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

The pyrethroid deltamethrin (DTM) is used to treat Atlantic salmon (Salmo salar) against salmon louse (Lepeophtheirus salmonis) infestations. However, DTM resistance has evolved in L. salmonis and is currently common in the North Atlantic. This study aimed to re-assess the association between DTM resistance and mitochondrial (mtDNA) mutations demonstrated in previous reports. Among 218 L. salmonis collected in Scotland in 2018–2019, 89.4% showed DTM resistance in bioassays, while 93.6% expressed at least one of four mtDNA single nucleotide polymorphisms (SNPs) previously shown to be resistance associated. Genotyping at further 14 SNP loci allowed to define three resistance-associated mtDNA haplotypes, named 2, 3 and 4, occurring in 72.0%, 14.2% and 7.3% of samples, respectively. L. salmonis strains IoA-02 (haplotype 2) and IoA-10 (haplotype 3) both showed high levels (~ 100-fold) of DTM resistance, which was inherited maternally in crossing experiments. MtDNA haplotypes 2 and 3 differed in genotype for 17 of 18 studied SNPs, but shared one mutation that causes an amino acid change (Leu107Ser) in the cytochrome c oxidase subunit 1 (COX1) and was present in all DTM resistant while lacking in all susceptible parasites. We conclude that Leu107Ser (COX1) is a main genetic determinant of DTM resistance in L. salmonis.

Investigation of deltamethrin resistance in salmon lice (Lepeophtheirus salmonis) provides no evidence for roles of mutations in voltage-gated sodium channels

BACKGROUND

The pyrethroid deltamethrin is used to treat infestations of farmed salmon by parasitic salmon lice, Lepeophtheirus salmonis (Krøyer). However, the efficacy of deltamethrin for salmon delousing is threatened by resistance development. In terrestrial arthropods, knockdown resistance (kdr) mutations of the voltage-gated sodium channel (Na_v ), the molecular target for pyrethroids, can cause deltamethrin resistance. A putative kdr mutation of an L. salmonis sodium channel homologue (LsNa_v 1.3 I936V) has been identified previously. At the same time, deltamethrin resistance of L. salmonis has been shown to be inherited maternally and to be associated with mitochondrial DNA (mtDNA) mutations. This study assessed potential roles of the above putative kdr mutation as a determinant of deltamethrin resistance in laboratory strains and field populations of L. salmonis.

RESULTS

The deltamethrin-resistant L. salmonis strain IoA-02 expresses the LsNa_v 1.3 I936V mutation but was susceptible to the non-ester pyrethroid etofenprox, a compound against which pyrethroid-resistant arthropods are usually cross-resistant if resistance is caused by Na_v mutations. In a family derived from a cross between an IoA-02 male and a drug-susceptible female lacking the kdr mutation, deltamethrin resistance was not associated with the genotype at the LsNa_v 1.3 locus (P > 0.05). Similarly, in Scottish field populations of L. salmonis, LsNa_v 1.3 I936V showed no association with deltamethrin resistance. By contrast, genotypes at the mtDNA loci A14013G and A9030G were significantly associated with deltamethrin resistance (P < 0.001).

CONCLUSION

In the studied L. salmonis isolates, deltamethrin resistance was unrelated to the LsNa_v 1.3 I936V mutation, but showed close association with mtDNA mutations.

Genome-wide survey of cytochrome P450 genes in the salmon louse Lepeophtheirus salmonis (Krøyer, 1837)

Background

The salmon louse (Lepeophtheirus salmonis) infests farmed and wild salmonid fishes, causing considerable economic damage to the salmon farming industry. Infestations of farmed salmon are controlled using a combination of non-medicinal approaches and veterinary drug treatments. While L. salmonis has developed resistance to most available salmon delousing agents, relatively little is known about the molecular mechanisms involved. Members of the cytochrome P450 (CYP) superfamily are typically monooxygenases, some of which are involved in the biosynthesis and metabolism of endogenous compounds, while others have central roles in the detoxification of xenobiotics. In terrestrial arthropods, insecticide resistance can be based on the enhanced expression of CYPs. The reported research aimed to characterise the CYP superfamily in L. salmonis and assess its potential roles in drug resistance.

Methods

Lepeophtheirus salmonis CYPs were identified by homology searches of the genome and transcriptome of the parasite. CYP transcript abundance in drug susceptible and multi-resistant L. salmonis was assessed by quantitative reverse transcription PCR, taking into account both constitutive expression and expression in parasites exposed to sublethal levels of salmon delousing agents, ecdysteroids and environmental chemicals.

Results

The above strategy led to the identification of 25 CYP genes/pseudogenes in L. salmonis, making its CYP superfamily the most compact characterised for any arthropod to date. Lepeophtheirus salmonis possesses homologues of a number of arthropod CYP genes with roles in ecdysteroid metabolism, such as the fruit fly genes disembodied, shadow, shade, spook and Cyp18a1. CYP transcript expression did not differ between one drug susceptible and one multi-resistant strain of L. salmonis. Exposure of L. salmonis to emamectin benzoate or deltamethrin caused the transcriptional upregulation of certain CYPs. In contrast, neither ecdysteroid nor benzo[a]pyrene exposure affected CYP transcription significantly.

