Introduction
During the later years, the composition of salmon diets has changed dramatically. Marine fish oils and fish meals have traditionally been the main ingredients, while the diets now are dominated by plant ingredients, and contain less than 30% fish oil and fish meal (Ytrestøyl et al., 2014). This has led to reduced levels of the marine fatty acids EPA and DHA in the salmon. The Atlantic salmon can produce EPA and DHA from shorter n-3 fatty acids, but the capacity is limited. The aim of this study was to investigate the genetic variation in ability of the Atlantic salmon to produce EPA and DHA and the phenotypic effects of changed metabolic capacity.
Material and methods
A total of 800 individuals from 103 Atlantic salmon families from SalmoBreed's breeding program were analysed for relative expression of the delta-6-desaturase (Δ6fad_b) gene. Parent fish were selected among the families with highest and lowest relative gene expression, and first generation progeny was produced. Six new full sib groups, 3 "High desaturase" and 3 "Low desaturase", were raised and used in a feeding experiment with different ratio of fish oil and rapeseed oil from 100 to 500 gram. Remaining fish from all genetic and dietary groups were transferred to a common sea net cage, and kept on a low-omega-3 diet up to 4 kg slaughter size.
The fish were analysed for an array of health related parameters, and fatty acid composition at 500 gram and at slaughter size 4 kg.
~360 000 single nucleotide polymorphisms (SNPs ) from the coding region of the genome were analysed for association to the expression of delta-6 desaturase.
Two significant SNPs were validated in 190 progeny from the main population slaughtered at 4 kg. The association of these two SNPs to a number of phenotypic traits related to size, fat distribution in liver, heart and muscle as and gut inflammation was performed. For estimation of the fatty acid composition in muscle and liver, selective phenotyping of pooled samples of individuals with + and - alleles was done.
Results
Genetic background had a significant effect on lipid distribution in the fish (liver and visceral fat level), capacity for production of EPA and DHA, and survival in sea. We observed that "High desaturase" tolerated the low-omega-3 diet better than "Low desaturase" fish. Total RNA sequencing of samples from the parent generation showed that 1181 genes had higher expression in "High desaturease" families, while 359 genes were higher expressed in "Low desaturase", families. Several genes for lipid metabolism, inflammatory factors, and immune related genes, showed co-variation.
Four significant QTLs were identified, 3 within the delta-6-desaturase gene, albeit not in the coding region.
Validation of the QTLs. showed significant effects of the delta-6 desaturase gene on length, fatness of liver, liver weight, liver index (relative size of the liver to body size) and inflammation in the middle intestine. The SNP analyses revealed large differences in the relative levels of EPA and DHA in liver of fish with the + and - alleles.
Conclusions
This study shows a potential of selective breeding as a promising tool to increase the innate capacity for production of EPA and DHA in Atlantic salmon while improving the metabolic health of the fish. .
References
Ytrestøyl, T., Aas, T.S. & Åsgård, T., 2014. Resource utilization of Norwegian salmon farming in 2012-2013. Nofima Report 36/2014.