Skin cell discovery could help Atlantic salmon fend off sea lice

Insights into how coho salmon fight off the parasites could pave the way for new genetic approaches to boost Atlantic salmon’s resistance to them, researchers say.

Their findings reveal that layers of skin cells – known as keratinocytes – play a key role in triggering localised swelling that helps coho salmon kill and remove sea lice.

The study entitled 'Keratinocytes Drive the Epithelial Hyperplasia Key to Sea Lice Resistance in Coho Salmon' was published in the journal BMC Biology. 

Damaging impacts

The parasites – which feed on fish’s skin and fins, causing open wounds that can lead to infection – reduce the market value of farmed fish and can have knock-on impacts on wild salmon populations.

Various treatments have been developed to tackle sea lice infestations in Atlantic salmon aquaculture – which costs the industry more than £700m a year – but these are often costly and ineffective. They can also be damaging to the environment and negatively affect animal welfare.

Parasite resistance

While Atlantic salmon are highly susceptible to sea lice, coho salmon have an innate resistance to them. In coho salmon, the attachment of sea lice triggers localised swelling. However, there is minimal swelling in Atlantic salmon, and the outermost layer of their skin rapidly breaks down.

Until now, the cell-level mechanisms underlying the differences in resistance between the species – which split from a common ancestor around 30 million years ago – have remained unknown.

An international team led by scientists at the Roslin Institute and Norwegian food science research institute Nofima analysed the molecular and cellular mechanisms that underlie coho salmon’s sea lice resistance and compared this to the response in Atlantic salmon.

Genetic shield

After exposing both species to the parasites, researchers investigated the cell types and gene expression patterns of their responses by analysing skin samples using a novel type of RNA sequencing that allows the characterisation of gene expression of individual cells.

They found that, while both species mount similar immune and wound-healing responses, keratinocytes in the three outermost layers of coho salmon’s skin play a central role in their response to infection.

Their findings suggest that keratinocytes in the bottom layer direct the expansion and movement of cells in the middle and top layers, which can eventually encapsulate and expel the parasites. Findings also revealed that sea lice can weaken the immune system of Atlantic salmon.

Using gene editing it could be possible to enhance Atlantic salmon’s sea lice resistance, by targeting genes that underly their susceptibility, and coho salmon’s resistance, to the parasites, the team says.

This work was funded by the Norwegian seafood industry research fund, FHF, and BBSRC. It also involved researchers from the University of Stirling, University of Santiago de Compostela, Spain, University of Prince Edward Island, Canada, Deakin University, Australia, and Benchmark Genetics.