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Nr. 2 / 2009

New methods in salmon breeding and egg production will be used in this year’s production of Aqua Gen eggs.  The world’s first selective breeding company in aquaculture will use so-called gene markers to choose brood fish having offspring with predictably high resistance to the viral disease IPN.  This marks a new era in selective breeding work, and will contribute to increased cost-effectiveness and better animal welfare in the fish farming industry.

Resistance to IPN - an Important Part of the Breeding Goal

Aqua Gen has chosen to put extra emphasis on IPN in its breeding programme. This is because IPN is widespread and occurs frequently both in fresh- and seawater. Since the disease is introduced early in the fish's life, it is difficult to develop vaccines and vaccination programs to control it.

In addition it has been documented several times that high IPN resistance is correlated to a high degree of general robustness and thereby a reduction of loss throughout the production cycle. Among other things it has been shown that salmon families with high resistance to IPN in challenge tests also show a higher survival in PD outbreaks in field trials, compared to families with low resistance to IPN ( Information Letter 1, 2008).

From Traditional Selection to Use of Gene Markers

Selection for increased disease resistance in fish is usually based on challenge tests carried out under controlled conditions. Challenge-tested fish are not possible to use as parents for the next generation of elite salmon, meaning that selection cannot be applied directly on the breeding candidates. To circumvent this problem, geneticists have been searching for genes controlling the degree of resistance to different diseases. Markers for such genes would be ideal criteria for selection, since they could be applied directly on the animals without requiring challenge testing. Thus, the accuracy of selection could be increased while the need to sacrifice fish in challenge tests would be lessened.

Such methods have been applied very successfully in the major livestock species. In spite of the prospects for more effective selection and better animal welfare, however, the methods have not been applied in fish breeding until now.

A Marker for the IPN Gene

After several years of focused research supported by the Research Council of Norway, a breakthrough for the use of gene markers in fish breeding came in 2007. At that time, researchers in Aqua Gen, Nofima Marin, and CIGENE identified markers for a gene that explains most (80 %) of the genetic variation in resistance to IPN in both fry and post-smolts. Based on these findings, Aqua Gen has developed a tool for directly selecting IPN-resistant fish. This tool can, with very high accuracy, determine whether individual fish have zero, one or two copies of the gene variant (allele) that gives high resistance, and it can be applied on practically any fish within the Aqua Gen population. Challenge tests have shown that the tool is very powerful in terms of predicting the degree of IPN resistance in individual animals (figure 1).

Figure 1. Survival in IPN infection trial of different fry groups of salmon categorised according to the total number of copies of the high-resistance allele (Q) in the fish’s parents. No parental pair was found to have four copies of Q, so there are no observations in this group.

Robust and Effective

To meet the diverse needs of the market, Aqua Gen has established different groups/lineages of brood fish. In the so-called Robust group, resistance to IPN and other diseases has been given extra weight. The other group, called Effective, has been selected mainly for important production characteristics such as growth, harvest results, fillet fat, and colour. By crossing brood fish from the Effective and Robust group (ExR), there will be an even emphasis on effective and robust characteristics in the offspring. A cross between two Effective parents (ExE) will emphasize effective characteristics, while a cross between robust parents (RxR) will emphasize robust characteristics in the offspring.

Selection Causes Changes in Allele Frequency

Since 1997 Aqua Gen has carried out IPN infection trials in order to make the Aqua Gen lineage more resistant to IPN. Using the gene-based tool described above, the genetic progress due to this selection can now be documented. In the 2005 cohort of the breeding nucleus (start-fed in 2005) we found that the frequency of the high-resistance allele (Q) was 0.30 (this implies that the frequency of the low-resistance allele (q) was 0.70). Three years later, after all the families in the 2005 cohort were tested and selected and formed the basis for the 2008 cohort, the frequency of the desired allele had increased to 0.50.

Similar trends were found on analysis of the Robust and Effective multiplier groups. In the Robust group, in which IPN resistance has been given much weight, the frequency of Q was 0.62, whereas in the Effective group, selected for IPN resistance to a lesser extent, the frequency of Q was 0.24.

These results clearly show that selection for IPN resistance is effective, and that selection based on survival in infection trials can be used to increase the degree of IPN resistance. Compared to the use of a powerful gene marker, however, the use of traditional infection trials is relatively time-consuming (Aqua Gen has used 15 years to achieve an allele frequency of 0.5), risky (since IPN challenge tests may sometimes fail) and less precise (selection based solely on families, not on the best individuals within the best families).

High IPN Resistance Combined with High Growth

This year Aqua Gen will offer eggs from parents which have been genetically tested using our gene-based tool. By combining our Effective and Robust groups with genetic testing, we will be able to achieve a much greater flexibility in product characteristics. Crossing Effective fathers and Effective mothers selected through gene-based testing will maximize exploitation of the Aqua Gen lineage's potential in the area of effective traits (growth, outcome at harvest, fat and colour), while at the same time ensuring that the level of IPN resistance is high.

As shown in Figure 2, challenge testing of fry which are descended from parents with different numbers of Q show great differences in survival. Parents with the qq x qq combination produced an average survival of 35 %. Survival increases to an average of 87 % in the fish group which comes from parents with the allele combination QQ x Qq. To use vaccination terminology on these challenge tests, this group has an RPS (Relative Percent Survival) of 80 % if one uses the group without the high-resistance allele as a control.

Figure 2. Average survival in IPN challenge tests and calculated protection against IPN with different groups of salmon fry, categorised according to the combination of alleles (= the genotype) of the fish’s parents. The group having no copies of Q in its parents is used as a control group (65 % mortality) in the calculation of the relative percent of survival (RPS).

Exhaustive analyses have also been carried out in the genetic database in order to map possible positive or negative correlations between the IPN gene and other important characteristics. The results show that the use of the gene marker as a supplement to the traditional selective breeding will have a minimal effect on the other characteristics in the selective breeding goal.

"World First with Gene Marker Selected Salmon Eggs" is found as a PDF here