Conclusions

The parasite L. salmonis is demonstrated to possess the most compact CYP superfamily characterised for any arthropod to date. The complement of CYP genes in L. salmonis includes conserved CYP genes involved in ecdysteroid biosynthesis and metabolism, as well as drug-inducible CYP genes. The present study does not provide evidence for a role of CYP genes in the decreased susceptibility of the multiresistant parasite strain studied.

Mutations in voltage-gated sodium channels from pyrethroid resistant salmon lice (Lepeophtheirus salmonis)

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.

Maternal inheritance of deltamethrin resistance in the salmon louse Lepeophtheirus salmonis (Krøyer) is associated with unique mtDNA haplotypes

Parasitic infections by the salmon louse, Lepeophtheirus salmonis (Krøyer), cause huge economic damage in salmon farming in the northern hemisphere, with combined treatment costs and production losses in 2014 having been estimated at US$ 350 million for Norway (annual production 1.25 million tonnes). The control of L. salmonis relies significantly on medicinal treatments, supplemented by non-pharmacological approaches. However, efficacy losses have been reported for several delousing agents, including the pyrethroid deltamethrin. The aim of the present study was to analyse the genetic basis of deltamethrin resistance in L. salmonis. Deltamethrin median effective concentrations (EC50) were 0.28 μg L-1 in the drug susceptible L. salmonis strain IoA-00 and 40.1 μg L-1 in the pyrethroid resistant strain IoA-02. IoA-00 and IoA-02 were crossed to produce families spanning one parental and three filial generations (P0, F1-F3). In three families derived from P0 crosses between an IoA-00 sire and an IoA-02 dam, 98.8% of F2 parasites (n = 173) were resistant, i.e. remained unaffected after exposure to 2.0 μg L-1 deltamethrin. F3 parasites from these crosses showed a deltamethrin EC50 of 9.66 μg L-1. In two families of the inverse orientation at P0 (IoA-02 sire x IoA-00 dam), 16.7% of F2 parasites were resistant (n = 84), while the deltamethrin EC50 in F3 animals was 0.26 μg L-1. The results revealed a predominantly maternal inheritance of deltamethrin resistance. The 15,947-nt mitochondrial genome was sequenced and compared among six unrelated L. salmonis strains and parasites sampled from wild salmon in 2010. IoA-02 and three further deltamethrin resistant strains, established from isolates originating from different regions of Scotland, showed almost identical mitochondrial haplotypes. In contrast, the mitochondrial genome was variable among susceptible strains and L. salmonis from wild hosts. Deltamethrin caused toxicity and depletion of whole body ATP levels in IoA-00 but not IoA-02 parasites. The maternal inheritance of deltamethrin resistance and its association with mitochondrial haplotypes suggests that pyrethroid toxicity in L. salmonis may involve molecular targets encoded by mitochondrial genes.

Time-to-response toxicity analysis as a method for drug susceptibility assessment in salmon lice

The salmon louse Lepeophtheirus salmonis (Krøyer, 1837) is an ectoparasite causing infections of wild and farmed Atlantic salmon (Salmo salar L.) in the Northern hemisphere. While L. salmonis control at commercial mariculture sites increasingly employs non-medicinal approaches, such as cage designs reducing infection rates and biological control through cleaner fish, anti-parasitic drugs are still a requirement for effective fish health care. With only a limited range of salmon delousing agents available, all of which have been in use for more than a decade, drug resistance formation has been reported for different products. Successful resistance management requires reliable susceptibility assessment, which is usually achieved through L. salmonis bioassays. These tests involve the exposure of parasites to different drug concentrations and require significant numbers of suitable L. salmonis stages. The present study reports an alternative bioassay that is based on time-to-response toxicity analyses and can be carried out with limited parasite numbers. The assay determines the median effective time (ET₅₀), i.e., the time required until impaired swimming and/or attachment behaviour becomes apparent in 50% of parasites, by conducting repeated examinations of test animals starting at the time point where exposure to a set drug concentration commences. This experimental approach further allows the estimation of the apparent drug susceptibility of individual L. salmonis by determining their time to response, which may prove useful in experiments designed to elucidate associations between genetic factors and the drug susceptibility phenotype of parasites. Three laboratory strains of L. salmonis differing in susceptibility to emamectin benzoate were characterised using standard 24 h bioassays and time-to-response toxicity assays. While both the median effective concentration (EC₅₀) and the ET₅₀ showed variability between experimental repeats, both types of bioassay consistently discriminated susceptible and drug-resistant L. salmonis laboratory strains.

