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Identification of a potential hatching enzyme involved in the hatching process of Lepeophteirus salmonis eggs


Gunnvør Norðberg



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The salmonid aquaculture industry has experienced an intensive increase in salmon farming over the past years; this has resulted in the awareness of problems such as infestation with sea lice (Lepeophtheirus salmonis). These marine ectoparasites have become a economical burden within the industry causing health issues of farmed salmonids worldwide, and are estimated to cost the world industry €300m a year. In the northern hemisphere it has been documented that salmon lice has become a threat to the wild salmonid stocks, since lice originating from farmed salmon infest the wild stocks. This has become a major concern to ecologists due to the negative impact of the abundance of the wild salmonid stocks.

Efforts have been done to maintain control over a high infestation pressure of l. salmonis in farmed salmon. Different treatment methods are available, but therapeutic pesticides represents the primary treatment used for delousing farmed salmon. These agents are however available in limited numbers and the potential risk for developing drug resistance in lice is high and has already been found. Another drawback concerning these pesticides is the lack of specificity and the risk of affecting non-target organisms and their habitats. It is therefore crucial to find alternative solutions to achieve safe control management, over this vigorous parasite responsible for environmental concerns addition to being an economical burden for the aquaculture industry.

Hatching enzymes (HEs) have not been investigated in sea lice before but their presence in close relatives of L. salmonis e.g. lice from phylum arthropoda, and shrimps from subphylum crustaceans indicate the likelihood to identify the presence of HEs in L. salmonis eggs, which is the aim of this experiment.

The identification of a HE in L. salmonis eggs will not only generate new insight into the biochemical properties of the hatching mechanism of L. salmonis but also a new potent target where modern pharmacological manipulation permits the development of novel therapeutic agents that are both selective and specific, addition to having biological efficacy.

In this study, three potential L. salmonis HE genes were investigated. The complete cDNAs of these genes were generated by reverse transcription polymerase chain reaction (rt-PCR) and rapid amplification of cDNA ends (RACE). To establish the functional characterization of potential HE genes, systemic RNA interference (RNAi) was utilised, and finally to detect temporal expression of the potential HE genes in L.salmonis eggs, quantitative reverse transcription real-time PCR (RTq-PCR) was performed.

All three genes were successfully cloned in this experiment. Two genes were found to be trypsin-like serine proteases as predicted and one gene believed to be a metalloprotease was instead found to belong to the super family of immunoglobulin. Their function in egg hatching was investigated. Only one of the three genes was successfully knocked down by RNAi, this was one of the trypsin-like serine proteases. Results showed that egg hatching remained normal and no phenotype on egg or nauplii was detected, and therefore it was concluded that this trypsin-like serine protease is not a hatching enzyme. If the other serine protease is a hatching enzyme remains currently unknown, and research shows that immunoglobulin are not HE candidates and this gene is rather involved in the immune system of L. salmonis.


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