Abstract
The pyrethroid deltamethrin and the macrocyclic lactone emamectin benzoate (EMB) are used to treat infestations of farmed salmon by parasitic salmon lice, Lepeophtheirus salmonis. While the efficacy of both compounds against Atlantic populations of the parasite has decreased as a result of the evolution of resistance, the molecular mechanisms of drug resistance in L. salmonis are currently not fully understood. The functionally diverse carboxylesterases (CaE) family includes members involved in pesticide resistance phenotypes of terrestrial arthropods. The present study had the objective to characterize the CaE family in L. salmonis and assess its role in drug resistance. L. salmonis CaE homologues were identified by homology searches in the parasite's transcriptome and genome. The transcript expression of CaEs predicted to be catalytically competent was studied using quantitative reverse-transcription PCR in drug susceptible and multi-resistant L. salmonis. The above strategy led to the identification of 21 CaEs genes/pseudogenes. Phylogenetic analyses assigned 13 CaEs to clades involved in neurodevelopmental signaling and cell adhesion, while three sequences were predicted to encode secreted enzymes. Ten CaEs were identified as being potentially catalytically competent. Transcript expression of acetylcholinesterase (ace1b) was significantly increased in multi-resistant lice compared to drug-susceptible L. salmonis, with transcript abundance further increased in preadult-II females following EMB exposure. In summary, results from the present study demonstrate that L. salmonis possesses fewer CaE gene family members than most arthropods characterized so far. Drug resistance in L. salmonis was associated with overexpression of ace1b.
Highlights
Sea lice of the family Caligidae (Copepoda) are ectoparasites of marine fish that feed on the mucus, skin, and blood of their hosts (Boxaspen, 2006)
L. salmonis carbox ylesterases (CaEs) were identified by homology searches in a reference transcriptome (EBI ENA reference ERS237607) and a genome assembly (LSalAtl2s, metazoan.ensembl.org) of the species
L. salmonis CaEs were subjected to phylogenetic analyses together with CaE sequences of D. melanogaster and A. mellifera (Fig. 1)
Summary
Sea lice of the family Caligidae (Copepoda) are ectoparasites of marine fish that feed on the mucus, skin, and blood of their hosts (Boxaspen, 2006). Depending on the severity of infections, sea lice can cause adverse effects in their fish hosts that include skin lesions, which are associated with a high risk of secondary infections, as well as osmoregulatory dysfunction, immunosuppression, increased stress, and reduced food conversion and growth rates (Grimnes and Jakobsen, 1996; Wootten et al, 1982). Sea lice are controlled using integrated pest management strategies (IPM) combining veteri nary drug treatments (Burridge et al, 2010) with a range of nonmedicinal control approaches, which include mechanical and thermal delousing (reviewed in Holan et al, 2017) as well as the deployment of different species of cleaner fish that remove caligids from farmed salmon (Brooker et al, 2018a), as well as. In-feed treatments include the macrocyclic lactone emamectin benzoate (EMB) and different benzoylureas, while bath treatments include the organophosphate azamethiphos, the disinfectant hydrogen peroxide and the pyrethroids cypermethrin and deltamethrin (DTM) (Helgesen et al, 2019)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
