Abstract
BackgroundThe 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.MethodsLepeophtheirus 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.ResultsThe 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.ConclusionsThe 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.
Highlights
The salmon louse (Lepeophtheirus salmonis) infests farmed and wild salmonid fishes, causing considera‐ ble economic damage to the salmon farming industry
25 putative cytochrome P450 (CYP) genes/pseudogenes were obtained in L. salmonis and named according to the current CYP nomenclature (Table 1)
The present study further identified putative L. salmonis homologues of spook/spookier (CYP307A1/2), shadow (CYP315A1) and a CYP18A1 homologue
Summary
The salmon louse (Lepeophtheirus salmonis) infests farmed and wild salmonid fishes, causing considera‐ ble economic damage to the salmon farming industry. Infestations of farmed salmon are controlled using a combina‐ tion 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. In the last five years, there has been an increased implementation of non-medicinal control strategies, which include co-culture with cleaner fish [7, 8], cage designs that reduce infection pressure [9], immunostimulants [10], treatments with freshwater and thermal delousing [11]. Licensed salmon delousing agents currently available in the UK include the organophosphate azamethiphos, the pyrethroid deltamethrin (DM) and the non-specific oxidant hydrogen peroxide, all of which are applied as bath treatments, and the macrocyclic lactone emamectin benzoate (EMB), which is administered as an in-feed medication [12]. Loss of treatment efficacy has been reported for most available drugs [14–17], likely indicative of the development of drug resistance [18–21]
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