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Background: Parasitic salmon lice (Lepeophtheirus salmonis) cause high economic losses in Atlantic salmon farming. Pyrethroids, which block arthropod voltage-gated sodium channels (Na v 1), are used for salmon delousing. However, pyrethroid resistance is common in L. salmonis. The present study characterized Na v 1 homologues in L. salmonis in order to identify channel mutations associated to resistance, called kdr (knockdown) mutations., Results: Genome scans identified three L. salmonis Na v 1 homologues, LsNa v 1.1, LsNa v 1.2 and LsNa v 1.3. Arthropod kdr mutations map to specific Na v 1 regions within domains DI-III, namely segments S5 and S6 and the linker helix connecting S4 and S5. The above channel regions were amplified by RT-PCR and sequenced in deltamethrin-susceptible and deltamethrin-resistant L. salmonis. While LsNa v 1.1 and LsNa v 1.2 lacked nucleotide polymorphisms showing association to resistance, LsNa v 1.3 showed a non-synonymous mutation in S5 of DII occurring in deltamethrin-resistant parasites. The mutation is homologous to a previously described kdr mutation (I936V, numbering according to Musca domestica Vssc1) and was present in two pyrethroid-resistant L. salmonis strains (allele frequencies of 0.800 and 0.357), but absent in two pyrethroid-susceptible strains., Conclusions: The present study indicates that a kdr-mutation in LsNa V 1.3 may contribute to deltamethrin resistance in L. salmonis. © 2018 Society of Chemical Industry., (© 2018 Society of Chemical Industry.)

Resistance towards deltamethrin (DMT) in the crustacean ectoparasite Lepeophtheirus salmonis (Caligidae) is a problem on fish farms lining the North Atlantic Ocean. Two Norwegian strains with different susceptibility towards DMT were crossed in the parental generation (P0), females from a sensitive strain were crossed with males from a resistant strain and vice versa. Individual susceptibility towards DMT was assessed in the second filial generation (F2). DMT resistance was only found in F2 descendants when the P0 females were from the resistant strain, pointing to maternal inheritance. Since maternal inheritance might be linked to the mitochondrial (mt) genome, the nucleotide sequences and the gene expressions of mt-genes were analysed. Twenty non-synonymous single nucleotide polymorphisms (SNPs) were identified in mt-transcripts from resistant F2 parasites, including SNPs in two cytochrome C oxidase subunits (COX1 and COX3) and two subunits of the NADH dehydrogenase complex (ND1 and ND5) previously linked to DMT resistance in the salmon louse. Differential expression analysis between the sensitive and resistant strain revealed strain effect in seven out of twelve mt-genes. The current study also show that DNA fragmentation (indicating apoptosis) was affected by DMT exposure in skeletal muscle tissue and that resistant parasites undergo less apoptosis than sensitive parasites.