The carboxylesterase family in salmon lice (Lepeophtheirus salmonis) and its role in drug resistance. The carboxylesterase family in salmon lice

The pyrethroid deltamethrin and the macrocyclic lactone emamectin benzoate (EMB) are used to treat infestations of farmed salmon by parasitic salmon lice, Lepeophtheirus salmonis. However, the efficacy of both compounds is threatened by resistance development. In terrestrial arthropods, pesticide resistance can be based on enhanced expression of carboxylesterases (CaEs). The CaE family is functionally diverse and comprises both catalytically competent and inactive members. The present study characterised the CaE family in L. salmonis and assessed its role in drug resistance.L. salmonis CaEs homologues were identified by homology searches in the parasite’s transcriptome and genome, and subjected to phylogenetic analyses. Potentially catalytically competent CaEs were characterised regarding their transcript expression in a drug susceptible and a multi‑resistant L. salmonis strain.The above strategy led to the identification of 21 genes/pseudogenes coding for CaEs. Thirteen sequences were assigned to clades involved in neurodevelopmental signalling and cell adhesion. Three sequences were predicted to encode secreted enzymes, including one secreted β esterase and two glutactins. Ten CaEs were identified as being potentially catalytically competent. Their transcript abundance did not differ between drug susceptible and multi-resistant L. salmonis. However, regulation of transcript expression of specific CaEs was observed following sublethal concentrations of deltamethrin and EMB.In summary, results from the present study suggest that L. salmonis possesses fewer CaE gene family members than most other arthropods characterised so far. The comparison of one multiresistant and one drug susceptible strain of L. salmonis failed to provide evidence for roles of CaEs in the resistance of sea lice to deltamethrin and EMB.