QTL Eggs – Documented IPN Protection from Day One

Nr. 1 / 2010

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In the autumn of 2009 Aqua Gen produced for the first time, eggs selected using a gene marker for increased IPN resistance. In the course of the egg season 2009/2010 nearly 50 million of this egg type were delivered to customers. The results of controlled laboratory experiments and feedback from hatcheries show that, to date, the salmon fry produced from these eggs have performed according to expectations and that so far there has been a very high protection against IPN.

Robust or Effective -With or Without QTL?

Aqua Gen’s selective breeding work is built around a genetic nucleus which is descended from the original gene pool that was collected from wild salmon in the early 1970’s. From this nucleus we produce two different brood fish lines (elite fish) called Robust and Effective. Robust elite fish are selected from families in the genetic nucleus which have a high survival rate in different infection tests, one of which is IPN. Effective elite fish are selected from families in the genetic nucleus which score high on growth and quality at harvest.

To date when Aqua Gen has produced eggs for customers from elite fish, there have been three alternative salmon
egg products to choose from: Aqua Gen® Atlantic ExR is a cross between brood fish from the Effective and Robust groups, with equal weighting for Effective and Robust traits in the offspring. Aqua Gen® Atlantic ExE is a cross between two Effective parents (ExE) which is weighted toward Effective traits. Aqua Gen® Atlantic RxR, a cross between Robust parents will be weighted toward Robust traits in the offspring. QTL-rogn2-250x145

Selective breeding to promote improved robustness characteristics in salmon has been a major area of investment for Aqua Gen for a long time. And resistance against the viral disease IPN has been a central part of this drive for stock improvement since 1997.

In 2007 a fundamental breakthrough in this work came when researchers at Aqua Gen, Nofima Marin and CIGENE (Ås) identified a QTL (Quantitative Trait Locus) which explains 80 % of the genetic variation for resistance to IPN in salmon. A QTL is an area on the genome which controls a certain characteristic, and with the help of genetic markers it is possible to control the inheritance of this QTL and thus the characteristic which it controls.

There are two alleles of the gene under consideration: Q for IPN resistant and q for IPN sensitive, and the resistance to IPN in a fish population is controlled mainly by the total percentage of the good allele (Q) in the population.

Using this new development Aqua Gen launched two new products in the egg season of 2009/2010 using a molecular genetic test for the Q allele carried out directly on the parents of the eggs which were to be sold. The two products were ExEQTL in which good growth and harvest quality characteristics are combined with the presence of the IPN-QTL, and RxEQTL where the robust characteristics of the parent fish are given more weight together with the use of the IPN-QTL.

QTL has comparable RPS to vaccination

It has been found that selection for IPN resistance using the QTL gene marker provides a level of protection against IPN which is comparable to the effect of vaccines. Through results from earlier infection tests on fry we can estimate the relative percent of survival (RPS) for both the standard Aqua Gen products, and the new QTL products. According to these calculations, the new products will have RPS values between 60 and 80 percent. In addition to this attribute, when compared to vaccination, the QTL eggs have several other distinct advantages: For example the fish will be protected against IPN from day one and at the same time no expensive and resourcedemanding procedures such as controlling outbreaks using heated water, will be required during production, in order to maintain a high level of protection.

Good Response from the Market

When the market was approached for the controlled launch of the new products in the autumn of 2009 a very good response came back and nearly 50 million QTL eggs, (corresponding to about 14 % of the total annual egg market in Norway) were finally delivered. To assemble even more documentation about the new products, a comprehensive follow-up project has been started, where IPN protection is tested in both large-scale commercial field conditions and under controlled laboratory conditions.

Documentation of QTL Eggs in Infection Trials 

Figure1. Results from IPN infection trials with fry from different egg types. Two parallels each with commercial eggs were selected with (RxEQTL) or without (RxE) the use of the gene marker. In addition there was one tank each of IPN resistant and IPN sensitive fish, as well as a tank with un-infected fish. Each tank contained 200 start-feeding fry which weighed about 0.2 grams at the start of the experiment.

Figure1. Results from IPN infection trials with fry from different egg types. Two parallels
each with commercial eggs were selected with (RxEQTL) or without (RxE) the use of the
gene marker. In addition there was one tank each of IPN resistant and IPN sensitive fish,
as well as a tank with un-infected fish. Each tank contained 200 start-feeding fry which
weighed about 0.2 grams at the start of the experiment.

In the spring of 2010 an infection trial was run at the Aquaculture Station in Tromsø under the direction of Nofima Marin. Different groups of start-feeding fry were exposed to the IPN virus by a validated bath challenge test. Two different groups of commercial eggs were examined; RxEQTL and RxE. These experimental groups were represented with two parallels each, and originated from egg batches that were delivered to customers. Two experimental control groups were also included, with IPN resistant and IPN sensitive fish, respectively. These control groups were selected on the basis of breeding values for IPN resistance calculated on the basis of previous challenge tests together with the use of gene markers. In addition to one parallel each of IPN resistant and IPN sensitive fish, an uninfected fish group was included.

Figure 1 shows the survival in the different groups after IPN infection. Despite fish in the RxEQTL group being exposed to large amounts of IPN virus, mortality was at the same level as the uninfected control fish. It is also important that results from this experiment also show that there are significantly fewer carriers at the end of the experiment in the group of RxEQTL fish compared with the RxE group. Surviving carriers are probably the greatest source of infection for IPN virus, and fewer carriers will cause a reduction in the total infection pressure in facilities which receive QTL eggs and the stock that is derived from them.

Documentation of QTL Eggs Under Field Conditions 

Figure 2. Results from field investigations of QTL eggs in hatcheries which previously experienced large losses due to IPN. The figure shows the accumulated number of IPN outbreaks for these facilities in 2010 compared with the same period in 2009. The numbers from 2010 comprises 38.6 million QTL eggs distributed over 22 egg deliveries in 15 hatcheries.

Figure 2. Results from field investigations of QTL eggs in hatcheries which previously
experienced large losses due to IPN. The figure shows the accumulated number of IPN
outbreaks for these facilities in 2010 compared with the same period in 2009. The numbers
from 2010 comprises 38.6 million QTL eggs distributed over 22 egg deliveries in 15
hatcheries.

Throughout 2010 as well as most of 2011 Aqua Gen will collect as much information as possible about the performance of stock derived from the QTL eggs which were delivered in the 2009/2010 season. All customers who have received QTL eggs have been invited to participate. The investigation of resistance to IPN is the main goal of the project, and four time point “milestones” for measuring are planned for status reporting. The project timescale is to run up until 90 days after smolt transfer for each batch, but some groups will be followed all the way to the harvest point. For every milestone the main question will be if an IPN
outbreak has occurred or not, but data about growth and mortality as a result of diseases other than IPN will also be recorded.

Several of the facilities which have focused heavily on QTL eggs are plants which previously have experienced severe IPN problems. It is therefore interesting to compare the results from this year’s deliveries of QTL eggs with earlier years’ experience with IPN in the same facilities.

So far we have received data up until July, 2010, but already now the results look very promising for QTL eggs (Figure 2). If one looks specifically at facilities which have previously had extensive IPN problems, one sees that in July, 2009, seven IPN outbreaks had already been registered. The corresponding data for the same facilities in 2010 is: no IPN outbreaks in fry from QTL eggs at the end of July.