Tests performed by researchers at Nofima, SalmoBreed and other partners found that for fish with high breeding value for PD, the survival rate was close to 90 per cent, while for fish with low breeding value, it was just below 60 per cent.
These findings were confirmed in two infection test models: intraperitoneal and cohabitant infection.
Project leader Hooman Moghadam from Nofima said: "We have so far done two infection tests with selected family groups of fish. When we see the big differences in survival rate between the groups already after a few generations' selection, we know that we are on the right track. And when in addition the results are confirmed through two different models of infection, our findings are confirmed with even stronger evidence. Therefore, we can be sure that breeding for increased PD survival is an important tool in the fight against PD.”
The NFR HAVBRUK2-funded research project, SalmoResist, aims to look at the connection between phenotypic properties and fish genotype. Fish from families with high and low breeding values have been tested in the infection tests for their ability to resist infection by PD virus. By analysing the genomes of these fishes, a phenotypic trait such as mortality can be linked to variations in the fish's DNA. This allows for a more efficient selection of PD-robust brood fish at an individual level. Such information can be utilised to increase the success rate through breeding and increase the safety of selection compared to traditional family breeding.
Genes control resistance to disease
Genetics manager Borghild Hillestad is responsible for SalmoBreed's participation in the SalmoResist project. Till 2015, SalmoBreed used a Quantitative trait loci (QTL) for selection of fish with higher tolerance against the PD virus. This has been the method that significantly reduced the number of outbreaks of the infectious pancreatic necrosis (IPN) disease in salmon farming. QTL-based selection methods are most effective when the trait is controlled to a high degree by one or a few genes.
"Finding such a powerful QTL as we have available for IPN is a rare event. The QTL for IPN controls around 80 per centof the fish's resistance to the disease," says Hillestad. On the other hand, she explains, there are many characteristics that are usually affected by a large number of genes. Separately, each gene has a small effect on a trait, but combined they explain a large part of genetic variation. This is where the genomic selection (GS) method comes into play and has proven to be far more precise and effective than QTL for many properties. To be able to use GS, a good understanding of the genetic variations in the genome of the species is essential. Using GS, the entire salmon genome is evaluated in order to identify variations associated with PD survival. By acquiring a deeper understanding of such factors, we have a much stronger capacity to breed effectively for resistance to the disease.
Genomic selection and PD are a good match
SalmoBreed produced families based on GS values of parent fish for the first time in 2015. Since 2008, every year infection tests have been conducted on all families, and breeding values have been prepared based on this information. The selection work based on these infection tests is confirmed through SalmoResist.
"Extreme values based on GS were used when the families were selected for the project. When we see the difference in survival between the groups in SalmoResist, we confirm that genomic selection is a good tool in the fight against PD,” says Hillestad.
She explains that through breeding from individuals that have the desired gene composition, SalmoBreed will be able to deliver fish with ever increasing resistance to PD, generation for generation. "Breeding is the only measure that gives an ’incremental interest effect’, " says Hillestad. "The progress we make through genomic selection in a generation form the basis for the next generation.”
As part of the SalmoResist project, and as a verification of infection tests, DNA samples will be collected from healthy and dead fish that have been exposed to the PD out in the field. By studying the genome variations, researchers will look for correlation and look for a confirmation of the relationship between DNA variation and high disease resistance.
"In the infection experiments, we can see how fish behave in a controlled environment," says Moghadam. "In addition, when we have knowledge of the link between genes and how fish respond to a PD-outbreak in the field, we can make even better decisions when we select a more robust fish.”