Molecular mechanism of synthetic pyrethroid and organophosphate resistance in field isolates of Rhipicephalus microplus tick collected from a northern state of India.

The frequently used chemical control method to manage Rhipicephalus microplus is limited by the emergence of resistance populations. Understanding of resistance mechanisms is essential to develop strategy for sustainable management. The present study was focused on working out the molecular mechanisms of resistance against synthetic pyrethroids (SPs) and organophosphates (OPs) in field isolates of R. microplus collected from six districts of Uttar Pradesh, India. Adult immersion test with discriminating concentrations (AIT-DC) was used to determine resistance status of isolates to SPs (deltamethrin, cypermethrin) and OPs (diazinon, coumaphos). All the six isolates were found resistant to SPs with resistance factor (RF) of 2.9-58.6 and to one of the OP compounds, diazinon having RF of 3.5-13.7 but susceptible to coumaphos (RF < 1.4). Three R. microplus genes, viz. para-sodium channel domain II S4-5 linker, carboxylesterase (372 bp) and acetylcholinesterase 2 (1692 bp) were sequenced and compared with respective sequences of reference susceptible IVRI-I, reference OP resistant population (IVRI-III), IVRI-IV and multi-acaricide resistant population (IVRI-V) of R. microplus. A C190A mutation in the domain II S4-5 linker region of sodium channel gene leading to L64I amino acid substitution was detected in all six isolates. The G1120A mutation in the carboxylesterase gene could not be detected in any isolate. Five nucleotide substitutions viz., G138A, G889A, T1090A, C1234T and G1403A were identified in the acetylcholinesterase 2 gene leading to four amino acid substitutions. The findings of the study corroborate the role of mutation in sodium channel and acetylcholinesterase 2 genes in SP and OP resistance in this part of India.