Sammendrag
Abstract
Background: Hydrogen peroxide (H2O2) is one of the delousing agents used to control sea lice infestations in salmo‑
nid aquaculture. However, some Lepeophtheirus salmonis populations have developed resistance towards H2O2. An
increased gene expression and activity of catalase, an enzyme that breaks down H2O2, 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 vali‑
date the use of catalase expression as a marker and to identify new candidate genes as additional markers to catalase,related to H2O2 resistance in L. salmonis.
Methods: A sensitive and an H2O2 resistant laboratory strain (P0 generation, not exposed to H2O2 for several years)
were batch crossed to generate a cohort with a wide range of H2O2 sensitivities (F2 generation). F2 adult females
were then exposed to H2O2 to separate sensitive and resistant individuals. Those F2 lice, the P0 lice and feld-collected
resistant lice (exposed to H2O2 in the feld) were used in an RNA sequencing study.
Results: Catalase was upregulated in resistant lice exposed to H2O2 compared to sensitive lice. This was, however, not
the case for unexposed resistant P0 lice. Several other genes were found diferentially expressed between sensitive
and resistant lice, but most of them seemed to be related to H2O2 exposure. However, fve 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 H2O2 exposure, since it was not upregulated in unex‑
posed resistant lice. This may pose a challenge for its use as a resistance marker. The fve new genes associated with
resistance are put forward as complementary candidate genes. The most promising was Glp1_v2, an aquaglycerop‑
orin that may serve as a passing channel for H2O2. Lower channel number can reduce the infux or distribution of
H2O2 in the salmon louse, being directly involved in the resistance mechanism.
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