Key Role of Mitochondrial Mutation Leu107Ser (COX1) in Deltamethrin Resistance in Salmon Lice (Lepeophtheirus Salmonis)

Deltamethrin (DTM) is used to treat Atlantic salmon ( Salmon salar ) against salmon lice ( Lepeophtheirus salmonis ) infestations. However, development of DTM resistance has been reported from North Atlantic L. salmonis populations, in which resistance is associated with mitochondrial (mtDNA) mutations. This study investigated the relationship between DTM resistance and mtDNA single nucleotide polymorphisms (SNPs). A total of 188 L. salmonis collected from Scottish aquaculture sites were assessed using DTM bioassays and genotyped at 18 SNP loci. Genotyping further included archived parasites of known DTM susceptibility status. The results identified eleven mtDNA haplotypes, three of which were associated with DTM resistance. Phylogenetic analyses of haplotypes suggested multiple origins of DTM resistance. L. salmonis laboratory strains IoA-00 and IoA-10 showed similarly high levels (~100-fold) of DTM resistance in bioassays. Both strains differed strongly in mtDNA haplotype, but shared the missense mutation Leu107Ser in the mitochondrial gene cytochrome c oxidase subunit 1 (COX1), which was detected in all further DTM resistant L. salmonis isolates assessed. In crossing experiments with a DTM-susceptible strains, maternal inheritance of DTM resistance is apparent with both IoA-10 (this study) and IoA-02 (earlier reports). We conclude that Leu107Ser (COX1) is a main genetic determinant of DTM resistance in L. salmonis.