Transgenic Hybridization of Atlantic Salmon and Brown Trout

by Morgan Beltz

With the possibility of genetically modified salmon being approved for the food industry, there is a growing concern of what would happen if a transgenic salmon breeds with a wild salmon. Oke et al. (2013) studied what the effects of breeding transgenic salmon and of cross breeding transgenic salmon with a brown trout would be in the wild. The authors created control and cross breeding scenarios which consisted of six brown trout families, two Atlantic salmon hybrid families (the mother was a transgenic salmon), four brown trout families (the mother was a brown trout), and seven salmon families (five families had transgenic mothers and two had transgenic fathers). The authors conducted this study in two different environments; one to mimic hatchery conditions, and the other, in stream mesocosms, to resemble natural conditions. The results show that the hybrids grew more rapidly than either transgenic salmon or non-transgenic fish in hatchery conditions. In the stream mesocosm the transgenic hybrid salmon and brown trout grew 86% and 87% faster than transgenic salmon. However, wild-type salmon grew at similar rates to wild-type hybrid brown trout and salmon. These results show the importance of not allowing… genetically modified fish to breed with the wild population.

With the possibility of genetically modified salmon being approved for the food industry, there is a growing concern of what would happen if a transgenic salmon breeds with a wild salmon. Oke et al. (2013) studied what the effects of breeding transgenic salmon and of cross breeding transgenic salmon with a brown trout would be in the wild. The authors created control and cross breeding scenarios which consisted of six brown trout families, two Atlantic salmon hybrid families (the mother was a transgenic salmon), four brown trout families (the mother was a brown trout), and seven salmon families (five families had transgenic mothers and two had transgenic fathers). The authors conducted this study in two different environments; one to mimic hatchery conditions, and the other, in stream mesocosms, to resemble natural conditions. The results show that the hybrids grew more rapidly than either transgenic salmon or non-transgenic fish in hatchery conditions. In the stream mesocosm the transgenic hybrid salmon and brown trout grew 86% and 87% faster than transgenic salmon. However, wild-type salmon grew at similar rates to wild-type hybrid brown trout and salmon. These results show the importance of not allowing genetically modified fish to breed with the wild population.

Oke et al. first examined the fish in hatchery-like conditions. Each fish family was raised in an individual tank and fed commercial dry fish feed for 100 days. The families were separated in order to trace the transgene from the parents into the hybrid offspring. Mortalities were removed daily, and every second one was examined for the presence of the transgene. At the beginning of the experiment, the mass and length of 30 individuals from each family were measured and the fish with transgenic parents were genotyped. After 100 days, fish were resampled for mass and length and genotyped to follow the transgene through breeding. Following hatchery-like early development the fish were put in stream mesocosms for 30 days. The authors looked at sympatric and allopatric treatments to see what the difference of having transgenic hybrid fish in natural conditions is. The sympatric treatments contained salmon and either an Atlantic salmon or brown trout transgenic hybrid, whereas the allopatric treatments only had salmon. The fish were fed by drip feeders with feed of variable shapes and colors to mimic natural conditions. Before and after the experiment fish were sampled for length and mass, and genotyped to map the transgene and its affect throughout the study.

The authors used the polymerase chain reaction (PCR) test on dried fish skin for genetic screening. This allowed the DNA strands to be separated and viewed for presence of the transgene. In the hatchery conditions the authors used a generalized linear model (GLM) to evaluate growth among the different cross breeds and genotypes. This model was also used to determine if mortalities were related to genotypes and fitness. The same model was used for the stream mesocosms, but an analysis of variance (ANOVA) was used to determine if growth rates differed among transgenic and non-transgenic salmon and hybrids.

The PCR analysis showed 43% of the 363 fish tested positive for the transgene. Of the Atlantic salmon hybrids, only 37% were positive for the transgene, significantly lower than the 50% expected. Forty-two percent of brown trout hybrids were positive for the transgene, still lower than the expected amount. In the hatchery conditions the authors found that growth rates differed significantly between the transgenic and non-transgenic fish. In all cases the transgenic fish grew significantly faster. These results also showed that the transgenic hybrids grew faster than the transgenic salmon. The authors also measured the mortality rate among the fish, finding that transgenic and non-transgenic brown trout hybrids had higher mortality rates than all the other crosses. The results also showed that the direction of hybridization affects mortality; transgenic Atlantic salmon (AS) hybrids had higher mortality than the wild-type AS hybrids, whereas wild-type brown trout (BT) hybrids had a higher mortality than transgenic BT hybrids.

The stream mesocosm resulted in growth being reduced all around in the sympatric scenarios. In the sympatric scenarios the wild-type salmon growth rate was reduced by 54% when compared to the allopatric scenario, and the transgenic salmon growth rate was reduced 82% relative to the allopatric scenario. When looking at the sympatric scenarios further, the transgenic hybrids grew 87% faster than transgenic salmon, and the wild-type hybrids and salmon grew at similar rates.

This study suggests that the effect of the transgene on juvenile salmon growth is dependent on the environment. Transgenic fish grew faster in hatchery conditions than in more natural conditions. However, the results also showed that the presence of transgenic hybrids suppresses the growth of wild-type fish. With being about 20% more common in the mesocosm, the transgenic hybrids would suppresses the growth of wild-type fish at a faster rate than anticipated. Although this study only looks at the juvenile stage, it shows the possible implications of transgenic hybrids affecting the natural stock. As the production of transgenic fish continues to develop, the authors note the importance of regulating hybridization to prevent a more rapid decline of natural salmon from occurring.

Oke, K., Westley, P., Moreau, D., Fleming, I., 2013. Hybridization between genetically modified Atlantic salmon and wild brown trout reveals novel ecological interactions. Proceedings of the Royal Society 280 (1763). http://bit.ly/1xmb3RQ

 

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