Deltamethrin resistance in salmon lice: Genetic markers and molecular mechanisms

Abstract

Resistance of salmon lice (Lepeophtheirus salmonis) against the pyrethroid deltamethrin used in bath treatments is common, but the mechanisms underlying resistance are unknown. In insects, deltamethrin resistance can involve mutations of voltage-gated sodium channels (Nav) considered the target-site of pyrethroids as well as metabolic detoxification through carboxylesterases (CaEs). In L. salmonis, deltamethrin resistance is mainly inherited maternally and associated with single nucleotide polymorphisms (SNPs) in the mitochondrial genome (mtDNA). In addition, a potential target-site mutation (I936V) has been identified in the L. salmonis Nav homologue LsNav1.3. This PhD thesis investigated the relative contribution of Nav target-site mutations and mtDNA mutations in deltamethrin resistance in L. salmonis. L. salmonis from farm sites were rated as deltamethrin resistant or susceptible in bioassays and genotyped for the LsNav1.3 mutation I936V and previously identified resistance-associated mitochondrial SNPs. The results provided no evidence for a role of I936V in deltamethrin resistance, while confirming its association with several mtDNA SNPs. This conclusion was further supported by the genotyping of deltamethrin resistant and susceptible L. salmonis derived from a crossing experiment. Further experiment assessed the association of mitochondrial SNPs with deltamethrin resistance. In L. salmonis from farm sites, several mitochondrial haplotypes associated with deltamethrin resistance were identified, suggesting that deltamethrin resistance evolved at least two times independently. The association of a previously unknown haplotype with deltamethrin resistance was demonstrated in toxicity and genetic studies that support the hypothesis that SNP T8600C in cytochrome c oxidase subunit 1 (Leu107Ser) is related to the resistance mechanism. Finally, potential roles of the CaE gene family in deltamethrin resistance were examined. L. salmonis CaE sequences were identified and annotated, and CaE sequences predicted to be catalytically competent were studied regarding their transcript and SNP expression in resistant and susceptible lice. Results suggested that CaEs are not major determinants of deltamethrin resistance.