Towards a consensus: Multiple experiments provide evidence for constitutive expression differences among sexes and populations of sea lice (Lepeophtheirus salmonis) related to emamectin benzoate resistance

Sea lice (Lepeophtheirus salmonis) are ectoparasitic copepods that impose a heavy financial burden on the salmon aquaculture industry. A parasiticide, emamectin benzoate (EMB; trade name SLICE (R)), waswidely used to control sea lice before EMB resistant strains of lice emerged. Several genetic mechanisms are likely responsible for EMB resistance however these are not yet fully understood. Resistance is further complicated by sex differences in EMB tolerance with males often showing better survival upon EMB exposure compared to females. Here, candidate EMB-resistance genes were used to explore differences between sex, population, and exposure to EMB using two in vitro bioassays and an in vivo experiment. Two acetylcholine receptor subunits (nAChR alpha 3 and nAChR alpha 7) showed opposite expression profiles across the assays, with male and EMB-resistant lice showing significant overexpression of nAChR alpha 7 and downregulation of nAChR alpha 3 compared to females and EMB-sensitive lice, respectively. Furthermore, a novel gene candidate LR9 showed induced expression upon EMB exposure with the highest expressing group being EMB-resistant males. An ABC transporter, pgp, also showed highest expression in EMB-resistant males but this finding was not consistent across all experiments. Other gene candidates like CYP18A and peroxinectin did not show similar expression profiles to work completed on other populations or species of sea lice. These data have provided a consensus with other transcriptomic studies showing that neuronal acetylcholine receptor subunits are differentially regulated between sexes and populations of sea lice. This unique expression profile, alongside analysis of other EMB resistance genes, provides a detailed snapshot of mechanisms responsible for resistance in sea lice. Statement of relevance: Sea lice are a major threat to salmon aquaculture and its' sustainability. With the advent of drug resistance, this problem has only become more troubling. The mechanisms of resistance are largely unknown, with a requirement on consensus findings across meta-populations of lice due to varying levels of resistance within them. This project shows consensus findings across multiple experiments for both previously explored targets as well as new, potential resistance markers. (C) 2015 Elsevier B.V. All rights reserved.