35], which allow for increased enzymatic detoxification [32]
Our results show that the expression patterns of these genes correlate with the toxicological profiles of the ticks, which strongly suggest a relationship between the XMEs gene expression and the acaricide resistance levels (Table 1, Figure 1)
Summary
Pesticide resistance in arthropods is a multifactorial phenomenon involving behavioral, biochemical, and metabolic mechanisms designed to counteract the effect of the pesticide on the target organism [1, 2]. The cytochrome P-450 monooxygenase (CYP) enzymatic family is a set of ubiquitous enzymes that participate in the regulation of endogenous bioactive molecules, such as hormones; they control the metabolism and detoxification of cell damaging chemicals such as plant toxins, drugs, and pesticides in a large variety of arthropods [5]. Previous reports suggest that Pyr acaricide toxicity is neutralized in R. microplus through the metabolic action of CYP [14]. Like AChE, CE enhances pesticide detoxification in several pesticide-resistant arthropods via ester-hydrolysis and metabolism of xenobiotic compound [20,21,22]; both enzymes share a common affinity for several synthetic substrates [23]. Further studies have shown that this CE gene expression and enzymatic activity are increased in Mexican pyrethroid-resistant strains of R. microplus ticks [29, 30] and a correlation between CE activity and cattle tick acaricide resistance [29,30,31,32]; a study on Australian R. microplus ticks suggested that altered AChE and CE may work simultaneously during OP and Pyr acaricide resistance [20]
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