STATEMENT OF RELEVANCE

Infections by sea lice cause significant costs to the global salmon farming industry, which have been estimated to exceed €300 million per year worldwide. Control of sea lice still relies to a significant extent on chemical delousing; however, chemical control is threatened by resistance formation. Resistance can be combated by rotation between different drugs and strategic implementation of non-medicinal strategies. However, resistance management requires reliable and feasible methods of susceptibility assessment. The present study is a technical note introducing a novel approach to susceptibility assessments in sea lice. The method can be applied in susceptibility assessments on farms, where it offers the advantage of a reduced requirement of parasites for testing. In addition, the novel method allows deriving the times of parasite require to show a response after drug treatment has started, thus providing a variable characterizing the drug susceptibility phenotype of individual parasites. Accordingly, the bioassay approach presented here will be useful for studies aiming at unravelling the genetic determinants of drug resistance.

Identification of a sex-linked SNP marker in the salmon louse (Lepeophtheirus salmonis) using RAD sequencing

The salmon louse (Lepeophtheirus salmonis (Krøyer, 1837)) is a parasitic copepod that can, if untreated, cause considerable damage to Atlantic salmon (Salmo salar Linnaeus, 1758) and incurs significant costs to the Atlantic salmon mariculture industry. Salmon lice are gonochoristic and normally show sex ratios close to 1:1. While this observation suggests that sex determination in salmon lice is genetic, with only minor environmental influences, the mechanism of sex determination in the salmon louse is unknown. This paper describes the identification of a sex-linked Single Nucleotide Polymorphism (SNP) marker, providing the first evidence for a genetic mechanism of sex determination in the salmon louse. Restriction site-associated DNA sequencing (RAD-seq) was used to isolate SNP markers in a laboratory-maintained salmon louse strain. A total of 85 million raw Illumina 100 base paired-end reads produced 281,838 unique RAD-tags across 24 unrelated individuals. RAD marker Lsa101901 showed complete association with phenotypic sex for all individuals analysed, being heterozygous in females and homozygous in males. Using an allele-specific PCR assay for genotyping, this SNP association pattern was further confirmed for three unrelated salmon louse strains, displaying complete association with phenotypic sex in a total of 96 genotyped individuals. The marker Lsa101901 was located in the coding region of the prohibitin-2 gene, which showed a sex-dependent differential expression, with mRNA levels determined by RT-qPCR about 1.8-fold higher in adult female than adult male salmon lice. This study’s observations of a novel sex-linked SNP marker are consistent with sex determination in the salmon louse being genetic and following a female heterozygous system. Marker Lsa101901 provides a tool to determine the genetic sex of salmon lice, and could be useful in the development of control strategies.

The zebrafish as a model for muscular dystrophy and congenital myopathy

The muscular dystrophies and congenital myopathies are inherited diseases of the skeletal muscle, which lead to a loss of muscle function and are often fatal. While many of the loci involved are already known, these conditions remain incurable, and genetic models are being developed in an effort to understand the pathological mechanisms involved. Recently several papers have shown that the zebrafish, which is now widely used in developmental genetic studies, will provide a useful addition to our toolkit in this regard. Here we describe these studies, including a zebrafish model of what is potentially the novel pathological mechanism of muscle attachment failure in Duchenne and other muscular dystrophies.

Dystrophin is required for the formation of stable muscle attachments in the zebrafish embryo

A class of recessive lethal zebrafish mutations has been identified in which normal skeletal muscle differentiation is followed by a tissue-specific degeneration that is reminiscent of the human muscular dystrophies. Here, we show that one of these mutations, sapje, disrupts the zebrafish orthologue of the X-linked human Duchenne muscular dystrophy (DMD) gene. Mutations in this locus cause Duchenne or Becker muscular dystrophies in human patients and are thought to result in a dystrophic pathology through disconnecting the cytoskeleton from the extracellular matrix in skeletal muscle by reducing the level of dystrophin protein at the sarcolemma. This is thought to allow tearing of this membrane, which in turn leads to cell death. Surprisingly, we have found that the progressive muscle degeneration phenotype of sapje mutant zebrafish embryos is caused by the failure of embryonic muscle end attachments. Although a role for dystrophin in maintaining vertebrate myotendinous junctions (MTJs) has been postulated previously and MTJ structural abnormalities have been identified in the Dystrophin-deficient mdx mouse model, in vivo evidence of pathology based on muscle attachment failure has thus far been lacking. This zebrafish mutation may therefore provide a model for a novel pathological mechanism of Duchenne muscular dystrophy and other muscle diseases.

Identification and developmental expression of a macrophage stimulating 1/ hepatocyte growth factor-like 1 orthologue in the zebrafish

The cytokine Macrophage Stimulating 1 (MST1/MSP/Hepatocyte Growth Factor-Like) is a ligand of the Met-related MST1-Receptor (MST1R/RON). Although MST1-deficient mice are viable, MST1R is essential in mice before gastrulation for implantation, and is a known oncogene in man. Here I report the identification, sequence, chromosomal location and embryonic expression of a novel zebrafish orthologue, termed macrophage stimulating 1 (mst1). mst1 shows a striking restriction of expression to the dorsal side of the embryo prior to gastrulation, and as gastrulation and somitogenesis proceed is expressed sequentially in the presumptive neurectoderm, the notochord, the somites, endodermal cells and in the syncytial yolk. This dynamic pattern is largely conserved in tetrapod vertebrates, suggesting that the appearance of MST1, may have played an early role in the evolution of the vertebrate body plan.