Mechanism behind Resistance against the Organophosphate Azamethiphos in Salmon Lice (<i>Lepeophtheirus salmonis</i>)

<div><p>Acetylcholinesterase (AChE) is the primary target for organophosphates (OP). Several mutations have been reported in AChE to be associated with the reduced sensitivity against OP in various arthropods. However, to the best of our knowledge, no such reports are available for <i>Lepeophtheirus salmonis</i>. Hence, in the present study, we aimed to determine the association of AChE(s) gene(s) with resistance against OP. We screened the AChE genes (<i>L</i>. <i>salmonis ace1a</i> and <i>ace1b</i>) in two salmon lice populations: one sensitive (n=5) and the other resistant (n=5) for azamethiphos, a commonly used OP in salmon farming. The screening led to the identification of a missense mutation <i>Phe362Tyr</i> in <i>L</i>. <i>salmonis ace1a</i>, (corresponding to <i>Phe331</i> in <i>Torpedo californica</i> AChE) in all the samples of the resistant population. We confirmed the potential role of the mutation, with reduced sensitivity against azamethiphos in <i>L</i>. <i>salmonis</i>, by screening for <i>Phe362Tyr</i> in 2 sensitive and 5 resistant strains. The significantly higher frequency of the mutant allele (<i>362Tyr</i>) in the resistant strains clearly indicated the possible association of <i>Phe362Tyr </i>mutation in <i>L</i>. <i>salmonis ace1a</i> with resistance towards azamethiphos. The 3D modelling, short term survival experiments and enzymatic assays further supported the imperative role of <i>Phe362Tyr </i>in reduced sensitivity of <i>L</i>. <i>salmonis</i> for azamethiphos. Based on all these observations, the present study, for the first time, presents the mechanism of resistance in <i>L</i>. <i>salmonis </i>against azamethiphos. In addition, we developed a rapid diagnostic tool for the high throughput screening of <i>Phe362Tyr</i> mutation using High Resolution Melt analysis.</p></div>