Transcriptomic responses to emamectin benzoate in Pacific and Atlantic Canada salmon lice Lepeophtheirus salmonis with differing levels of drug resistance

Abstract

Salmon lice Lepeophtheirus salmonis are an ecologically and economically important parasite of wild and farmed salmon. In Scotland, Norway, and Eastern Canada, L. salmonis have developed resistance to emamectin benzoate (EMB), one of the few parasiticides available for salmon lice. Drug resistance mechanisms can be complex, potentially differing among populations and involving multiple genes with additive effects (i.e., polygenic resistance). Indicators of resistance development may enable early detection and countermeasures to avoid the spread of resistance. Here, we collect sensitive Pacific L. salmonis and sensitive and resistant Atlantic L. salmonis from salmon farms, propagate in laboratory (F1), expose to EMB in bioassays, and evaluate either baseline (Atlantic only) or induced transcriptomic differences between populations. In all populations, induced responses were minor and a cellular stress response was not identified. Pacific lice did not upregulate any genes in response to EMB, but downregulated degradative enzymes and transport proteins at 50 ppb EMB. Baseline differences between sensitive and now resistant Atlantic lice were much greater than responses to exposures. All resistant lice overexpressed degradative enzymes, and resistant males, the most resistant group, overexpressed collagenases to the greatest extent. These results indicate an accumulation of baseline expression differences related to resistance.

In silico transcriptome analysis of cuticle-related genes associated with delousing drug responses in the sea louse Caligus rogercresseyi

“…This is then partially reabsorbed by the body and synthesized in the new exoskeleton (Duan et al, 2014;Merzendorfer and Zimoch, 2003). In L. salmonis, EMB-resistant strains present low transcript levels of degrading enzymes, including of chitinases and collagenases (Sutherland et al, 2015). This is similar to what was observed in C. rogercresseyi in the present study, where expression of chitinase was higher in sea lice individuals with a greater response to the two evaluated antiparasitics, suggesting an effect on cuticle degradation by the antiparasitics.…”

Drug resistance in sea lice: a threat to salmonid aquaculture

“…Still, the drug's mode of action is relatively unknown [54]. Similarly, the mechanisms connected to the pharmacology of EMB in sea lice are still subject to speculation [55]. This provides indications that investigating medicinal compounds, their effects on sea lice, and possible resistance mechanisms, are potentially substantial tasks to deal with.…”

Host–parasite transcriptomics during immunostimulant-enhanced rejection of salmon lice (Lepeophtheirus salmonis) by Atlantic salmon (Salmo salar)

“…For example, putative collagenases were underexpressed in LD-feeding lice but overexpressed in EMB-resistant lice (Sutherland et al 2015), such as trypsin-like serine protease, and nas-6 and -14. Also, collagenases were overexpressed in EMB-resistant males (i.e., the most resistant group of L. salmonis in Sutherland et al 2015), and here, lice-infecting LD salmon overexpressed setd7, a histone methyltransferase chromatin remodeler involved in induction of collagenase transcription (Martens et al 2003). Interestingly, a link between immunostimulation and EMB response has been previously identified in L. salmonis (Atlantic subspecies); lice feeding on CpG-ODN immunostimulated hosts showed higher EMB tolerance than those on normal hosts ).…”