Genomic analysis of the carboxylesterase family in the salmon louse (Lepeophtheirus salmonis)

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 .

Key role of mitochondrial mutation Leu107Ser (COX1) in deltamethrin resistance in salmon lice (Lepeophtheirus salmonis)

“…However, while DTM resistance of L. salmonis field populations determined in acute bioassays was associated with genotypes at two mtDNA SNP loci, the SNP in LsNa v 1.3 showed no such association and did not explain mt-DNA independent DTM resistance observed in a crossing experiment 25 . Similarly, genome-wide characterisation and transcript expression studies of L. salmonis CYP and CaE genes failed to provide evidence for an upregulation of CYPs or CaEs in DTM resistant parasites 26 , 27 .…”