Candidate genes for monitoring hydrogen peroxide resistance in the salmon louse, Lepeophtheirus salmonis.

Background: Hydrogen peroxide (HO) is one of the delousing agents used to control sea lice infestations in salmonid aquaculture. However, some Lepeophtheirus salmonis populations have developed resistance towards HO. An increased gene expression and activity of catalase, an enzyme that breaks down HO, have been detected in resistant lice, being therefore introduced as a resistance marker in the salmon industry. In the present study the aim was to validate the use of catalase expression as a marker and to identify new candidate genes as additional markers to catalase, related to HO resistance in L. salmonis.

Methods: A sensitive and an HO resistant laboratory strain (P0 generation, not exposed to HO for several years) were batch crossed to generate a cohort with a wide range of HO sensitivities (F2 generation). F2 adult females were then exposed to HO to separate sensitive and resistant individuals. Those F2 lice, the P0 lice and field-collected resistant lice (exposed to HO in the field) were used in an RNA sequencing study.

Results: Catalase was upregulated in resistant lice exposed to HO compared to sensitive lice. This was, however, not the case for unexposed resistant P0 lice. Several other genes were found differentially expressed between sensitive and resistant lice, but most of them seemed to be related to HO exposure. However, five genes were consistently up- or downregulated in the resistant lice independent of exposure history. The upregulated genes were: one gene in the DNA polymerase family, one gene encoding a Nesprin-like protein and an unannotated gene encoding a small protein. The downregulated genes encoded endoplasmic reticulum resident protein 29 and an aquaporin (Glp1_v2).

Conclusions: Catalase expression seems to be induced by HO exposure, since it was not upregulated in unexposed resistant lice. This may pose a challenge for its use as a resistance marker. The five new genes associated with resistance are put forward as complementary candidate genes. The most promising was Glp1_v2, an aquaglyceroporin that may serve as a passing channel for HO. Lower channel number can reduce the influx or distribution of HO in the salmon louse, being directly involved in the resistance mechanism.